Patterns and shapes

The pattern syntax builds on Python’s existing syntax for sequence unpacking (e.g., a, b = value).

A match statement compares a value (the subject) to several different shapes (the patterns) until a shape fits. Each pattern describes the type and structure of the accepted values as well as the variables where to capture its contents.

Patterns can specify the shape to be:

• a sequence to be unpacked, as already mentioned
• a mapping with specific keys
• an instance of a given class with (optionally) specific attributes
• a specific value
• a wildcard

Patterns can be composed in several ways.

Syntax

Syntactically, a match statement contains:

• a subject expression
• one or more case clauses

Each case clause specifies:

• a pattern (the overall shape to be matched)
• an optional “guard” (a condition to be checked if the pattern matches)
• a code block to be executed if the case clause is selected

Motivation

The rest of the PEP:

• motivates why we believe pattern matching makes a good addition to Python
• explains our design choices
• contains a precise syntactic and runtime specification
• gives guidance for static type checkers (and one small addition to the typing module)
• discusses the main objections and alternatives that have been brought up during extensive discussion of the proposal, both within the group of authors and in the python-dev community

Finally, we discuss some possible extensions that might be considered in the future, once the community has ample experience with the currently proposed syntax and semantics.

Pattern, a new syntactic construct, and destructuring

A new syntactic construct called pattern is introduced in this PEP. Syntactically, patterns look like a subset of expressions. The following are examples of patterns:

• [first, second, *rest]
• Point2d(x, 0)
• {"name": "Bruce", "age": age}
• 42

The above expressions may look like examples of object construction with a constructor which takes some values as parameters and builds an object from those components.

When viewed as a pattern, the above patterns mean the inverse operation of construction, which we call destructuring. Destructuring takes a subject value and extracts its components.

The syntactic similarity between object construction and destructuring is intentional. It also follows the existing Pythonic style of contexts which makes assignment targets (write contexts) look like expressions (read contexts).

Pattern matching never creates objects. This is in the same way that [a, b] = my_list doesn’t create a new [a, b] list, nor reads the values of a and b.

Matching process

During this matching process, the structure of the pattern may not fit the subject, and matching fails.

For example, matching the pattern Point2d(x, 0) to the subject Point2d(3, 0) successfully matches. The match also binds the pattern’s free variable x to the subject’s value 3.

As another example, if the subject is [3, 0], the match fails because the subject’s type list is not the pattern’s Point2d.

As a third example, if the subject is Point2d(3, 7), the match fails because the subject’s second coordinate 7 is not the same as the pattern’s 0.

The match statement tries to match a single subject to each of the patterns in its case clauses. At the first successful match to a pattern in a case clause:

• the variables in the pattern are assigned, and
• a corresponding block is executed.

Each case clause can also specify an optional boolean condition, known as a guard.

Let’s look at a more detailed example of a match statement. The match statement is used within a function to define the building of 3D points. In this example, the function can accept as input any of the following: tuple with 2 elements, tuple with 3 elements, an existing Point2d object or an existing Point3d object:

def make_point_3d(pt):
    match pt:
        case (x, y):
            return Point3d(x, y, 0)
        case (x, y, z):
            return Point3d(x, y, z)
        case Point2d(x, y):
            return Point3d(x, y, 0)
        case Point3d(_, _, _):
            return pt
        case _:
            raise TypeError("not a point we support")

Without pattern matching, this function’s implementation would require several isinstance() checks, one or two len() calls, and a more convoluted control flow. The match example version and the traditional Python version without match translate into similar code under the hood. With familiarity of pattern matching, a user reading this function using match will likely find this version clearer than the traditional approach.

Patterns

The pattern is a new syntactic construct, that could be considered a loose generalization of assignment targets. The key properties of a pattern are what types and shapes of subjects it accepts, what variables it captures and how it extracts them from the subject. For example, the pattern [a, b] matches only sequences of exactly 2 elements, extracting the first element into a and the second one into b.

This PEP defines several types of patterns. These are certainly not the only possible ones, so the design decision was made to choose a subset of functionality that is useful now but conservative. More patterns can be added later as this feature gets more widespread use. See the rejected ideas and deferred ideas sections for more details.

The patterns listed here are described in more detail below, but summarized together in this section for simplicity:

• A literal pattern is useful to filter constant values in a structure. It looks like a Python literal (including some values like True, False and None). It only matches objects equal to the literal, and never binds.
• A capture pattern looks like x and is equivalent to an identical assignment target: it always matches and binds the variable with the given (simple) name.
• The wildcard pattern is a single underscore: _. It always matches, but does not capture any variable (which prevents interference with other uses for _ and allows for some optimizations).
• A constant value pattern works like the literal but for certain named constants. Note that it must be a qualified (dotted) name, given the possible ambiguity with a capture pattern. It looks like Color.RED and only matches values equal to the corresponding value. It never binds.
• A sequence pattern looks like [a, *rest, b] and is similar to a list unpacking. An important difference is that the elements nested within it can be any kind of patterns, not just names or sequences. It matches only sequences of appropriate length, as long as all the sub-patterns also match. It makes all the bindings of its sub-patterns.
• A mapping pattern looks like {"user": u, "emails": [*es]}. It matches mappings with at least the set of provided keys, and if all the sub-patterns match their corresponding values. It binds whatever the sub-patterns bind while matching with the values corresponding to the keys. Adding **rest at the end of the pattern to capture extra items is allowed.
• A class pattern is similar to the above but matches attributes instead of keys. It looks like datetime.date(year=y, day=d). It matches instances of the given type, having at least the specified attributes, as long as the attributes match with the corresponding sub-patterns. It binds whatever the sub-patterns bind when matching with the values of the given attributes. An optional protocol also allows matching positional arguments.
• An OR pattern looks like [*x] | {"elems": [*x]}. It matches if any of its sub-patterns match. It uses the binding for the leftmost pattern that matched.
• A walrus pattern looks like d := datetime(year=2020, month=m). It matches only if its sub-pattern also matches. It binds whatever the sub-pattern match does, and also binds the named variable to the entire object.

The match statement

A simplified, approximate grammar for the proposed syntax is:

...
compound_statement:
    | if_stmt
    ...
    | match_stmt
match_stmt: "match" expression ':' NEWLINE INDENT case_block+ DEDENT
case_block: "case" pattern [guard] ':' block
guard: 'if' expression
pattern: walrus_pattern | or_pattern
walrus_pattern: NAME ':=' or_pattern
or_pattern: closed_pattern ('|' closed_pattern)*
closed_pattern:
    | literal_pattern
    | capture_pattern
    | wildcard_pattern
    | constant_pattern
    | sequence_pattern
    | mapping_pattern
    | class_pattern

See Appendix A for the full, unabridged grammar. The simplified grammars in this section are there for helping the reader, not as a full specification.

We propose that the match operation should be a statement, not an expression. Although in many languages it is an expression, being a statement better suits the general logic of Python syntax. See rejected ideas for more discussion. The allowed patterns are described in detail below in the patterns subsection.

The match and case keywords are proposed to be soft keywords, so that they are recognized as keywords at the beginning of a match statement or case block respectively, but are allowed to be used in other places as variable or argument names.

The proposed indentation structure is as following:

match some_expression:
    case pattern_1:
        ...
    case pattern_2:
        ...

Here, some_expression represents the value that is being matched against, which will be referred to hereafter as the subject of the match.

Match semantics

The proposed large scale semantics for choosing the match is to choose the first matching pattern and execute the corresponding suite. The remaining patterns are not tried. If there are no matching patterns, the statement ‘falls through’, and execution continues at the following statement.

Essentially this is equivalent to a chain of if ... elif ... else statements. Note that unlike for the previously proposed switch statement, the pre-computed dispatch dictionary semantics does not apply here.

There is no default or else case - instead the special wildcard _ can be used (see the section on capture_pattern) as a final ‘catch-all’ pattern.

Name bindings made during a successful pattern match outlive the executed suite and can be used after the match statement. This follows the logic of other Python statements that can bind names, such as for loop and with statement. For example:

match shape:
    case Point(x, y):
        ...
    case Rectangle(x, y, _, _):
        ...
print(x, y)  # This works

During failed pattern matches, some sub-patterns may succeed. For example, while matching the value [0, 1, 2] with the pattern (0, x, 1), the sub-pattern x may succeed if the list elements are matched from left to right. The implementation may choose to either make persistent bindings for those partial matches or not. User code including a match statement should not rely on the bindings being made for a failed match, but also shouldn’t assume that variables are unchanged by a failed match. This part of the behavior is left intentionally unspecified so different implementations can add optimizations, and to prevent introducing semantic restrictions that could limit the extensibility of this feature.

Note that some pattern types below define more specific rules about when the binding is made.

Allowed patterns

We introduce the proposed syntax gradually. Here we start from the main building blocks. The following patterns are supported:

literal_pattern:
    | number
    | string
    | 'None'
    | 'True'
    | 'False'

match number:
    case 0:
        print("Nothing")
    case 1:
        print("Just one")
    case 2:
        print("A couple")
    case -1:
        print("One less than nothing")
    case 1-1j:
        print("Good luck with that...")

case True:
    ...

case 1:
    ...

capture_pattern: NAME

match greeting:
    case "":
        print("Hello!")
    case name:
        print(f"Hi {name}!")

match greeting:
    case "":
        print("Hello!")
    case name:
        print(f"Hi {name}!")
if name == "Santa":      # <-- might raise UnboundLocalError
    ...                  # but works fine if greeting was not empty

match data:
    case [x, x]:  # Error!
        ...

wildcard_pattern: "_"

match data:
    case [_, _]:
        print("Some pair")
        print(_)  # Error!

constant_pattern: NAME ('.' NAME)+

from enum import Enum

class Sides(str, Enum):
    SPAM = "Spam"
    EGGS = "eggs"
    ...

match entree[-1]:
    case Sides.SPAM:  # Compares entree[-1] == Sides.SPAM.
        response = "Have you got anything without Spam?"
    case side:  # Assigns side = entree[-1].
        response = f"Well, could I have their Spam instead of the {side} then?"

sequence_pattern:
    | '[' [values_pattern] ']'
    | '(' [value_pattern ',' [values pattern]] ')'
values_pattern: ','.value_pattern+ ','?
value_pattern: '*' capture_pattern | pattern

match collection:
    case 1, [x, *others]:
        print("Got 1 and a nested sequence")
    case (1, x):
        print(f"Got 1 and {x}")

mapping_pattern: '{' [items_pattern] '}'
items_pattern: ','.key_value_pattern+ ','?
key_value_pattern:
    | (literal_pattern | constant_pattern) ':' or_pattern
    | '**' capture_pattern

import constants

match config:
    case {"route": route}:
        process_route(route)
    case {constants.DEFAULT_PORT: sub_config, **rest}:
        process_config(sub_config, rest)

class_pattern:
    | name_or_attr '(' ')'
    | name_or_attr '(' ','.pattern+ ','? ')'
    | name_or_attr '(' ','.keyword_pattern+ ','? ')'
    | name_or_attr '(' ','.pattern+ ',' ','.keyword_pattern+ ','? ')'
keyword_pattern: NAME '=' or_pattern

match shape:
    case Point(x, y):
        ...
    case Rectangle(x0, y0, x1, y1, painted=True):
        ...

Combining multiple patterns (OR patterns)

Multiple alternative patterns can be combined into one using |. This means the whole pattern matches if at least one alternative matches. Alternatives are tried from left to right and have a short-circuit property, subsequent patterns are not tried if one matched. Examples:

match something:
    case 0 | 1 | 2:
        print("Small number")
    case [] | [_]:
        print("A short sequence")
    case str() | bytes():
        print("Something string-like")
    case _:
        print("Something else")

The alternatives may bind variables, as long as each alternative binds the same set of variables (excluding _). For example:

match something:
    case 1 | x:  # Error!
        ...
    case x | 1:  # Error!
        ...
    case one := [1] | two := [2]:  # Error!
        ...
    case Foo(arg=x) | Bar(arg=x):  # Valid, both arms bind 'x'
        ...
    case [x] | x:  # Valid, both arms bind 'x'
        ...

Guards

Each top-level pattern can be followed by a guard of the form if expression. A case clause succeeds if the pattern matches and the guard evaluates to a true value. For example:

match input:
    case [x, y] if x > MAX_INT and y > MAX_INT:
        print("Got a pair of large numbers")
    case x if x > MAX_INT:
        print("Got a large number")
    case [x, y] if x == y:
        print("Got equal items")
    case _:
        print("Not an outstanding input")

If evaluating a guard raises an exception, it is propagated onwards rather than fail the case clause. Names that appear in a pattern are bound before the guard succeeds. So this will work:

values = [0]

match values:
    case [x] if x:
        ...  # This is not executed
    case _:
        ...
print(x)  # This will print "0"

Note that guards are not allowed for nested patterns, so that [x if x > 0] is a SyntaxError and 1 | 2 if 3 | 4 will be parsed as (1 | 2) if (3 | 4).

Walrus patterns

It is often useful to match a sub-pattern and bind the corresponding value to a name. For example, it can be useful to write more efficient matches, or simply to avoid repetition. To simplify such cases, any pattern (other than the walrus pattern itself) can be preceded by a name and the walrus operator (:=). For example:

match get_shape():
    case Line(start := Point(x, y), end) if start == end:
        print(f"Zero length line at {x}, {y}")

The name on the left of the walrus operator can be used in a guard, in the match suite, or after the match statement. However, the name will only be bound if the sub-pattern succeeds. Another example:

match group_shapes():
    case [], [point := Point(x, y), *other]:
        print(f"Got {point} in the second group")
        process_coordinates(x, y)
        ...

Technically, most such examples can be rewritten using guards and/or nested match statements, but this will be less readable and/or will produce less efficient code. Essentially, most of the arguments in PEP 572 apply here equally.

The wildcard _ is not a valid name here.

The Match Protocol

The equivalent of an isinstance call is used to decide whether an object matches a given class pattern and to extract the corresponding attributes. Classes requiring different matching semantics (such as duck-typing) can do so by defining __instancecheck__ (a pre-existing metaclass hook) or by using typing.Protocol.

The procedure is as following:

• The class object for Class in Class(<sub-patterns>) is looked up and isinstance(obj, Class) is called, where obj is the value being matched. If false, the match fails.
• Otherwise, if any sub-patterns are given in the form of positional or keyword arguments, these are matched from left to right, as follows. The match fails as soon as a sub-pattern fails; if all sub-patterns succeed, the overall class pattern match succeeds.
• If there are match-by-position items and the class has a __match_args__ attribute, the item at position i is matched against the value looked up by attribute __match_args__[i]. For example, a pattern Point2d(5, 8), where Point2d.__match_args__ == ["x", "y"], is translated (approximately) into obj.x == 5 and obj.y == 8.
• If there are more positional items than the length of __match_args__, a TypeError is raised.
• If the __match_args__ attribute is absent on the matched class, and one or more positional item appears in a match, TypeError is also raised. We don’t fall back on using __slots__ or __annotations__ – “In the face of ambiguity, refuse the temptation to guess.”
• If there are any match-by-keyword items the keywords are looked up as attributes on the subject. If the lookup succeeds the value is matched against the corresponding sub-pattern. If the lookup fails, the match fails.

Such a protocol favors simplicity of implementation over flexibility and performance. For other considered alternatives, see extended matching.

For the most commonly-matched built-in types (bool, bytearray, bytes, dict, float, frozenset, int, list, set, str, and tuple), a single positional sub-pattern is allowed to be passed to the call. Rather than being matched against any particular attribute on the subject, it is instead matched against the subject itself. This creates behavior that is useful and intuitive for these objects:

• bool(False) matches False (but not 0).
• tuple((0, 1, 2)) matches (0, 1, 2) (but not [0, 1, 2]).
• int(i) matches any int and binds it to the name i.

Overlapping sub-patterns

Certain classes of overlapping matches are detected at runtime and will raise exceptions. In addition to basic checks described in the previous subsection:

• The interpreter will check that two match items are not targeting the same attribute, for example Point2d(1, 2, y=3) is an error.
• It will also check that a mapping pattern does not attempt to match the same key more than once.

Special attribute __match_args__

The __match_args__ attribute is always looked up on the type object named in the pattern. If present, it must be a list or tuple of strings naming the allowed positional arguments.

In deciding what names should be available for matching, the recommended practice is that class patterns should be the mirror of construction; that is, the set of available names and their types should resemble the arguments to __init__().

Only match-by-name will work by default, and classes should define __match_args__ as a class attribute if they would like to support match-by-position. Additionally, dataclasses and named tuples will support match-by-position out of the box. See below for more details.

Exceptions and side effects

While matching each case, the match statement may trigger execution of other functions (for example __getitem__(), __len__() or a property). Almost every exception caused by those propagates outside of the match statement normally. The only case where an exception is not propagated is an AttributeError raised while trying to lookup an attribute while matching attributes of a Class Pattern; that case results in just a matching failure, and the rest of the statement proceeds normally.

The only side-effect carried on explicitly by the matching process is the binding of names. However, the process relies on attribute access, instance checks, len(), equality and item access on the subject and some of its components. It also evaluates constant value patterns and the left side of class patterns. While none of those typically create any side-effects, some of these objects could. This proposal intentionally leaves out any specification of what methods are called or how many times. User code relying on that behavior should be considered buggy.

The standard library

To facilitate the use of pattern matching, several changes will be made to the standard library:

• Namedtuples and dataclasses will have auto-generated __match_args__.
• For dataclasses the order of attributes in the generated __match_args__ will be the same as the order of corresponding arguments in the generated __init__() method. This includes the situations where attributes are inherited from a superclass.

In addition, a systematic effort will be put into going through existing standard library classes and adding __match_args__ where it looks beneficial.

Exhaustiveness checks

From a reliability perspective, experience shows that missing a case when dealing with a set of possible data values leads to hard to debug issues, thus forcing people to add safety asserts like this:

def get_first(data: Union[int, list[int]]) -> int:
    if isinstance(data, list) and data:
        return data[0]
    elif isinstance(data, int):
        return data
    else:
        assert False, "should never get here"

PEP 484 specifies that static type checkers should support exhaustiveness in conditional checks with respect to enum values. PEP 586 later generalized this requirement to literal types.

This PEP further generalizes this requirement to arbitrary patterns. A typical situation where this applies is matching an expression with a union type:

def classify(val: Union[int, Tuple[int, int], List[int]]) -> str:
    match val:
        case [x, y] if x > 0 and y > 0:
            return f"A pair of {x} and {y}"
        case [x, *other]:
            return f"A sequence starting with {x}"
        case int():
            return f"Some integer"
        # Type-checking error: some cases unhandled.

The exhaustiveness checks should also apply where both pattern matching and enum values are combined:

from enum import Enum
from typing import Union

class Level(Enum):
    BASIC = 1
    ADVANCED = 2
    PRO = 3

class User:
    name: str
    level: Level

class Admin:
    name: str

account: Union[User, Admin]

match account:
    case Admin(name=name) | User(name=name, level=Level.PRO):
        ...
    case User(level=Level.ADVANCED):
        ...
    # Type-checking error: basic user unhandled

Obviously, no Matchable protocol (in terms of PEP 544) is needed, since every class is matchable and therefore is subject to the checks specified above.

Sealed classes as algebraic data types

Quite often it is desirable to apply exhaustiveness to a set of classes without defining ad-hoc union types, which is itself fragile if a class is missing in the union definition. A design pattern where a group of record-like classes is combined into a union is popular in other languages that support pattern matching and is known under a name of algebraic data types.

We propose to add a special decorator class @sealed to the typing module, that will have no effect at runtime, but will indicate to static type checkers that all subclasses (direct and indirect) of this class should be defined in the same module as the base class.

The idea is that since all subclasses are known, the type checker can treat the sealed base class as a union of all its subclasses. Together with dataclasses this allows a clean and safe support of algebraic data types in Python. Consider this example:

from dataclasses import dataclass
from typing import sealed

@sealed
class Node:
    ...

class Expression(Node):
    ...

class Statement(Node):
    ...

@dataclass
class Name(Expression):
    name: str

@dataclass
class Operation(Expression):
    left: Expression
    op: str
    right: Expression

@dataclass
class Assignment(Statement):
    target: str
    value: Expression

@dataclass
class Print(Statement):
    value: Expression

With such definition, a type checker can safely treat Node as Union[Name, Operation, Assignment, Print], and also safely treat e.g. Expression as Union[Name, Operation]. So this will result in a type checking error in the below snippet, because Name is not handled (and type checker can give a useful error message):

def dump(node: Node) -> str:
    match node:
        case Assignment(target, value):
            return f"{target} = {dump(value)}"
        case Print(value):
            return f"print({dump(value)})"
        case Operation(left, op, right):
            return f"({dump(left)} {op} {dump(right)})"

Type erasure

Class patterns are subject to runtime type erasure. Namely, although one can define a type alias IntQueue = Queue[int] so that a pattern like IntQueue() is syntactically valid, type checkers should reject such a match:

queue: Union[Queue[int], Queue[str]]
match queue:
    case IntQueue():  # Type-checking error here
        ...

Note that the above snippet actually fails at runtime with the current implementation of generic classes in the typing module, as well as with builtin generic classes in the recently accepted PEP 585, because they prohibit isinstance checks.

To clarify, generic classes are not prohibited in general from participating in pattern matching, just that their type parameters can’t be explicitly specified. It is still fine if sub-patterns or literals bind the type variables. For example:

from typing import Generic, TypeVar, Union

T = TypeVar('T')

class Result(Generic[T]):
    first: T
    other: list[T]

result: Union[Result[int], Result[str]]

match result:
    case Result(first=int()):
        ...  # Type of result is Result[int] here
    case Result(other=["foo", "bar", *rest]):
        ...  # Type of result is Result[str] here

Note about constants

The fact that a capture pattern is always an assignment target may create unwanted consequences when a user by mistake tries to “match” a value against a constant instead of using the constant value pattern. As a result, at runtime such a match will always succeed and moreover override the value of the constant. It is important therefore that static type checkers warn about such situations. For example:

from typing import Final

MAX_INT: Final = 2 ** 64

value = 0

match value:
    case MAX_INT:  # Type-checking error here: cannot assign to final name
        print("Got big number")
    case _:
        print("Something else")

Note that the CPython reference implementation also generates a SyntaxWarning message for this case.

Precise type checking of star matches

Type checkers should perform precise type checking of star items in pattern matching giving them either a heterogeneous list[T] type, or a TypedDict type as specified by PEP 589. For example:

stuff: Tuple[int, str, str, float]

match stuff:
    case a, *b, 0.5:
        # Here a is int and b is list[str]
        ...

Don’t do this, pattern matching is hard to learn

In our opinion, the proposed pattern matching is not more difficult than adding isinstance() and getattr() to iterable unpacking. Also, we believe the proposed syntax significantly improves readability for a wide range of code patterns, by allowing to express what one wants to do, rather than how to do it. We hope the few real code snippets we included in the PEP above illustrate this comparison well enough. For more real code examples and their translations see Ref. [1].

Don’t do this, use existing method dispatching mechanisms

We recognize that some of the use cases for the match statement overlap with what can be done with traditional object-oriented programming (OOP) design techniques using class inheritance. The ability to choose alternate behaviors based on testing the runtime type of a match subject might even seem heretical to strict OOP purists.

However, Python has always been a language that embraces a variety of programming styles and paradigms. Classic Python design idioms such as “duck”-typing go beyond the traditional OOP model.

We believe that there are important use cases where the use of match results in a cleaner and more maintainable architecture. These use cases tend to be characterized by a number of features:

• Algorithms which cut across traditional lines of data encapsulation. If an algorithm is processing heterogeneous elements of different types (such as evaluating or transforming an abstract syntax tree, or doing algebraic manipulation of mathematical symbols), forcing the user to implement the algorithm as individual methods on each element type results in logic that is smeared across the entire codebase instead of being neatly localized in one place.
• Program architectures where the set of possible data types is relatively stable, but there is an ever-expanding set of operations to be performed on those data types. Doing this in a strict OOP fashion requires constantly adding new methods to both the base class and subclasses to support the new methods, “polluting” the base class with lots of very specialized method definitions, and causing widespread disruption and churn in the code. By contrast, in a match-based dispatch, adding a new behavior merely involves writing a new match statement.
• OOP also does not handle dispatching based on the shape of an object, such as the length of a tuple, or the presence of an attribute – instead any such dispatching decision must be encoded into the object’s type. Shape-based dispatching is particularly interesting when it comes to handling “duck”-typed objects.

Where OOP is clearly superior is in the opposite case: where the set of possible operations is relatively stable and well-defined, but there is an ever-growing set of data types to operate on. A classic example of this is UI widget toolkits, where there is a fixed set of interaction types (repaint, mouse click, keypress, and so on), but the set of widget types is constantly expanding as developers invent new and creative user interaction styles. Adding a new kind of widget is a simple matter of writing a new subclass, whereas with a match-based approach you end up having to add a new case clause to many widespread match statements. We therefore don’t recommend using match in such a situation.

Allow more flexible assignment targets instead

There was an idea to instead just generalize the iterable unpacking to much more general assignment targets, instead of adding a new kind of statement. This concept is known in some other languages as “irrefutable matches”. We decided not to do this because inspection of real-life potential use cases showed that in vast majority of cases destructuring is related to an if condition. Also many of those are grouped in a series of exclusive choices.

Make it an expression

In most other languages pattern matching is represented by an expression, not statement. But making it an expression would be inconsistent with other syntactic choices in Python. All decision making logic is expressed almost exclusively in statements, so we decided to not deviate from this.

Use a hard keyword

There were options to make match a hard keyword, or choose a different keyword. Although using a hard keyword would simplify life for simple-minded syntax highlighters, we decided not to use hard keyword for several reasons:

• Most importantly, the new parser doesn’t require us to do this. Unlike with async that caused hardships with being a soft keyword for few releases, here we can make match a permanent soft keyword.
• match is so commonly used in existing code, that it would break almost every existing program and will put a burden to fix code on many people who may not even benefit from the new syntax.
• It is hard to find an alternative keyword that would not be commonly used in existing programs as an identifier, and would still clearly reflect the meaning of the statement.

Use as or | instead of case for case clauses

The pattern matching proposed here is a combination of multi-branch control flow (in line with switch in Algol-derived languages or cond in Lisp) and object-deconstruction as found in functional languages. While the proposed keyword case highlights the multi-branch aspect, alternative keywords such as as would equally be possible, highlighting the deconstruction aspect. as or with, for instance, also have the advantage of already being keywords in Python. However, since case as a keyword can only occur as a leading keyword inside a match statement, it is easy for a parser to distinguish between its use as a keyword or as a variable.

Other variants would use a symbol like | or =>, or go entirely without special marker.

Since Python is a statement-oriented language in the tradition of Algol, and as each composite statement starts with an identifying keyword, case seemed to be most in line with Python’s style and traditions.

Use a flat indentation scheme

There was an idea to use an alternative indentation scheme, for example where every case clause would not be indented with respect to the initial match part:

match expression:
case pattern_1:
    ...
case pattern_2:
    ...

The motivation is that although flat indentation saves some horizontal space, it may look awkward to an eye of a Python programmer, because everywhere else colon is followed by an indent. This will also complicate life for simple-minded code editors. Finally, the horizontal space issue can be alleviated by allowing “half-indent” (i.e. two spaces instead of four) for match statements.

In sample programs using match, written as part of the development of this PEP, a noticeable improvement in code brevity is observed, more than making up for the additional indentation level.

Another proposal considered was to use flat indentation but put the expression on the line after match:, like this:

match:
    expression
case pattern_1:
    ...
case pattern_2:
    ...

This was ultimately rejected because the first block would be a novelty in Python’s grammar: a block whose only content is a single expression rather than a sequence of statements.

Alternatives for constant value pattern

This is probably the trickiest item. Matching against some pre-defined constants is very common, but the dynamic nature of Python also makes it ambiguous with capture patterns. Five other alternatives were considered:

• Use some implicit rules. For example, if a name was defined in the global scope, then it refers to a constant, rather than representing a capture pattern:

  # Here, the name "spam" must be defined in the global scope (and
  # not shadowed locally). "side" must be local.
  
  match entree[-1]:
      case spam: ...  # Compares entree[-1] == spam.
      case side: ...  # Assigns side = entree[-1].
  

  This however can cause surprises and action at a distance if someone defines an unrelated coinciding name before the match statement.

• Use a rule based on the case of a name. In particular, if the name starts with a lowercase letter it would be a capture pattern, while if it starts with uppercase it would refer to a constant:

  match entree[-1]:
      case SPAM: ...  # Compares entree[-1] == SPAM.
      case side: ...  # Assigns side = entree[-1].
  

  This works well with the recommendations for naming constants from PEP 8. The main objection is that there’s no other part of core Python where the case of a name is semantically significant. In addition, Python allows identifiers to use different scripts, many of which (e.g. CJK) don’t have a case distinction.

• Use extra parentheses to indicate lookup semantics for a given name. For example:

  match entree[-1]:
      case (spam): ...  # Compares entree[-1] == spam.
      case side: ...    # Assigns side = entree[-1].
  

  This may be a viable option, but it can create some visual noise if used often. Also honestly it looks pretty unusual, especially in nested contexts.

  This also has the problem that we may want or need parentheses to disambiguate grouping in patterns, e.g. in Point(x, y=(y := complex())).

• Introduce a special symbol, for example ., ?, $, or ^ to indicate that a given name is a value to be matched against, not to be assigned to. An earlier version of this proposal used a leading-dot rule:

  match entree[-1]:
      case .spam: ...  # Compares entree[-1] == spam.
      case side: ...   # Assigns side = entree[-1].
  

  While potentially useful, it introduces strange-looking new syntax without making the pattern syntax any more expressive. Indeed, named constants can be made to work with the existing rules by converting them to Enum types, or enclosing them in their own namespace (considered by the authors to be one honking great idea):

  match entree[-1]:
      case Sides.SPAM: ...  # Compares entree[-1] == Sides.SPAM.
      case side: ...        # Assigns side = entree[-1].
  

  If needed, the leading-dot rule (or a similar variant) could be added back later with no backward-compatibility issues.

• There was also an idea to make lookup semantics the default, and require $ or ? to be used in capture patterns:

  match entree[-1]:
      case spam: ...   # Compares entree[-1] == spam.
      case side?: ...  # Assigns side = entree[-1].
  

  There are a few issues with this:

  • Capture patterns are more common in typical code, so it is undesirable to require special syntax for them.
  • The authors are not aware of any other language that adorns captures in this way.
  • None of the proposed syntaxes have any precedent in Python; no other place in Python that binds names (e.g. import, def, for) uses special marker syntax.
  • It would break the syntactic parallels of the current grammar:

    match coords:
        case ($x, $y):
            return Point(x, y)  # Why not "Point($x, $y)"?
    

In the end, these alternatives were rejected because of the mentioned drawbacks.

Disallow float literals in patterns

Because of the inexactness of floats, an early version of this proposal did not allow floating-point constants to be used as match patterns. Part of the justification for this prohibition is that Rust does this.

However, during implementation, it was discovered that distinguishing between float values and other types required extra code in the VM that would slow matches generally. Given that Python and Rust are very different languages with different user bases and underlying philosophies, it was felt that allowing float literals would not cause too much harm, and would be less surprising to users.

Range matching patterns

This would allow patterns such as 1...6. However, there are a host of ambiguities:

• Is the range open, half-open, or closed? (I.e. is 6 included in the above example or not?)
• Does the range match a single number, or a range object?
• Range matching is often used for character ranges (‘a’…’z’) but that won’t work in Python since there’s no character data type, just strings.
• Range matching can be a significant performance optimization if you can pre-build a jump table, but that’s not generally possible in Python due to the fact that names can be dynamically rebound.

Rather than creating a special-case syntax for ranges, it was decided that allowing custom pattern objects (InRange(0, 6)) would be more flexible and less ambiguous; however those ideas have been postponed for the time being (See deferred ideas).

Use dispatch dict semantics for matches

Implementations for classic switch statement sometimes use a pre-computed hash table instead of a chained equality comparisons to gain some performance. In the context of match statement this is technically also possible for matches against literal patterns. However, having subtly different semantics for different kinds of patterns would be too surprising for potentially modest performance win.

We can still experiment with possible performance optimizations in this direction if they will not cause semantic differences.

Use continue and break in case clauses.

Another rejected proposal was to define new meanings for continue and break inside of match, which would have the following behavior:

• continue would exit the current case clause and continue matching at the next case clause.
• break would exit the match statement.

However, there is a serious drawback to this proposal: if the match statement is nested inside of a loop, the meanings of continue and break are now changed. This may cause unexpected behavior during refactorings; also, an argument can be made that there are other means to get the same behavior (such as using guard conditions), and that in practice it’s likely that the existing behavior of continue and break are far more useful.

AND (&) patterns

This proposal defines an OR-pattern (|) to match one of several alternates; why not also an AND-pattern (&)? Especially given that some other languages (F# for example) support this.

However, it’s not clear how useful this would be. The semantics for matching dictionaries, objects and sequences already incorporates an implicit ‘and’: all attributes and elements mentioned must be present for the match to succeed. Guard conditions can also support many of the use cases that a hypothetical ‘and’ operator would be used for.

In the end, it was decided that this would make the syntax more complex without adding a significant benefit.

Negative match patterns

A negation of a match pattern using the operator ! as a prefix would match exactly if the pattern itself does not match. For instance, !(3 | 4) would match anything except 3 or 4.

This was rejected because there is documented evidence that this feature is rarely useful (in languages which support it) or used as double negation !! to control variable scopes and prevent variable bindings (which does not apply to Python). It can also be simulated using guard conditions.

Check exhaustiveness at runtime

The question is what to do if no case clause has a matching pattern, and there is no default case. An earlier version of the proposal specified that the behavior in this case would be to throw an exception rather than silently falling through.

The arguments back and forth were many, but in the end the EIBTI (Explicit Is Better Than Implicit) argument won out: it’s better to have the programmer explicitly throw an exception if that is the behavior they want.

For cases such as sealed classes and enums, where the patterns are all known to be members of a discrete set, static checkers can warn about missing patterns.

Type annotations for pattern variables

The proposal was to combine patterns with type annotations:

match x:
    case [a: int, b: str]: print(f"An int {a} and a string {b}:)
    case [a: int, b: int, c: int]: print(f"Three ints", a, b, c)
    ...

This idea has a lot of problems. For one, the colon can only be used inside of brackets or parens, otherwise the syntax becomes ambiguous. And because Python disallows isinstance() checks on generic types, type annotations containing generics will not work as expected.

Allow *rest in class patterns

It was proposed to allow *rest in a class pattern, giving a variable to be bound to all positional arguments at once (similar to its use in unpacking assignments). It would provide some symmetry with sequence patterns. But it might be confused with a feature to provide the values for all positional arguments at once. And there seems to be no practical need for it, so it was scrapped. (It could easily be added at a later stage if a need arises.)

Disallow _.a in constant value patterns

The first public draft said that the initial name in a constant value pattern must not be _ because _ has a special meaning in pattern matching, so this would be invalid:

case _.a: ...

(However, a._ would be legal and load the attribute with name _ of the object a as usual.)

There was some pushback against this on python-dev (some people have a legitimate use for _ as an important global variable, esp. in i18n) and the only reason for this prohibition was to prevent some user confusion. But it’s not the hill to die on.

Use some other token as wildcard

It has been proposed to use ... (i.e., the ellipsis token) or * (star) as a wildcard. However, both these look as if an arbitrary number of items is omitted:

case [a, ..., z]: ...
case [a, *, z]: ...

Both look like the would match a sequence of at two or more items, capturing the first and last values.

In addition, if * were to be used as the wildcard character, we would have to come up with some other way to capture the rest of a sequence, currently spelled like this:

case [first, second, *rest]: ...

Using an ellipsis would also be more confusing in documentation and examples, where ... is routinely used to indicate something obvious or irrelevant. (Yes, this would also be an argument against the other uses of ... in Python, but that water is already under the bridge.)

Another proposal was to use ?. This could be acceptable, although it would require modifying the tokenizer.

Also, _ is already used as a throwaway target in other contexts, and this use is pretty similar. This example is from difflib.py in the stdlib:

for tag, _, _, j1, j2 in group: ...

Perhaps the most convincing argument is that _ is used as the wildcard in every other language we’ve looked at supporting pattern matching: C#, Elixir, Erlang, F#, Haskell, Mathematica, OCaml, Ruby, Rust, Scala, and Swift. Now, in general, we should not be concerned too much with what another language does, since Python is clearly different from all these languages. However, if there is such an overwhelming and strong consensus, Python should not go out of its way to do something completely different – particularly given that _ works well in Python and is already in use as a throwaway target.

Note that _ is not assigned to by patterns – this avoids conflicts with the use of _ as a marker for translatable strings and an alias for gettext.gettext, as recommended by the gettext module documentation.

Use some other syntax instead of | for OR patterns

A few alternatives to using | to separate the alternatives in OR patterns have been proposed. Instead of:

case 401|403|404:
    print("Some HTTP error")

the following proposals have been fielded:

• Use a comma:

  case 401, 403, 404:
    print("Some HTTP error")
  

  This looks too much like a tuple – we would have to find a different way to spell tuples, and the construct would have to be parenthesized inside the argument list of a class pattern. In general, commas already have many different meanings in Python, we shouldn’t add more.

• Allow stacked cases:

  case 401:
  case 403:
  case 404:
    print("Some HTTP error")
  

  This is how this would be done in C, using its fall-through semantics for cases. However, we don’t want to mislead people into thinking that match/case uses fall-through semantics (which are a common source of bugs in C). Also, this would be a novel indentation pattern, which might make it harder to support in IDEs and such (it would break the simple rule “add an indentation level after a line ending in a colon”). Finally, this wouldn’t support OR patterns nested inside other patterns.

• Use case in followed by a comma-separated list:

  case in 401, 403, 404:
    print("Some HTTP error")
  

  This wouldn’t work for OR patterns nested inside other patterns, like:

  case Point(0|1, 0|1):
      print("A corner of the unit square")
  

• Use the or keyword:

  case 401 or 403 or 404:
      print("Some HTTP error")
  

  This could work, and the readability is not too different from using |. Some users expressed a preference for or because they associate | with bitwise OR. However:

  1. Many other languages that have pattern matching use | (the list includes Elixir, Erlang, F#, Mathematica, OCaml, Ruby, Rust, and Scala).
  2. | is shorter, which may contribute to the readability of nested patterns like Point(0|1, 0|1).
  3. Some people mistakenly believe that | has the wrong priority; but since patterns don’t support other operators it has the same priority as in expressions.
  4. Python users use or very frequently, and may build an impression that it is strongly associated with Boolean short-circuiting.
  5. | is used between alternatives in regular expressions and in EBNF grammars (like Python’s own).
  6. | not just used for bitwise OR – it’s used for set unions, dict merging (PEP 584) and is being considered as an alternative to typing.Union (PEP 604).
  7. | works better as a visual separator, especially between strings. Compare:

     case "spam" or "eggs" or "cheese":
     

     to:

     case "spam" | "eggs" | "cheese":
     

Add an else clause

We decided not to add an else clause for several reasons.

• It is redundant, since we already have case _:
• There will forever be confusion about the indentation level of the else: – should it align with the list of cases or with the match keyword?
• Completionist arguments like “every other statement has one” are false – only those statements have an else clause where it adds new functionality.

One-off syntax variant

While inspecting some code-bases that may benefit the most from the proposed syntax, it was found that single clause matches would be used relatively often, mostly for various special-casing. In other languages this is supported in the form of one-off matches. We proposed to support such one-off matches too:

if match value as pattern [and guard]:
    ...

or, alternatively, without the if:

match value as pattern [if guard]:
    ...

as equivalent to the following expansion:

match value:
    case pattern [if guard]:
        ...

To illustrate how this will benefit readability, consider this (slightly simplified) snippet from real code:

if isinstance(node, CallExpr):
    if (isinstance(node.callee, NameExpr) and len(node.args) == 1 and
            isinstance(node.args[0], NameExpr)):
        call = node.callee.name
        arg = node.args[0].name
        ...  # Continue special-casing 'call' and 'arg'
...  # Follow with common code

This can be rewritten in a more straightforward way as:

if match node as CallExpr(callee=NameExpr(name=call), args=[NameExpr(name=arg)]):
    ...  # Continue special-casing 'call' and 'arg'
...  # Follow with common code

This one-off form would not allow elif match statements, as it was only meant to handle a single pattern case. It was intended to be special case of a match statement, not a special case of an if statement:

if match value_1 as patter_1 [and guard_1]:
    ...
elif match value_2 as pattern_2 [and guard_2]:  # Not allowed
    ...
elif match value_3 as pattern_3 [and guard_3]:  # Not allowed
    ...
else:  # Also not allowed
    ...

This would defeat the purpose of one-off matches as a complement to exhaustive full matches - it’s better and clearer to use a full match in this case.

Similarly, if not match would not be allowed, since match ... as ... is not an expression. Nor do we propose a while match construct present in some languages with pattern matching, since although it may be handy, it will likely be used rarely.

Other pattern-based constructions

Many other languages supporting pattern-matching use it as a basis for multiple language constructs, including a matching operator, a generalized form of assignment, a filter for loops, a method for synchronizing communication, or specialized if statements. Some of these were mentioned in the discussion of the first draft. Another question asked was why this particular form (joining binding and conditional selection) was chosen while other forms were not.

Introducing more uses of patterns would be too bold and premature given the experience we have using patterns, and would make this proposal too complicated. The statement as presented provides a form of the feature that is sufficiently general to be useful while being self-contained, and without having a massive impact on the syntax and semantics of the language as a whole.

After some experience with this feature, the community may have a better feeling for what other uses of pattern matching could be valuable in Python.

Algebraic matching of repeated names

A technique occasionally seen in functional languages like Erlang and Elixir is to use a match variable multiple times in the same pattern:

match value:
    case Point(x, x):
        print("Point is on a diagonal!")

The idea here is that the first appearance of x would bind the value to the name, and subsequent occurrences would verify that the incoming value was equal to the value previously bound. If the value was not equal, the match would fail.

However, there are a number of subtleties involved with mixing load-store semantics for capture patterns. For the moment, we decided to make repeated use of names within the same pattern an error; we can always relax this restriction later without affecting backwards compatibility.

Note that you can use the same name more than once in alternate choices:

match value:
    case x | [x]:
        # etc.

Custom matching protocol

During the initial design discussions for this PEP, there were a lot of ideas thrown around about custom matchers. There were a couple of motivations for this:

• Some classes might want to expose a different set of “matchable” names than the actual class properties.
• Some classes might have properties that are expensive to calculate, and therefore shouldn’t be evaluated unless the match pattern actually needed access to them.
• There were ideas for exotic matchers such as IsInstance(), InRange(), RegexMatchingGroup() and so on.
• In order for built-in types and standard library classes to be able to support matching in a reasonable and intuitive way, it was believed that these types would need to implement special matching logic.

These customized match behaviors would be controlled by a special __match__ method on the class name. There were two competing variants:

• A ‘full-featured’ match protocol which would pass in not only the subject to be matched, but detailed information about which attributes the specified pattern was interested in.
• A simplified match protocol, which only passed in the subject value, and which returned a “proxy object” (which in most cases could be just the subject) containing the matchable attributes.

Here’s an example of one version of the more complex protocol proposed:

match expr:
    case BinaryOp(left=Number(value=x), op=op, right=Number(value=y)):
        ...

from types import PatternObject
BinaryOp.__match__(
    (),
    {
        "left": PatternObject(Number, (), {"value": ...}, -1, False),
        "op": ...,
        "right": PatternObject(Number, (), {"value": ...}, -1, False),
    },
    -1,
    False,
)

One drawback of this protocol is that the arguments to __match__ would be expensive to construct, and could not be pre-computed due to the fact that, because of the way names are bound, there are no real constants in Python. It also meant that the __match__ method would have to re-implement much of the logic of matching which would otherwise be implemented in C code in the Python VM. As a result, this option would perform poorly compared to an equivalent if-statement.

The simpler protocol suffered from the fact that although it was more performant, it was much less flexible, and did not allow for many of the creative custom matchers that people were dreaming up.

Late in the design process, however, it was realized that the need for a custom matching protocol was much less than anticipated. Virtually all the realistic (as opposed to fanciful) uses cases brought up could be handled by the built-in matching behavior, although in a few cases an extra guard condition was required to get the desired effect.

Moreover, it turned out that none of the standard library classes really needed any special matching support other than an appropriate __match_args__ property.

The decision to postpone this feature came with a realization that this is not a one-way door; that a more flexible and customizable matching protocol can be added later, especially as we gain more experience with real-world use cases and actual user needs.

The authors of this PEP expect that the match statement will evolve over time as usage patterns and idioms evolve, in a way similar to what other “multi-stage” PEPs have done in the past. When this happens, the extended matching issue can be revisited.

Parameterized Matching Syntax

(Also known as “Class Instance Matchers”.)

This is another variant of the “custom match classes” idea that would allow diverse kinds of custom matchers mentioned in the previous section – however, instead of using an extended matching protocol, it would be achieved by introducing an additional pattern type with its own syntax. This pattern type would accept two distinct sets of parameters: one set which consists of the actual parameters passed into the pattern object’s constructor, and another set representing the binding variables for the pattern.

The __match__ method of these objects could use the constructor parameter values in deciding what was a valid match.

This would allow patterns such as InRange<0, 6>(value), which would match a number in the range 0..6 and assign the matched value to ‘value’. Similarly, one could have a pattern which tests for the existence of a named group in a regular expression match result (different meaning of the word ‘match’).

Although there is some support for this idea, there was a lot of bikeshedding on the syntax (there are not a lot of attractive options available) and no clear consensus was reached, so it was decided that for now, this feature is not essential to the PEP.

Pattern Utility Library

Both of the previous ideas would be accompanied by a new Python standard library module which would contain a rich set of useful matchers. However, it is not really possible to implement such a library without adopting one of the extended pattern proposals given in the previous sections, so this idea is also deferred.