Notice: While Javascript is not essential for this website, your interaction with the content will be limited. Please turn Javascript on for the full experience.

PEP 505 -- None-aware operators

PEP:505
Title:None-aware operators
Author:Mark E. Haase <mehaase at gmail.com>
Status:Draft
Type:Standards Track
Created:18-Sep-2015
Python-Version:3.6

Abstract

Several modern programming languages have so-called "null-coalescing" or "null- aware" operators, including C# [1], Dart [2], Perl, Swift, and PHP (starting in version 7). These operators provide syntactic sugar for common patterns involving null references.

  • The "null-coalescing" operator is a binary operator that returns its left operand if it is not null. Otherwise it returns its right operand.
  • The "null-aware member access" operator accesses an instance member only if that instance is non-null. Otherwise it returns null. (This is also called a "safe navigation" operator.)
  • The "null-aware index access" operator accesses an element of a collection only if that collection is non-null. Otherwise it returns null. (This is another type of "safe navigation" operator.)

The purpose of this PEP is to explore the possibility of implementing similar operators in Python. It provides some background material and then offers several competing alternatives for implementation.

The initial reaction to this idea is majority negative. Even if ultimately rejected, this PEP still serves a purpose: to fully document the reasons why Python should not add this behavior, so that it can be pointed to in the future when the question inevitably arises again. (This is the null alternative, so to speak!)

This proposal advances multiple alternatives, and it should be considered severable. It may be accepted in whole or in part. For example, the safe navigation operators might be rejected even if the null-coalescing operator is approved, or vice-versa.

Of course, Python does not have null; it has None, which is conceptually distinct. Although this PEP is inspired by "null-aware" operators in other languages, it uses the term "None-aware" operators to describe some hypothetical Python implementations.

Background

Specialness of None

The Python language does not currently define any special behavior for None. This PEP suggests making None a special case. This loss of generality is a noticeable drawback of the proposal. A generalization of None-aware operators is set forth later in this document in order to avoid this specialization.

Utility of None

One common criticism of adding special syntax for None is that None shouldn't be used in the first place: it's a code smell. A related criticism is that None-aware operators are used to silence errors (such as the novice misunderstanding of an implicit return None) akin to PHP's @ operator. Therefore, the utility of None must be debated before discussing whether to add new behavior around it.

Python does not have any concept of null. Every Python identifier must refer to an instance, so there cannot be any null references. Python does have a special instance called None that can be used to represent missing values, but None is conceptually distinct from null.

The most frequent use of None in Python is to provide a default value for optional arguments when some other default object is unwieldy. For example: def get(url, proxy=None):. In this case, proxy is an optional argument. If proxy is None, then the request should be sent directly to the server; otherwise, the request should be routed through the specified proxy server. This use of None is preferred here to some other sentinel value or the Null Object Pattern. [3]

Examples of this form abound. Consider types.py in the standard library:

def prepare_class(name, bases=(), kwds=None):
    if kwds is None:
        kwds = {}
    else:
        kwds = dict(kwds)
    ...

Another frequent use of None is interfacing with external systems. Many of those other systems have a concept of null. Therefore, Python code must have a way of representing null, and typically it is represented by None. For example, databases can have null values, and most Python database drivers will convert null to None when retrieving data from a database, and will convert from None back to null when sending data to a database.

This convention of interchanging null and None is widespread in Python. It is canonized in the Python DBAPI (PEP-249). [4] The json module in the standard library and the third party PyYAML package both use None to represent their respective languages' null.

The C language null often bleeds into Python, too, particularly for thin wrappers around C libraries. For example, in pyopenssl, the X509 class has a get_notBefore() method that returns either a timestamp or None. This function is a thin wrapper around an OpenSSL function with the return type ASN1_TIME *. Because this C pointer may be null, the Python wrapper must be able to represent null, and None is the chosen representation.

The representation of null is particularly noticeable when Python code is marshalling data between two systems. For example, consider a Python server that fetches data from a database and converts it to JSON for consumption by another process. In this case, it's often desirable that null in the database can be easily translated to null in JSON. If None is not used for this purpose, then each package will have to define its own representation of null, and converting between these representations adds unnecessary complexity to the Python glue code.

Therefore, the preference for avoiding None is nothing more than a preference. None has legitimate uses, particularly in specific types of software. Any hypothetical None-aware operators should be construed as syntactic sugar for simplifying common patterns involving None, and should not be construed as error handling behavior.

Behavior In Other Languages

Given that null-aware operators exist in other modern languages, it may be helpful to quickly understand how they work in those languages:

/* Null-coalescing. */

String s1 = null;
String s2 = "hello";
String s3 = s1 ?? s2;
Console.WriteLine("s3 is: " + s3);
// s3 is: hello

/* Null-aware member access, a.k.a. safe navigation. */

Console.WriteLine("s1.Length is: " + s1?.Length);
Console.WriteLine("s2.Length is: " + s2?.Length);
// s1.Length is:
// s2.Length is: 5

/* Null-aware index access, a.k.a. safe navigation. */

Dictionary<string,string> d1 = null;
Dictionary<string,string> d2 = new Dictionary<string, string>
{
    { "foo", "bar" },
    { "baz", "bat" }
};

Console.WriteLine("d1[\"foo\"] is: " + d1?["foo"]);
Console.WriteLine("d2[\"foo\"] is: " + d2?["foo"]);
// d1["foo"] is:
// d2["foo"] is: bar

/* Short Circuiting */

Console.WriteLine("s1 trim/upper is: " + s1?.Trim().Length);
Console.WriteLine("s2 trim/upper is: " + s2?.Trim().Length);
// s1 trimmed length is:
// s2 trimmed length is: 5

String s4 = s1 ?? s2 ?? DoError();
Console.WriteLine("s4 is: " + s4)
// s4 is: hello

A working example can be viewed online.

Of utmost importance, notice the short circuiting behavior. The short circuiting of ?? is similar to short circuiting of other boolean operators such as || or && and should not be surprising. Helpfully, ?. is also short circuiting: s1?.Trim() evaluates to null, but s1?.Trim().Length does not attempt to dereference the null pointer.

Rationale

Existing Alternatives

Python does not have any specific None-aware operators, but it does have operators that can be used for a similar purpose. This section describes why these alternatives may be undesirable for some common None patterns.

or Operator

Similar behavior can be achieved with the or operator, but or checks whether its left operand is false-y, not specifically None. This can lead to surprising behavior. Consider the scenario of computing the price of some products a customer has in his/her shopping cart:

>>> price = 100
>>> default_quantity = 1
# If user didn't specify a quantity, then assume the default.
>>> requested_quantity = None
>>> (requested_quantity or default_quantity) * price
100
# The user added 5 items to the cart.
>>> requested_quantity = 5
>>> (requested_quantity or default_quantity) * price
500
# User removed 5 items from cart.
>>> requested_quantity = 0
>>> (requested_quantity or default_quantity) * price # oops!
100

An experienced Python developer should know how or works and be capable of avoiding bugs like this. However, getting in the habit of using or for this purpose still might cause an experienced developer to occasionally make this mistake, especially when refactoring existing code and not carefully paying attention to the possible values of the left-hand operand.

For inexperienced developers, the problem is worse. The top Google hit for "python null coalesce" is a StackOverflow page, and the top answer says to use or. The top answer goes on to explain the caveats of using or like this, but how many beginning developers go on to read all those caveats? The accepted answer on a more recent question says to use or without any caveats at all. These two questions have a combined 26,000 views!

The common usage of or for the purpose of providing default values is undeniable, and yet it is also booby-trapped for unsuspecting newcomers. This suggests that a safe operator for providing default values would have positive utility. While some critics claim that None-aware operators will be abused for error handling, they are no more prone to abuse than or is.

Ternary Operator

Another common way to initialize default values is to use the ternary operator. Here is an excerpt from the popular Requests package:

data = [] if data is None else data
files = [] if files is None else files
headers = {} if headers is None else headers
params = {} if params is None else params
hooks = {} if hooks is None else hooks

This particular formulation has the undesirable effect of putting the operands in an unintuitive order: the brain thinks, "use data if possible and use [] as a fallback," but the code puts the fallback before the preferred value.

The author of this package could have written it like this instead:

data = data if data is not None else []
files = files if files is not None else []
headers = headers if headers is not None else {}
params = params if params is not None else {}
hooks = hooks if hooks is not None else {}

This ordering of the operands is more intuitive, but it requires 4 extra characters (for "not "). It also highlights the repetition of identifiers: data if data, files if files, etc. This example benefits from short identifiers, but what if the tested expression is longer and/or has side effects? This is addressed in the next section.

Motivating Examples

The purpose of this PEP is to simplify some common patterns involving None. This section presents some examples of common None patterns and explains the drawbacks.

This first example is from a Python web crawler that uses the popular Flask framework as a front-end. This function retrieves information about a web site from a SQL database and formats it as JSON to send to an HTTP client:

class SiteView(FlaskView):
    @route('/site/<id_>', methods=['GET'])
    def get_site(self, id_):
        site = db.query('site_table').find(id_)

        return jsonify(
            first_seen=site.first_seen.isoformat() if site.first_seen is not None else None,
            id=site.id,
            is_active=site.is_active,
            last_seen=site.last_seen.isoformat() if site.last_seen is not None else None,
            url=site.url.rstrip('/')
        )

Both first_seen and last_seen are allowed to be null in the database, and they are also allowed to be null in the JSON response. JSON does not have a native way to represent a datetime, so the server's contract states that any non-null date is represented as an ISO-8601 string.

Note that this code is invalid by PEP-8 standards: several lines are over the line length limit. In fact, including it in this document violates the PEP formatting standard! But it's not unreasonably indented, nor are any of the identifiers excessively long. The excessive line length is due to the repetition of identifiers on both sides of the ternary if and the verbosity of the ternary itself (10 characters out of a 78 character line length).

One way to fix this code is to replace each ternary with a full if/else block:

class SiteView(FlaskView):
    @route('/site/<id_>', methods=['GET'])
    def get_site(self, id_):
        site = db.query('site_table').find(id_)

        if site.first_seen is None:
            first_seen = None
        else:
            first_seen = site.first_seen.isoformat()

        if site.last_seen is None:
            last_seen = None
        else:
            last_seen = site.last_seen.isoformat()

        return jsonify(
            first_seen=first_seen,
            id=site.id,
            is_active=site.is_active,
            last_seen=last_seen,
            url=site.url.rstrip('/')
        )

This version definitely isn't bad. It is easy to read and understand. On the other hand, adding 8 lines of code to express this common behavior feels a bit heavy, especially for a deliberately simplified example. If a larger, more complicated data model was being used, then it would get tedious to continually write in this long form. The readability would start to suffer as the number of lines in the function grows, and a refactoring would be forced.

Another alternative is to rename some of the identifiers:

class SiteView(FlaskView):
    @route('/site/<id_>', methods=['GET'])
    def get_site(self, id_):
        site = db.query('site_table').find(id_)

        fs = site.first_seen
        ls = site.last_seen

        return jsonify(
            first_seen=fs.isodate() if fs is not None else None,
            id=site.id,
            is_active=site.is_active,
            last_seen=ls.isodate() if ls is not None else None,,
            url=site.url.rstrip('/')
        )

This adds fewer lines of code than the previous example, but it comes at the expense of introducing extraneous identifiers that amount to nothing more than aliases. These new identifiers are short enough to fit a ternary expression onto one line, but the identifiers are also less intuitive, e.g. fs versus first_seen.

As a quick preview, consider an alternative rewrite using a new operator:

class SiteView(FlaskView):
    @route('/site/<id_>', methods=['GET'])
    def get_site(self, id_):
        site = db.query('site_table').find(id_)

        return jsonify(
            first_seen=site.first_seen?.isoformat(),
            id=site.id,
            is_active=site.is_active,
            last_seen=site.last_seen?.isoformat(),
            url=site.url.rstrip('/')
        )

The ?. operator behaves as a "safe navigation" operator, allowing a more concise syntax where the expression site.first_seen is not duplicated.

The next example is from a trending project on GitHub called Grab, which is a Python scraping library:

class BaseUploadObject(object):
    def find_content_type(self, filename):
        ctype, encoding = mimetypes.guess_type(filename)
        if ctype is None:
            return 'application/octet-stream'
        else:
            return ctype

class UploadContent(BaseUploadObject):
    def __init__(self, content, filename=None, content_type=None):
        self.content = content
        if filename is None:
            self.filename = self.get_random_filename()
        else:
            self.filename = filename
        if content_type is None:
            self.content_type = self.find_content_type(self.filename)
        else:
            self.content_type = content_type

class UploadFile(BaseUploadObject):
    def __init__(self, path, filename=None, content_type=None):
        self.path = path
        if filename is None:
            self.filename = os.path.split(path)[1]
        else:
            self.filename = filename
        if content_type is None:
            self.content_type = self.find_content_type(self.filename)
        else:
            self.content_type = content_type

Note

I don't know the author of the Grab project. I used it as an example because it is a trending repo on GitHub and it has good examples of common None patterns.

This example contains several good examples of needing to provide default values. It is a bit verbose as it is, and it is certainly not improved by the ternary operator:

class BaseUploadObject(object):
    def find_content_type(self, filename):
        ctype, encoding = mimetypes.guess_type(filename)
        return 'application/octet-stream' if ctype is None else ctype

class UploadContent(BaseUploadObject):
    def __init__(self, content, filename=None, content_type=None):
        self.content = content
        self.filename = self.get_random_filename() if filename \
            is None else filename
        self.content_type = self.find_content_type(self.filename) \
            if content_type is None else content_type

class UploadFile(BaseUploadObject):
    def __init__(self, path, filename=None, content_type=None):
        self.path = path
        self.filename = os.path.split(path)[1] if filename is \
            None else filename
        self.content_type = self.find_content_type(self.filename) \
            if content_type is None else content_type

The first ternary expression is tidy, but it reverses the intuitive order of the operands: it should return ctype if it has a value and use the string literal as fallback. The other ternary expressions are unintuitive and so long that they must be wrapped. The overall readability is worsened, not improved.

This code might be improved, though, if there was a syntactic shortcut for this common need to supply a default value:

class BaseUploadObject(object):
    def find_ctype(self, filename):
        ctype, encoding = mimetypes.guess_type(filename)
        return ctype ?? 'application/octet-stream'

class UploadContent(BaseUploadObject):
    def __init__(self, content, filename=None, content_type=None):
        self.content = content
        self.filename = filename ?? self.get_random_filename()
        self.content_type = content_type ?? self.find_ctype(self.filename)

class UploadFile(BaseUploadObject):
    def __init__(self, path, filename=None, content_type=None):
        self.path = path
        self.filename = filename ?? os.path.split(path)[1]
        self.content_type = content_type ?? self.find_ctype(self.filename)

This syntax has an intuitive ordering of the operands, e.g. ctype -- the preferred value -- comes before the fallback value. The terseness of the syntax also makes for fewer lines of code and less code to visually parse.

Note

I cheated on the last example: I renamed find_content_type to find_ctype in order to fit two of the lines under 80 characters. If you find this underhanded, you can go back and apply the same renaming to the previous 2 examples. You'll find that it doesn't change the conclusions.

Usage Of None In The Standard Library

The previous sections show some code patterns that are claimed to be "common", but how common are they? The attached script find-pep505.py is meant to answer this question. It uses the ast module to search for variations of the following patterns in any *.py file:

>>> # None-coalescing if block
...
>>> if a is None:
...     a = b

>>> # [Possible] None-coalescing "or" operator
...
>>> a or 'foo'
>>> a or []
>>> a or {}

>>> # None-coalescing ternary
...
>>> a if a is not None else b
>>> b if a is None else a

>>> # Safe navigation "and" operator
...
>>> a and a.foo
>>> a and a['foo']
>>> a and a.foo()

>>> # Safe navigation if block
...
>>> if a is not None:
...     a.foo()

>>> # Safe navigation ternary
...
>>> a.foo if a is not None else b
>>> b if a is None else a.foo

This script takes one or more names of Python source files to analyze:

$ python3 find-pep505.py test.py
$ find /usr/lib/python3.4 -name '*.py' | xargs python3 find-pep505.py

The script prints out any matches it finds. Sample:

None-coalescing if block: /usr/lib/python3.4/inspect.py:594
    if _filename is None:
         _filename = getsourcefile(object) or getfile(object)

[Possible] None-coalescing `or`: /usr/lib/python3.4/lib2to3/refactor.py:191
        self.explicit = explicit or []

None-coalescing ternary: /usr/lib/python3.4/decimal.py:3909
        self.clamp = clamp if clamp is not None else dc.clamp

Safe navigation `and`: /usr/lib/python3.4/weakref.py:512
        obj = info and info.weakref()

Safe navigation `if` block: /usr/lib/python3.4/http/cookiejar.py:1895
                        if k is not None:
                            lc = k.lower()
                        else:
                            lc = None

Safe navigation ternary: /usr/lib/python3.4/sre_parse.py:856
        literals = [None if s is None else s.encode('latin-1') for s in literals]

Note

Coalescing with or is marked as a "possible" match, because it's not trivial to infer whether or is meant to coalesce False-y values (correct) or if it meant to coalesce None (incorrect). On the other hand, we assume that and is always incorrect for safe navigation.

The script has been tested against test.py and the Python 3.4 standard library, but it should work on any arbitrary Python 3 source code. The complete output from running it against the standard library is attached to this proposal as find-pep505.out.

The script counts how many matches it finds and prints the totals at the end:

Total None-coalescing `if` blocks: 426
Total [possible] None-coalescing `or`: 119
Total None-coalescing ternaries: 21
Total Safe navigation `and`: 9
Total Safe navigation `if` blocks: 55
Total Safe navigation ternaries: 7

This is a total of 637 possible matches for these common code patterns in the standard library. Allowing for some false positives and false negatives, it is fair to say that these code patterns are definitely common in the standard library.

Rejected Ideas

Several related ideas were discussed on python-ideas, and some of these were roundly rejected by BDFL, the community, or both. For posterity's sake, some of those ideas are recorded here.

None-aware Function Call

The None-aware syntax applies to attribute and index access, so it seems natural to ask if it should also apply to function invocation syntax. It might be written as foo?(), where foo is only called if it is not None. This idea was quickly rejected, for several reasons.

First, no other mainstream language has such syntax. Second, Python evaluates arguments to a function before it looks up the function itself, so foo?(bar()) would still call bar() even if foo is None. This behaviour is unexpected for a so-called "short-circuiting" operator.

? Unary Postfix Operator

To generalize the None-aware behavior and limit the number of new operators introduced, a unary, postfix operator spelled ? was suggested. The idea is that ? might return a special object that could would override dunder methods that return self. For example, foo? would evaluate to foo if it is not None, otherwise it would evaluate to an instance of NoneQuestion:

class NoneQuestion():
    def __call__(self, *args, **kwargs):
        return self

    def __getattr__(self, name):
        return self

    def __getitem__(self, key):
        return self

With this new operator and new type, an expression like foo?.bar[baz] evaluates to NoneQuestion if foo is None. This is a nifty generalization, but it's difficult to use in practice since most existing code won't know what NoneQuestion is.

Going back to one of the motivating examples above, consider the following:

>>> import json
>>> created = None
>>> json.dumps({'created': created?.isoformat()})``

The JSON serializer does not know how to serialize NoneQuestion, nor will any other API. This proposal actually requires lots of specialized logic throughout the standard library and any third party library.

At the same time, the ? operator may also be too general, in the sense that it can be combined with any other operator. What should the following expressions mean?:

>>> x? + 1
>>> x? -= 1
>>> x? == 1
>>> ~x?

This degree of generalization is not useful. The operators actually proposed herein are intentionally limited to a few operators that are expected to make it easier to write common code patterns.

Haskell-style Maybe

Haskell has a concept called Maybe that encapsulates the idea of an optional value without relying on any special keyword (e.g. null) or any special instance (e.g. None). In Haskell, the purpose of Maybe is to avoid separate handling of "something" and nothing". The concept is so heavily intertwined with Haskell's lazy evaluation that it doesn't translate cleanly into Python.

There is a Python package called pymaybe that provides a rough approximation. The documentation shows the following example that appears relevant to the discussion at hand:

>>> maybe('VALUE').lower()
'value'

>>> maybe(None).invalid().method().or_else('unknown')
'unknown'

The function maybe() returns either a Something instance or a Nothing instance. Similar to the unary postfix operator described in the previous section, Nothing overrides dunder methods in order to allow chaining on a missing value.

Note that or_else() is eventually required to retrieve the underlying value from pymaybe's wrappers. Furthermore, pymaybe does not short circuit any evaluation. Although pymaybe has some strengths and may be useful in its own right, it also demonstrates why a pure Python implementation of coalescing is not nearly as powerful as support built into the language.

Specification

This PEP suggests 3 new operators be added to Python:

  1. None-coalescing operator
  2. None-aware attribute access
  3. None-aware index access/slicing

We will continue to assume the same spellings as in the previous sections in order to focus on behavior before diving into the much more contentious issue of how to spell these operators.

A generalization of these operators is also proposed below under the heading "Generalized Coalescing".

Operator Spelling

Despite significant support for the proposed operators, the majority of discussion on python-ideas fixated on the spelling. Many alternative spellings were proposed, both punctuation and keywords, but each alternative drew some criticism. Spelling the operator as a keyword is problematic, because adding new keywords to the language is not backwards compatible.

It is not impossible to add a new keyword, however, and we can look at several other PEPs for inspiration. For example, PEP-492 introduced the new keywords async and await into Python 3.5. These new keywords are fully backwards compatible, because that PEP also introduces a new lexical context such that async and await are only treated as keywords when used inside of an async def function. In other locations, async and await may be used as identifiers.

It is also possible to craft a new operator out of existing keywords, as was the case with PEP-308, which created a ternary operator by cobbling together the if and else keywords into a new operator.

In addition to the lexical acrobatics required to create a new keyword, keyword operators are also undesirable for creating an assignment shortcut syntax. In Dart, for example, x ??= y is an assignment shortcut that approximately means x = x ?? y except that x is only evaluated once. If Python's coalesce operator is a keyword, e.g. foo, then the assignment shortcut would be very ugly: x foo= y.

Spelling new logical operators with punctuation is unlikely, for several reasons. First, Python eschews punctuation for logical operators. For example, it uses not instead of !, or instead of ||, and … if … else … instead of … ? … : ….

Second, nearly every single punctuation character on a standard keyboard already has special meaning in Python. The only exceptions are $, !, ?, and backtick (as of Python 3). This leaves few options for a new, single-character operator.

Third, other projects in the Python universe assign special meaning to punctuation. For example, IPython assigns special meaning to %, %%, ?, ??, $, and $$, among others. Out of deference to those projects and the large communities using them, introducing conflicting syntax into Python is undesirable.

The spellings ?? and ?. will be familiar to programmers who have seen them in other popular programming languages. Any alternative punctuation will be just as ugly but without the benefit of familiarity from other languages. Therefore, this proposal spells the new operators using the same punctuation that already exists in other languages.

None-Coalescing Operator

The None-coalescing operator is a short-circuiting, binary operator that behaves in the following way.

  1. Evaluate the left operand first.
  2. If the left operand is not None, then return it immediately.
  3. Else, evaluate the right operand and return the result.

Consider the following examples. We will continue to use the spelling ?? here, but keep in mind that alternative spellings will be discussed below:

>>> 1 ?? 2
1
>>> None ?? 2
2

Importantly, note that the right operand is not evaluated unless the left operand is None:

>>> def err(): raise Exception('foo')
>>> 1 ?? err()
1
>>> None ?? err()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 1, in err
Exception: foo

The operator is left associative. Combined with its short circuiting behavior, this makes the operator easy to chain:

>>> timeout = None
>>> local_timeout = 60
>>> global_timeout = 300
>>> timeout ?? local_timeout ?? global_timeout
60

>>> local_timeout = None
>>> timeout ?? local_timeout ?? global_timeout
300

The operator has higher precedence than the comparison operators ==, >, is, etc., but lower precedence than any bitwise or arithmetic operators. This precedence is chosen for making "default value" expressions intuitive to read and write:

>>> not None ?? True
>>> not (None ?? True) # Same precedence

>>> 1 == None ?? 1
>>> 1 == (None ?? 1) # Same precedence

>>> 'foo' in None ?? ['foo', 'bar']
>>> 'foo' in (None ?? ['foo', 'bar']) # Same precedence

>>> 1 + None ?? 2
>>> 1 + (None ?? 2) # Same precedence

Recall the example above of calculating the cost of items in a shopping cart, and the easy-to-miss bug. This type of bug is not possible with the None- coalescing operator, because there is no implicit type coersion to bool:

>>> requested_quantity = 0
>>> default_quantity = 1
>>> price = 100
>>> requested_quantity ?? default_quantity * price
0

The None-coalescing operator also has a corresponding assignment shortcut. The following assignments are semantically similar, except that foo is only looked up once when using the assignment shortcut:

>>> foo ??= []
>>> foo = foo ?? []

The None coalescing operator improves readability, especially when handling default function arguments. Consider again the example from the Requests library, rewritten to use None-coalescing:

def __init__(self, data=None, files=None, headers=None, params=None, hooks=None):
    self.data = data ?? []
    self.files = files ?? []
    self.headers = headers ?? {}
    self.params = params ?? {}
    self.hooks = hooks ?? {}

The operator makes the intent easier to follow (by putting operands in an intuitive order) and is more concise than the ternary operator, while still preserving the short circuit semantics of the code that it replaces.

None-Aware Attribute Access Operator

The None-aware attribute access operator (also called "safe navigation") checks its left operand. If the left operand is None, then the operator evaluates to None. If the the left operand is not None, then the operator accesses the attribute named by the right operand:

>>> from datetime import date
>>> d = date.today()
>>> d.year
2015

>>> d = None
>>> d.year
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'NoneType' object has no attribute 'year'

>>> d?.year
None

The operator has the same precedence and associativity as the plain attribute access operator ., but this operator is also short-circuiting in a unique way: if the left operand is None, then any series of attribute access, index access, slicing, or function call operators immediately to the right of it are not evaluated:

>>> name = ' The Black Knight '
>>> name.strip()[4:].upper()
'BLACK KNIGHT'

>>> name = None
>>> name?.strip()[4:].upper()
None

If this operator did not short circuit in this way, then the second example would partially evaluate name?.strip() to None() and then fail with TypeError: 'NoneType' object is not callable.

To put it another way, the following expressions are semantically similar, except that name is only looked up once on the first line:

>>> name?.strip()[4:].upper()
>>> name.strip()[4:].upper() if name is not None else None

Note

C# implements its safe navigation operators with the same short-circuiting semantics, but Dart does not. In Dart, the second example (suitably translated) would fail. The C# semantics are obviously superior, given the original goal of writing common cases more concisely. The Dart semantics are nearly useless.

This operator short circuits one or more attribute access, index access, slicing, or function call operators that are adjacent to its right, but it does not short circuit any other operators (logical, bitwise, arithmetic, etc.), nor does it escape parentheses:

>>> d = date.today()
>>> d?.year.numerator + 1
2016

>>> d = None
>>> d?.year.numerator + 1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'NoneType' and 'int'

>>> (d?.year).numerator + 1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'NoneType' object has no attribute 'numerator'

Note that the error in the second example is not on the attribute access numerator. In fact, that attribute access is never performed. The error occurs when adding None + 1, because the None-aware attribute access does not short circuit +.

The third example fails because the operator does not escape parentheses. In that example, the attribute access numerator is evaluated and fails because None does not have that attribute.

Finally, observe that short circuiting adjacent operators is not at all the same thing as propagating None throughout an expression:

>>> user?.first_name.upper()

If user is not None, then user.first_name is evaluated. If user.first_name evaluates to None, then user.first_name.upper() is an error! In English, this expression says, "user is optional but if it has a value, then it must have a first_name, too."

If first_name is supposed to be optional attribute, then the expression must make that explicit:

>>> user?.first_name?.upper()

The operator is not intended as an error silencing mechanism, and it would be undesirable if its presence infected nearby operators.

None-Aware Index Access/Slicing Operator

The None-aware index access/slicing operator (also called "safe navigation") is nearly identical to the None-aware attribute access operator. It combines the familiar square bracket syntax [] with new punctuation or a new keyword, the spelling of which is discussed later:

>>> person = {'name': 'Mark', 'age': 32}
>>> person['name']
'Mark'

>>> person = None
>>> person['name']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'NoneType' object is not subscriptable

>>> person?.['name']
None

The None-aware slicing operator behaves similarly:

>>> name = 'The Black Knight'
>>> name[4:]
'Black Knight'

>>> name = None
>>> name[4:]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'NoneType' object is not subscriptable

>>> name?.[4:]
None

These operators have the same precedence as the plain index access and slicing operators. They also have the same short-circuiting behavior as the None-aware attribute access.

Generalized Coalescing

Making None a special case is too specialized and magical. The behavior can be generalized by making the None-aware operators invoke a dunder method, e.g. __coalesce__(self) that returns True if an object should be coalesced and False otherwise.

With this generalization, object would implement a dunder method equivalent to this:

def __coalesce__(self):
    return False

NoneType would implement a dunder method equivalent to this:

def __coalesce__(self):
    return True

If this generalization is accepted, then the operators will need to be renamed such that the term None is not used, e.g. "Coalescing Operator", "Coalesced Member Access Operator", etc.

The coalesce operator would invoke this dunder method. The following two expressions are semantically similar, except foo is only looked up once when using the coalesce operator:

>>> foo ?? bar
>>> bar if foo.__coalesce__() else foo

The coalesced attribute and index access operators would invoke the same dunder method:

>>> user?.first_name.upper()
>>> None if user.__coalesce__() else user.first_name.upper()

This generalization allows for domain-specific null objects to be coalesced just like None. For example the pyasn1 package has a type called Null that represents an ASN.1 null:

>>> from pyasn1.type import univ
>>> univ.Null() ?? univ.Integer(123)
Integer(123)

In addition to making the proposed operators less specialized, this generalization also makes it easier to work with the Null Object Pattern, [3] for those developers who prefer to avoid using None.

Implementation

The author of this PEP is not competent with grammars or lexers, and given the contentiousness of this proposal, the implementation details for CPython will be deferred until we have a clearer idea that one or more of the proposed enhancements will be approved.

...TBD...

Source: https://github.com/python/peps/blob/master/pep-0505.txt