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PEP 3101 -- Advanced String Formatting

PEP:3101
Title:Advanced String Formatting
Author:Talin <viridia at gmail.com>
Status:Final
Type:Standards Track
Created:16-Apr-2006
Python-Version:3.0
Post-History:28-Apr-2006, 6-May-2006, 10-Jun-2007, 14-Aug-2007, 14-Sep-2008

Abstract

This PEP proposes a new system for built-in string formatting operations, intended as a replacement for the existing '%' string formatting operator.

Rationale

Python currently provides two methods of string interpolation:

  • The '%' operator for strings. [1]
  • The string.Template module. [2]

The primary scope of this PEP concerns proposals for built-in string formatting operations (in other words, methods of the built-in string type).

The '%' operator is primarily limited by the fact that it is a binary operator, and therefore can take at most two arguments. One of those arguments is already dedicated to the format string, leaving all other variables to be squeezed into the remaining argument. The current practice is to use either a dictionary or a tuple as the second argument, but as many people have commented [3], this lacks flexibility. The "all or nothing" approach (meaning that one must choose between only positional arguments, or only named arguments) is felt to be overly constraining.

While there is some overlap between this proposal and string.Template, it is felt that each serves a distinct need, and that one does not obviate the other. This proposal is for a mechanism which, like '%', is efficient for small strings which are only used once, so, for example, compilation of a string into a template is not contemplated in this proposal, although the proposal does take care to define format strings and the API in such a way that an efficient template package could reuse the syntax and even some of the underlying formatting code.

Specification

The specification will consist of the following parts:

  • Specification of a new formatting method to be added to the built-in string class.
  • Specification of functions and flag values to be added to the string module, so that the underlying formatting engine can be used with additional options.
  • Specification of a new syntax for format strings.
  • Specification of a new set of special methods to control the formatting and conversion of objects.
  • Specification of an API for user-defined formatting classes.
  • Specification of how formatting errors are handled.

Note on string encodings: When discussing this PEP in the context of Python 3.0, it is assumed that all strings are unicode strings, and that the use of the word 'string' in the context of this document will generally refer to a Python 3.0 string, which is the same as Python 2.x unicode object.

In the context of Python 2.x, the use of the word 'string' in this document refers to an object which may either be a regular string or a unicode object. All of the function call interfaces described in this PEP can be used for both strings and unicode objects, and in all cases there is sufficient information to be able to properly deduce the output string type (in other words, there is no need for two separate APIs). In all cases, the type of the format string dominates - that is, the result of the conversion will always result in an object that contains the same representation of characters as the input format string.

String Methods

The built-in string class (and also the unicode class in 2.6) will gain a new method, 'format', which takes an arbitrary number of positional and keyword arguments:

"The story of {0}, {1}, and {c}".format(a, b, c=d)

Within a format string, each positional argument is identified with a number, starting from zero, so in the above example, 'a' is argument 0 and 'b' is argument 1. Each keyword argument is identified by its keyword name, so in the above example, 'c' is used to refer to the third argument.

There is also a global built-in function, 'format' which formats a single value:

print(format(10.0, "7.3g"))

This function is described in a later section.

Format Strings

Format strings consist of intermingled character data and markup.

Character data is data which is transferred unchanged from the format string to the output string; markup is not transferred from the format string directly to the output, but instead is used to define 'replacement fields' that describe to the format engine what should be placed in the output string in place of the markup.

Brace characters ('curly braces') are used to indicate a replacement field within the string:

"My name is {0}".format('Fred')

The result of this is the string:

"My name is Fred"

Braces can be escaped by doubling:

"My name is {0} :-{{}}".format('Fred')

Which would produce:

"My name is Fred :-{}"

The element within the braces is called a 'field'. Fields consist of a 'field name', which can either be simple or compound, and an optional 'format specifier'.

Simple and Compound Field Names

Simple field names are either names or numbers. If numbers, they must be valid base-10 integers; if names, they must be valid Python identifiers. A number is used to identify a positional argument, while a name is used to identify a keyword argument.

A compound field name is a combination of multiple simple field names in an expression:

"My name is {0.name}".format(open('out.txt', 'w'))

This example shows the use of the 'getattr' or 'dot' operator in a field expression. The dot operator allows an attribute of an input value to be specified as the field value.

Unlike some other programming languages, you cannot embed arbitrary expressions in format strings. This is by design - the types of expressions that you can use is deliberately limited. Only two operators are supported: the '.' (getattr) operator, and the '[]' (getitem) operator. The reason for allowing these operators is that they don't normally have side effects in non-pathological code.

An example of the 'getitem' syntax:

"My name is {0[name]}".format(dict(name='Fred'))

It should be noted that the use of 'getitem' within a format string is much more limited than its conventional usage. In the above example, the string 'name' really is the literal string 'name', not a variable named 'name'. The rules for parsing an item key are very simple. If it starts with a digit, then it is treated as a number, otherwise it is used as a string.

Because keys are not quote-delimited, it is not possible to specify arbitrary dictionary keys (e.g., the strings "10" or ":-]") from within a format string.

Implementation note: The implementation of this proposal is not required to enforce the rule about a simple or dotted name being a valid Python identifier. Instead, it will rely on the getattr function of the underlying object to throw an exception if the identifier is not legal. The str.format() function will have a minimalist parser which only attempts to figure out when it is "done" with an identifier (by finding a '.' or a ']', or '}', etc.).

Format Specifiers

Each field can also specify an optional set of 'format specifiers' which can be used to adjust the format of that field. Format specifiers follow the field name, with a colon (':') character separating the two:

"My name is {0:8}".format('Fred')

The meaning and syntax of the format specifiers depends on the type of object that is being formatted, but there is a standard set of format specifiers used for any object that does not override them.

Format specifiers can themselves contain replacement fields. For example, a field whose field width is itself a parameter could be specified via:

"{0:{1}}".format(a, b)

These 'internal' replacement fields can only occur in the format specifier part of the replacement field. Internal replacement fields cannot themselves have format specifiers. This implies also that replacement fields cannot be nested to arbitrary levels.

Note that the doubled '}' at the end, which would normally be escaped, is not escaped in this case. The reason is because the '{{' and '}}' syntax for escapes is only applied when used outside of a format field. Within a format field, the brace characters always have their normal meaning.

The syntax for format specifiers is open-ended, since a class can override the standard format specifiers. In such cases, the str.format() method merely passes all of the characters between the first colon and the matching brace to the relevant underlying formatting method.

Standard Format Specifiers

If an object does not define its own format specifiers, a standard set of format specifiers is used. These are similar in concept to the format specifiers used by the existing '%' operator, however there are also a number of differences.

The general form of a standard format specifier is:

[[fill]align][sign][#][0][minimumwidth][.precision][type]

The brackets ([]) indicate an optional element.

Then the optional align flag can be one of the following:

'<' - Forces the field to be left-aligned within the available
      space (This is the default.)
'>' - Forces the field to be right-aligned within the
      available space.
'=' - Forces the padding to be placed after the sign (if any)
      but before the digits.  This is used for printing fields
      in the form '+000000120'. This alignment option is only
      valid for numeric types.
'^' - Forces the field to be centered within the available
      space.

Note that unless a minimum field width is defined, the field width will always be the same size as the data to fill it, so that the alignment option has no meaning in this case.

The optional 'fill' character defines the character to be used to pad the field to the minimum width. The fill character, if present, must be followed by an alignment flag.

The 'sign' option is only valid for numeric types, and can be one of the following:

'+'  - indicates that a sign should be used for both
       positive as well as negative numbers
'-'  - indicates that a sign should be used only for negative
       numbers (this is the default behavior)
' '  - indicates that a leading space should be used on
       positive numbers

If the '#' character is present, integers use the 'alternate form' for formatting. This means that binary, octal, and hexadecimal output will be prefixed with '0b', '0o', and '0x', respectively.

'width' is a decimal integer defining the minimum field width. If not specified, then the field width will be determined by the content.

If the width field is preceded by a zero ('0') character, this enables zero-padding. This is equivalent to an alignment type of '=' and a fill character of '0'.

The 'precision' is a decimal number indicating how many digits should be displayed after the decimal point in a floating point conversion. For non-numeric types the field indicates the maximum field size - in other words, how many characters will be used from the field content. The precision is ignored for integer conversions.

Finally, the 'type' determines how the data should be presented.

The available integer presentation types are:

'b' - Binary. Outputs the number in base 2.
'c' - Character. Converts the integer to the corresponding
      Unicode character before printing.
'd' - Decimal Integer. Outputs the number in base 10.
'o' - Octal format. Outputs the number in base 8.
'x' - Hex format. Outputs the number in base 16, using lower-
      case letters for the digits above 9.
'X' - Hex format. Outputs the number in base 16, using upper-
      case letters for the digits above 9.
'n' - Number. This is the same as 'd', except that it uses the
      current locale setting to insert the appropriate
      number separator characters.
'' (None) - the same as 'd'

The available floating point presentation types are:

'e' - Exponent notation. Prints the number in scientific
      notation using the letter 'e' to indicate the exponent.
'E' - Exponent notation. Same as 'e' except it converts the
      number to uppercase.
'f' - Fixed point. Displays the number as a fixed-point
      number.
'F' - Fixed point. Same as 'f' except it converts the number
      to uppercase.
'g' - General format. This prints the number as a fixed-point
      number, unless the number is too large, in which case
      it switches to 'e' exponent notation.
'G' - General format. Same as 'g' except switches to 'E'
      if the number gets to large.
'n' - Number. This is the same as 'g', except that it uses the
      current locale setting to insert the appropriate
      number separator characters.
'%' - Percentage. Multiplies the number by 100 and displays
      in fixed ('f') format, followed by a percent sign.
'' (None) - similar to 'g', except that it prints at least one
      digit after the decimal point.

Objects are able to define their own format specifiers to replace the standard ones. An example is the 'datetime' class, whose format specifiers might look something like the arguments to the strftime() function:

"Today is: {0:%a %b %d %H:%M:%S %Y}".format(datetime.now())

For all built-in types, an empty format specification will produce the equivalent of str(value). It is recommended that objects defining their own format specifiers follow this convention as well.

Explicit Conversion Flag

The explicit conversion flag is used to transform the format field value before it is formatted. This can be used to override the type-specific formatting behavior, and format the value as if it were a more generic type. Currently, two explicit conversion flags are recognized:

!r - convert the value to a string using repr().
!s - convert the value to a string using str().

These flags are placed before the format specifier:

"{0!r:20}".format("Hello")

In the preceding example, the string "Hello" will be printed, with quotes, in a field of at least 20 characters width.

A custom Formatter class can define additional conversion flags. The built-in formatter will raise a ValueError if an invalid conversion flag is specified.

Controlling Formatting on a Per-Type Basis

Each Python type can control formatting of its instances by defining a __format__ method. The __format__ method is responsible for interpreting the format specifier, formatting the value, and returning the resulting string.

The new, global built-in function 'format' simply calls this special method, similar to how len() and str() simply call their respective special methods:

def format(value, format_spec):
    return value.__format__(format_spec)

It is safe to call this function with a value of "None" (because the "None" value in Python is an object and can have methods.)

Several built-in types, including 'str', 'int', 'float', and 'object' define __format__ methods. This means that if you derive from any of those types, your class will know how to format itself.

The object.__format__ method is the simplest: It simply converts the object to a string, and then calls format again:

class object:
    def __format__(self, format_spec):
        return format(str(self), format_spec)

The __format__ methods for 'int' and 'float' will do numeric formatting based on the format specifier. In some cases, these formatting operations may be delegated to other types. So for example, in the case where the 'int' formatter sees a format type of 'f' (meaning 'float') it can simply cast the value to a float and call format() again.

Any class can override the __format__ method to provide custom formatting for that type:

class AST:
    def __format__(self, format_spec):
        ...

Note for Python 2.x: The 'format_spec' argument will be either a string object or a unicode object, depending on the type of the original format string. The __format__ method should test the type of the specifiers parameter to determine whether to return a string or unicode object. It is the responsibility of the __format__ method to return an object of the proper type.

Note that the 'explicit conversion' flag mentioned above is not passed to the __format__ method. Rather, it is expected that the conversion specified by the flag will be performed before calling __format__.

User-Defined Formatting

There will be times when customizing the formatting of fields on a per-type basis is not enough. An example might be a spreadsheet application, which displays hash marks '#' when a value is too large to fit in the available space.

For more powerful and flexible formatting, access to the underlying format engine can be obtained through the 'Formatter' class that lives in the 'string' module. This class takes additional options which are not accessible via the normal str.format method.

An application can subclass the Formatter class to create its own customized formatting behavior.

The PEP does not attempt to exactly specify all methods and properties defined by the Formatter class; instead, those will be defined and documented in the initial implementation. However, this PEP will specify the general requirements for the Formatter class, which are listed below.

Although string.format() does not directly use the Formatter class to do formatting, both use the same underlying implementation. The reason that string.format() does not use the Formatter class directly is because "string" is a built-in type, which means that all of its methods must be implemented in C, whereas Formatter is a Python class. Formatter provides an extensible wrapper around the same C functions as are used by string.format().

Formatter Methods

The Formatter class takes no initialization arguments:

fmt = Formatter()

The public API methods of class Formatter are as follows:

-- format(format_string, *args, **kwargs)
-- vformat(format_string, args, kwargs)

'format' is the primary API method. It takes a format template, and an arbitrary set of positional and keyword arguments. 'format' is just a wrapper that calls 'vformat'.

'vformat' is the function that does the actual work of formatting. It is exposed as a separate function for cases where you want to pass in a predefined dictionary of arguments, rather than unpacking and repacking the dictionary as individual arguments using the *args and **kwds syntax. 'vformat' does the work of breaking up the format template string into character data and replacement fields. It calls the 'get_positional' and 'get_index' methods as appropriate (described below.)

Formatter defines the following overridable methods:

-- get_value(key, args, kwargs)
-- check_unused_args(used_args, args, kwargs)
-- format_field(value, format_spec)

'get_value' is used to retrieve a given field value. The 'key' argument will be either an integer or a string. If it is an integer, it represents the index of the positional argument in 'args'; If it is a string, then it represents a named argument in 'kwargs'.

The 'args' parameter is set to the list of positional arguments to 'vformat', and the 'kwargs' parameter is set to the dictionary of positional arguments.

For compound field names, these functions are only called for the first component of the field name; subsequent components are handled through normal attribute and indexing operations.

So for example, the field expression '0.name' would cause 'get_value' to be called with a 'key' argument of 0. The 'name' attribute will be looked up after 'get_value' returns by calling the built-in 'getattr' function.

If the index or keyword refers to an item that does not exist, then an IndexError/KeyError should be raised.

'check_unused_args' is used to implement checking for unused arguments if desired. The arguments to this function is the set of all argument keys that were actually referred to in the format string (integers for positional arguments, and strings for named arguments), and a reference to the args and kwargs that was passed to vformat. The set of unused args can be calculated from these parameters. 'check_unused_args' is assumed to throw an exception if the check fails.

'format_field' simply calls the global 'format' built-in. The method is provided so that subclasses can override it.

To get a better understanding of how these functions relate to each other, here is pseudocode that explains the general operation of vformat:

def vformat(format_string, args, kwargs):

  # Output buffer and set of used args
  buffer = StringIO.StringIO()
  used_args = set()

  # Tokens are either format fields or literal strings
  for token in self.parse(format_string):
    if is_format_field(token):
      # Split the token into field value and format spec
      field_spec, _, format_spec = token.partition(":")

      # Check for explicit type conversion
      explicit, _, field_spec  = field_spec.rpartition("!")

      # 'first_part' is the part before the first '.' or '['
      # Assume that 'get_first_part' returns either an int or
      # a string, depending on the syntax.
      first_part = get_first_part(field_spec)
      value = self.get_value(first_part, args, kwargs)

      # Record the fact that we used this arg
      used_args.add(first_part)

      # Handle [subfield] or .subfield. Assume that 'components'
      # returns an iterator of the various subfields, not including
      # the first part.
      for comp in components(field_spec):
        value = resolve_subfield(value, comp)

      # Handle explicit type conversion
      if explicit == 'r':
        value = repr(value)
      elif explicit == 's':
        value = str(value)

      # Call the global 'format' function and write out the converted
      # value.
      buffer.write(self.format_field(value, format_spec))

    else:
      buffer.write(token)

  self.check_unused_args(used_args, args, kwargs)
  return buffer.getvalue()

Note that the actual algorithm of the Formatter class (which will be implemented in C) may not be the one presented here. (It's likely that the actual implementation won't be a 'class' at all - rather, vformat may just call a C function which accepts the other overridable methods as arguments.) The primary purpose of this code example is to illustrate the order in which overridable methods are called.

Customizing Formatters

This section describes some typical ways that Formatter objects can be customized.

To support alternative format-string syntax, the 'vformat' method can be overridden to alter the way format strings are parsed.

One common desire is to support a 'default' namespace, so that you don't need to pass in keyword arguments to the format() method, but can instead use values in a pre-existing namespace. This can easily be done by overriding get_value() as follows:

class NamespaceFormatter(Formatter):
   def __init__(self, namespace={}):
       Formatter.__init__(self)
       self.namespace = namespace

   def get_value(self, key, args, kwds):
       if isinstance(key, str):
           try:
               # Check explicitly passed arguments first
               return kwds[key]
           except KeyError:
               return self.namespace[key]
       else:
           Formatter.get_value(key, args, kwds)

One can use this to easily create a formatting function that allows access to global variables, for example:

fmt = NamespaceFormatter(globals())

greeting = "hello"
print(fmt.format("{greeting}, world!"))

A similar technique can be done with the locals() dictionary to gain access to the locals dictionary.

It would also be possible to create a 'smart' namespace formatter that could automatically access both locals and globals through snooping of the calling stack. Due to the need for compatibility with the different versions of Python, such a capability will not be included in the standard library, however it is anticipated that someone will create and publish a recipe for doing this.

Another type of customization is to change the way that built-in types are formatted by overriding the 'format_field' method. (For non-built-in types, you can simply define a __format__ special method on that type.) So for example, you could override the formatting of numbers to output scientific notation when needed.

Error handling

There are two classes of exceptions which can occur during formatting: exceptions generated by the formatter code itself, and exceptions generated by user code (such as a field object's 'getattr' function).

In general, exceptions generated by the formatter code itself are of the "ValueError" variety -- there is an error in the actual "value" of the format string. (This is not always true; for example, the string.format() function might be passed a non-string as its first parameter, which would result in a TypeError.)

The text associated with these internally generated ValueError exceptions will indicate the location of the exception inside the format string, as well as the nature of the exception.

For exceptions generated by user code, a trace record and dummy frame will be added to the traceback stack to help in determining the location in the string where the exception occurred. The inserted traceback will indicate that the error occurred at:

File "<format_string>;", line XX, in column_YY

where XX and YY represent the line and character position information in the string, respectively.

Alternate Syntax

Naturally, one of the most contentious issues is the syntax of the format strings, and in particular the markup conventions used to indicate fields.

Rather than attempting to exhaustively list all of the various proposals, I will cover the ones that are most widely used already.

  • Shell variable syntax: $name and $(name) (or in some variants, ${name}). This is probably the oldest convention out there, and is used by Perl and many others. When used without the braces, the length of the variable is determined by lexically scanning until an invalid character is found.

    This scheme is generally used in cases where interpolation is implicit - that is, in environments where any string can contain interpolation variables, and no special substitution function need be invoked. In such cases, it is important to prevent the interpolation behavior from occurring accidentally, so the '$' (which is otherwise a relatively uncommonly-used character) is used to signal when the behavior should occur.

    It is the author's opinion, however, that in cases where the formatting is explicitly invoked, that less care needs to be taken to prevent accidental interpolation, in which case a lighter and less unwieldy syntax can be used.

  • printf and its cousins ('%'), including variations that add a field index, so that fields can be interpolated out of order.

  • Other bracket-only variations. Various MUDs (Multi-User Dungeons) such as MUSH have used brackets (e.g. [name]) to do string interpolation. The Microsoft .Net libraries uses braces ({}), and a syntax which is very similar to the one in this proposal, although the syntax for format specifiers is quite different. [4]

  • Backquoting. This method has the benefit of minimal syntactical clutter, however it lacks many of the benefits of a function call syntax (such as complex expression arguments, custom formatters, etc.).

  • Other variations include Ruby's #{}, PHP's {$name}, and so on.

Some specific aspects of the syntax warrant additional comments:

1) Backslash character for escapes. The original version of this PEP used backslash rather than doubling to escape a bracket. This worked because backslashes in Python string literals that don't conform to a standard backslash sequence such as \n are left unmodified. However, this caused a certain amount of confusion, and led to potential situations of multiple recursive escapes, i.e. \\\\{ to place a literal backslash in front of a bracket.

2) The use of the colon character (':') as a separator for format specifiers. This was chosen simply because that's what .Net uses.

Alternate Feature Proposals

Restricting attribute access: An earlier version of the PEP restricted the ability to access attributes beginning with a leading underscore, for example "{0}._private". However, this is a useful ability to have when debugging, so the feature was dropped.

Some developers suggested that the ability to do 'getattr' and 'getitem' access should be dropped entirely. However, this is in conflict with the needs of another set of developers who strongly lobbied for the ability to pass in a large dict as a single argument (without flattening it into individual keyword arguments using the **kwargs syntax) and then have the format string refer to dict entries individually.

There has also been suggestions to expand the set of expressions that are allowed in a format string. However, this was seen to go against the spirit of TOOWTDI, since the same effect can be achieved in most cases by executing the same expression on the parameter before it's passed in to the formatting function. For cases where the format string is being use to do arbitrary formatting in a data-rich environment, it's recommended to use a template engine specialized for this purpose, such as Genshi [5] or Cheetah [6].

Many other features were considered and rejected because they could easily be achieved by subclassing Formatter instead of building the feature into the base implementation. This includes alternate syntax, comments in format strings, and many others.

Security Considerations

Historically, string formatting has been a common source of security holes in web-based applications, particularly if the string formatting system allows arbitrary expressions to be embedded in format strings.

The best way to use string formatting in a way that does not create potential security holes is to never use format strings that come from an untrusted source.

Barring that, the next best approach is to ensure that string formatting has no side effects. Because of the open nature of Python, it is impossible to guarantee that any non-trivial operation has this property. What this PEP does is limit the types of expressions in format strings to those in which visible side effects are both rare and strongly discouraged by the culture of Python developers. So for example, attribute access is allowed because it would be considered pathological to write code where the mere access of an attribute has visible side effects (whether the code has invisible side effects - such as creating a cache entry for faster lookup - is irrelevant.)

Sample Implementation

An implementation of an earlier version of this PEP was created by Patrick Maupin and Eric V. Smith, and can be found in the pep3101 sandbox at:

http://svn.python.org/view/sandbox/trunk/pep3101/

Backwards Compatibility

Backwards compatibility can be maintained by leaving the existing mechanisms in place. The new system does not collide with any of the method names of the existing string formatting techniques, so both systems can co-exist until it comes time to deprecate the older system.

References

[1]Python Library Reference - String formating operations http://docs.python.org/library/stdtypes.html#string-formatting-operations
[2]Python Library References - Template strings http://docs.python.org/library/string.html#string.Template
[3][Python-3000] String formating operations in python 3k https://mail.python.org/pipermail/python-3000/2006-April/000285.html
[4]Composite Formatting - [.Net Framework Developer's Guide] http://msdn.microsoft.com/library/en-us/cpguide/html/cpconcompositeformatting.asp?frame=true
[5]Genshi templating engine. http://genshi.edgewall.org/
[6]Cheetah - The Python-Powered Template Engine. http://www.cheetahtemplate.org/
Source: https://github.com/python/peps/blob/master/pep-3101.txt