PEP: 498
Title: Literal String Formatting
Version: $Revision$
Last-Modified: $Date$
Author: Eric V. Smith <eric@trueblade.com>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 01-Aug-2015
Python-Version: 3.6
Post-History: 07-Aug-2015

Abstract
========

Python supports multiple ways to format text strings. These include
%-formatting [#]_, str.format [#]_, and string.Template [#]_. Each of
these methods have their advantages, but in addition have
disadvantages that make them cumbersome to use in practice. This PEP
proposed to add a new string formatting mechanism: Literal String
Formatting. In this PEP, such strings will be refered to as
"f-strings", taken from the leading character used to denote such
strings.

This PEP does not propose to remove or deprecate any of the existing
string formatting mechanisms.

f-strings provide a way to embed expressions inside string literals,
using a minimal syntax. It should be noted that an f-string is really
an expression evaluated at run time, not a constant value. In Python
source code, an f-string is a literal string, prefixed with 'f', that
contains expressions inside braces. The expressions are replaced with
their values. Some examples are::

  >>> import datetime
  >>> name = 'Fred'
  >>> age = 50
  >>> anniversary = datetime.date(1991, 10, 12)
  >>> f'My name is {name}, my age next year is {age+1}, my anniversary is {anniversary:%A, %B %d, %Y}.'
  'My name is Fred, my age next year is 51, my anniversary is Saturday, October 12, 1991.'
  >>> f'He said his name is {name!r}.'
  "He said his name is 'Fred'."

A similar feature was proposed in PEP 215 [#]_. PEP 215 proposed to
support a subset of Python expressions, and did not support the
type-specific string formatting (the __format__ method) which was
introduced with PEP 3101 [#]_.

Rationale
=========

This PEP is driven by the desire to have a simpler way to format
strings in Python. The existing ways of formatting are either error
prone, inflexible, or cumbersome.

%-formatting is limited as to the types it supports. Only ints, strs,
and doubles can be formatted. All other types are either not
supported, or converted to one of these types before formatting. In
addition, there's a well-known trap where a single value is passed::

  >>> msg = 'disk failure'
  >>> 'error: %s' % msg
  'error: disk failure'

But if msg were ever to be a tuple, the same code would fail::

  >>> msg = ('disk failure', 32)
  >>> 'error: %s' % msg
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  TypeError: not all arguments converted during string formatting

To be defensive, the following code should be used::

  >>> 'error: %s' % (msg,)
  "error: ('disk failure', 32)"

str.format() was added to address some of these problems with
%-formatting. In particular, it uses normal function call syntax (and
therefor supports multiple parameters) and it is extensible through
the __format__() method on the object being converted to a string. See
PEP-3101 for a detailed rationale. This PEP reuses much of the
str.format() syntax and machinery, in order to provide continuity with
an existing Python string formatting mechanism.

However, str.format() is not without its issues. Chief among them is
its verbosity. For example, the text 'value' is repeated here::

  >>> value = 4 * 20
  >>> 'The value is {value}.'.format(value=value)
  'The value is 80.'

Even in its simplest form, there is a bit of boilerplate, and the
value that's inserted into the placeholder is sometimes far removed
from where the placeholder is situated::

  >>> 'The value is {}.'.format(value)
  'The value is 80.'

With an f-string, this becomes::

  >>> f'The value is {value}.'
  'The value is 80.'

f-strings provide a concise, readable way to include expressions
inside strings.

In this sense, string.Template and %-formatting have similar
shortcomings to str.format(), but also support fewer formatting
options. In particular, they do not support the __format__ protocol,
so that there is no way to control how a specific object is converted
to a string, nor can it be extended to additional types that want to
control how they are converted to strings (such as Decimal and
datetime). This example is not possible with string.Template::

  >>> value = 1234
  >>> f'input={value:#0.6x}'
  'input=0x04d2'

And neither %-formatting nor string.Template can control formatting
such as::

  >>> date = datetime.date(1991, 10, 12)
  >>> f'{date} was on a {date:%A}'
  '1991-10-12 was on a Saturday'

No use of globals() or locals()
-------------------------------

In the discussions on python-dev [#]_, a number of solutions where
presented that used locals() and globals() or their equivalents. All
of these have various problems. Among these are referencing variables
that are not otherwise used in a closure. Consider::

  >>> def outer(x):
  ...     def inner():
  ...         return 'x={x}'.format_map(locals())
  ...     return inner
  ...
  >>> outer(42)()
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
    File "<stdin>", line 3, in inner
  KeyError: 'x'

This returns an error because the compiler has not added a reference
to x inside the closure. You need to manually add a reference to x in
order for this to work::

  >>> def outer(x):
  ...     def inner():
  ...         x
  ...         return 'x={x}'.format_map(locals())
  ...     return inner
  ...
  >>> outer(42)()
  'x=42'

In addition, using locals() or globals() introduces an information
leak. A called routine that has access to the callers locals() or
globals() has access to far more information than needed to do the
string interpolation.

Guido stated [#]_ that any solution to better string interpolation
would not use locals() or globals().

Specification
=============

In source code, f-strings are string literals that are prefixed by the
letter 'f'. 'f' may be combined with 'r', in either order, to produce
raw f-string literals. 'f' may not be combined with 'b': there are no
binary f-strings. 'f' may also be combined with 'u', in either order,
although adding 'u' has no effect.

f-strings are parsed in to literals and expressions. Expressions
appear within curly braces '{' and '}. The parts of the string outside
of braces are literals.  The expressions are evaluated, formatted with
the existing __format__ protocol, then the results are concatenated
together with the string literals. While scanning the string for
expressions, any doubled braces '{{' or '}}' are replaced by the
corresponding single brace. Doubled opening braces do not signify the
start of an expression.

Following the expression, an optional type conversion may be
specified.  The allowed conversions are '!s', '!r', or '!a'. These are
treated the same as in str.format: '!s' calls str() on the expression,
'!r' calls repr() on the expression, and '!a' calls ascii() on the
expression. These conversions are applied before the call to
__format__. The only reason to use '!s' is if you want to specify a
format specifier that applies to str, not to the type of the
expression.

Similar to str.format, optional format specifiers maybe be included
inside the f-string, separated from the expression (or the type
conversion, if specified) by a colon. If a format specifier is not
provied, an empty string is used.

So, an f-string looks like::

  f ' <text> { <expression> <optional !s, !r, or !a> <optional : format specifier> } text ... '

The resulting expression's __format__ method is called with the format
specifier. The resulting value is used when building the value of the
f-string.

Expressions cannot contain ':' or '!' outside of strings or parens,
brackets, or braces. The exception is that the '!=' operator is
special cased.

Code equivalence
----------------

The exact code used to implement f-strings is not specified. However,
it is guaranteed that any embedded value that is converted to a string
will use that value's __format__ method. This is the same mechanism
that str.format() uses to convert values to strings.

For example, this code::

  f'abc{expr1:spec1}{expr2!r:spec2}def{expr3:!s}ghi'

Might be be evaluated as::

  ''.join(['abc', expr1.__format__(spec1), repr(expr2).__format__(spec2), 'def', str(spec3).__format__(''), 'ghi'])

Expression evaluation
---------------------

The expressions that are extracted from the string are evaluated in
the context where the f-string appeared. This means the expression has
full access to local and global variables. Any valid Python expression
can be used, including function and method calls.

Because the f-strings are evaluated where the string appears in the
source code, there is no additional expressiveness available with
f-strings. There are also no additional security concerns: you could
have also just written the same expression, not inside of an
f-string::

  >>> def foo():
  ...   return 20
  ...
  >>> f'result={foo()}'
  'result=20'

Is equivalent to::

  >>> 'result=' + str(foo())
  'result=20'

After stripping leading and trailing whitespace (see below), the
expression is parsed with the equivalent of ast.parse(expression,
'<fstring>', 'eval') [#]_. Note that this restricts the expression: it
cannot contain any newlines, for example::

  >>> x = 0
  >>> f'''{x
  ... +1}'''
    File "<fstring>", line 2
      +1
      ^
  SyntaxError: invalid syntax

But note that this works, since the newline is removed from the
string, and the spaces in front of the '1' are allowed in an
expression::

  >>> f'{x+\
  ...    1}'
  '2'

Format specifiers
-----------------

Format specifiers may also contain evaluated expressions. This allows
code such as::

  >>> width = 10
  >>> precision = 4
  >>> value = decimal.Decimal('12.34567')
  >>> f'result: {value:{width}.{precision}}'
  'result:      12.35'

Once expressions in a format specifier are evaluated (if necessary),
format specifiers are not interpreted by the f-string evaluator. Just as
in str.format(), they are merely passed in to the __format__() method
of the object being formatted.

Concatenating strings
---------------------

Adjacent f-strings and regular strings are concatenated. Regular
strings are concatenated at compile time, and f-strings are
concatenated at run time. For example, the expression::

  >>> x = 10
  >>> y = 'hi'
  >>> 'a' 'b' f'{x}' '{c}' f'str<{y:^4}>' 'd' 'e'

yields the value::

  'ab10{c}str< hi >de'

While the exact method of this run time concatenation is unspecified,
the above code might evaluate to::

  ''.join(['ab', x.__format__(''), '{c}', 'str<', y.__format__('^4'), 'de'])

Error handling
--------------

Either compile time or run time errors can occur when processing
f-strings. Compile time errors are limited to those errors that can be
detected when scanning an f-string. These errors all raise
SyntaxError.

Unmatched braces::

  >>> f'x={x'
    File "<stdin>", line 1
  SyntaxError: missing '}' in format string expression

Invalid expressions::

  >>> f'x={!x}'
    File "<fstring>", line 1
      !x
      ^
  SyntaxError: invalid syntax

Run time errors occur when evaluating the expressions inside an
f-string. Note that an f-string can be evaluated multiple times, and
work sometimes and raise an error at other times::

  >>> d = {0:10, 1:20}
  >>> for i in range(3):
  ...     print(f'{i}:{d[i]}')
  ...
  0:10
  1:20
  Traceback (most recent call last):
    File "<stdin>", line 2, in <module>
  KeyError: 2

or::

  >>> for x in (32, 100, 'fifty'):
  ...   print(f'x = {x:+3}')
  ...
  'x = +32'
  'x = +100'
  Traceback (most recent call last):
    File "<stdin>", line 2, in <module>
  ValueError: Sign not allowed in string format specifier

Leading and trailing whitespace in expressions is skipped
---------------------------------------------------------

For ease of readability, leading and trailing whitespace in
expressions is ignored.

Discussion
==========

python-ideas discussion
-----------------------

Most of the discussions on python-ideas [#]_ focused on three issues:

 - How to denote f-strings,
 - How to specify the location of expressions in f-strings, and
 - Whether to allow full Python expressions.

How to denote f-strings
***********************

Because the compiler must be involved in evaluating the expressions
contained in the interpolated strings, there must be some way to
denote to the compiler which strings should be evaluated. This PEP
chose a leading 'f' character preceeding the string literal. This is
similar to how 'b' and 'r' prefixes change the meaning of the string
itself, at compile time. Other prefixes were suggested, such as 'i'. No
option seemed better than the other, so 'f' was chosen.

Another option was to support special functions, known to the
compiler, such as Format(). This seems like too much magic for Python:
not only is there a chance for collision with existing identifiers,
the PEP author feels that it's better to signify the magic with a
string prefix character.

How to specify the location of expressions in f-strings
*******************************************************

This PEP supports the same syntax as str.format() for distinguishing
replacement text inside strings: expressions are contained inside
braces. There were other options suggested, such as string.Template's
$identifier or ${expression}.

While $identifier is no doubt more familiar to shell scripters and
users of some other languages, in Python str.format() is heavily
used. A quick search of Python's standard library shows only a handful
of uses of string.Template, but hundreds of uses of str.format().

Another proposed alternative was to have the substituted text between
\\{ and } or between \\{ and \\}. While this syntax would probably be
desirable if all string literals were to support interpolation, this
PEP only supports strings that are already marked with the leading
'f'. As such, the PEP is using unadorned braces to denoted substituted
text, in order to leverage end user familiarity with str.format().

Supporting full Python expressions
**********************************

Many people on the python-ideas discussion wanted support for either
only single identifiers, or a limited subset of Python expressions
(such as the subset supported by str.format()). This PEP supports full
Python expressions inside the braces. Without full expressions, some
desirable usage would be cumbersome. For example::

  >>> f'Column={col_idx+1}'
  >>> f'number of items: {len(items)}'

would become::

  >>> col_number = col_idx+1
  >>> f'Column={col_number}'
  >>> n_items = len(items)
  >>> f'number of items: {n_items}'

While it's true that very ugly expressions could be included in the
f-strings, this PEP takes the position that such uses should be
addressed in a linter or code review::

  >>> f'mapping is { {a:b for (a, b) in ((1, 2), (3, 4))}}'
  'mapping is {1: 2, 3: 4}'

Similar support in other languages
----------------------------------

Wikipedia has a good discussion of string interpolation in other
programming languages [#]_. This feature is implemented in many
languages, with a variety of syntaxes and restrictions.

Differences between f-string and str.format expressions
-------------------------------------------------------

There is one small difference between the limited expressions allowed
in str.format() and the full expressions allowed inside f-strings. The
difference is in how index lookups are performed. In str.format(),
index values that do not look like numbers are converted to strings::

  >>> d = {'a': 10, 'b': 20}
  >>> 'a={d[a]}'.format(d=d)
  'a=10'

Notice that the index value is converted to the string "a" when it is
looked up in the dict.

However, in f-strings, you would need to use a literal for the value
of 'a'::

  >>> f'a={d["a"]}'
  'a=10'

This difference is required because otherwise you would not be able to
use variables as index values::

  >>> a = 'b'
  >>> f'a={d[a]}'
  'a=20'

See [#]_ for a further discussion. It was this observation that led to
full Python expressions being supported in f-strings.

Furthermore, the limited expressions that str.format() understands
need not be valid Python expressions. For example::

  >>> '{i[";]}'.format(i={'";':4})
  '4'

For this reason, the str.format() "expression parser" is not suitable
for use when implementing f-strings.

Triple-quoted f-strings
-----------------------

Triple quoted f-strings are allowed. These strings are parsed just as
normal triple-quoted strings are. After parsing, the normal f-string
logic is applied, and __format__() on each value is called.

Raw f-strings
-------------

Raw and f-strings may be combined. For example they could be used to
build up regular expressions::

  >>> header = 'Subject'
  >>> fr'{header}:\s+'
  'Subject:\\s+'

In addition, raw f-strings may be combined with triple-quoted strings.

No binary f-strings
-------------------

For the same reason that we don't support bytes.format(), you may not
combine 'f' with 'b' string literals. The primary problem is that an
object's __format__() method may return Unicode data that is not
compatible with a bytes string.

Binary f-strings would first require a solution for
bytes.format(). This idea has been proposed in the past, most recently
in PEP 461 [#]_. The discussions of such a feature usually suggest either

  - adding a method such as __bformat__() so an object can control how
    it is converted to bytes, or

  - having bytes.format() not be as general purpose or extensible as
    str.format().

Both of these remain as options in the future, if such functionality
is desired.

!s, !r, and !a are redundant
----------------------------

The !s, !r, and !a are not strictly required. Because arbitrary
expressions are allowed inside the f-strings, this code::

  >>> a = 'some string'
  >>> f'{a!r}'
  "'some string'"

Is identical to::

  >>> f'{repr(a)}'
  "'some string'"

Similarly, !s can be replaced by calls to str() and !a by calls to
ascii().

However, !s, !r, and !a are supported by this PEP in order to minimize
the differences with str.format(). !s, !r, and !a are required in
str.format() because it does not allow the execution of arbitrary
expressions.

Lambdas inside expressions
--------------------------

Because lambdas use the ':' character, they cannot appear outside of
parenthesis in an expression. The colon is interpreted as the start of
the format specifier, which means the start of the lambda expression
is seen and is syntactically invalid. As there's no practical use for
a plain lambda in an f-string expression, this is not seen as much of
a limitation.

If you feel you must use lambdas, they may be used inside of parens::

  >>> f'{(lambda x: x*2)(3)}'
  '6'

Examples from Python's source code
==================================

Here are some examples from Python source code that currently use
str.format(), and how they would look with f-strings. This PEP does
not recommend wholesale converting to f-strings, these are just
examples of real-world usages of str.format() and how they'd look if
written from scratch using f-strings.

Lib/asyncio/locks.py::

  extra = '{},waiters:{}'.format(extra, len(self._waiters))
  extra = f'{extra},waiters:{len(self._waiters)}'

Lib/configparser.py::

  message.append(" [line {0:2d}]".format(lineno))
  message.append(f" [line {lineno:2d}]")

Tools/clinic/clinic.py::

  methoddef_name = "{}_METHODDEF".format(c_basename.upper())
  methoddef_name = f"{c_basename.upper()}_METHODDEF"

python-config.py::

  print("Usage: {0} [{1}]".format(sys.argv[0], '|'.join('--'+opt for opt in valid_opts)), file=sys.stderr)
  print(f"Usage: {sys.argv[0]} [{'|'.join('--'+opt for opt in valid_opts)}]", file=sys.stderr)

Implementation limitations
==========================

Maximum of 255 expressions
--------------------------

Due to a CPython limit with the number of parameters to a function, an
f-string may not contain more that 255 expressions. This includes
expressions inside format specifiers. So this code would count as
having 2 expressions::

  f'{x:.{width}}'

Evaluation order of expressions
-------------------------------

The expressions in an f-string are evaluated in left-to-right
order. This is detectable only if the expressions have side effects::

  >>> def fn(l, incr):
  ...    result = l[0]
  ...    l[0] += incr
  ...    return result
  ...
  >>> lst = [0]
  >>> f'{fn(lst,2)} {fn(lst,3)}'
  '0 2'
  >>> f'{fn(lst,2)} {fn(lst,3)}'
  '5 7'
  >>> lst
  [10]

Expressions used multiple times
-------------------------------

xxx

References
==========

.. [#] %-formatting
       (https://docs.python.org/3/library/stdtypes.html#printf-style-string-formatting)

.. [#] str.format
       (https://docs.python.org/3/library/string.html#formatstrings)

.. [#] string.Template documentation
       (https://docs.python.org/3/library/string.html#template-strings)

.. [#] PEP 215: String Interpolation
       (https://www.python.org/dev/peps/pep-0215/)

.. [#] PEP 3101: Advanced String Formatting
       (https://www.python.org/dev/peps/pep-3101/)

.. [#] Formatting using locals() and globals()
       (https://mail.python.org/pipermail/python-ideas/2015-July/034671.html)

.. [#] Avoid locals() and globals()
       (https://mail.python.org/pipermail/python-ideas/2015-July/034701.html)

.. [#] str.format_map() documentation
       (https://docs.python.org/3/library/stdtypes.html#str.format_map)

.. [#] Format string syntax
       (https://docs.python.org/3/library/string.html#format-string-syntax)

.. [#] ast.parse() documentation
       (https://docs.python.org/3/library/ast.html#ast.parse)

.. [#] Start of python-ideas discussion
       (https://mail.python.org/pipermail/python-ideas/2015-July/034657.html)

.. [#] Wikipedia article on string interpolation
       (https://en.wikipedia.org/wiki/String_interpolation)

.. [#] Differences in str.format() and f-string expressions
       (https://mail.python.org/pipermail/python-ideas/2015-July/034726.html)

.. [#] PEP 461 rejects bytes.format()
       (https://www.python.org/dev/peps/pep-0461/#proposed-variations)

Copyright
=========

This document has been placed in the public domain.


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