707 lines
22 KiB
Plaintext
707 lines
22 KiB
Plaintext
PEP: 498
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Title: Literal String Formatting
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Version: $Revision$
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Last-Modified: $Date$
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Author: Eric V. Smith <eric@trueblade.com>
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Status: Draft
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Type: Standards Track
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Content-Type: text/x-rst
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Created: 01-Aug-2015
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Python-Version: 3.6
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Post-History: 07-Aug-2015
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Abstract
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========
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Python supports multiple ways to format text strings. These include
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%-formatting [#]_, str.format [#]_, and string.Template [#]_. Each of
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these methods have their advantages, but in addition have
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disadvantages that make them cumbersome to use in practice. This PEP
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proposed to add a new string formatting mechanism: Literal String
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Formatting. In this PEP, such strings will be refered to as
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"f-strings", taken from the leading character used to denote such
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strings.
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This PEP does not propose to remove or deprecate any of the existing
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string formatting mechanisms.
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f-strings provide a way to embed expressions inside string literals,
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using a minimal syntax. It should be noted that an f-string is really
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an expression evaluated at run time, not a constant value. In Python
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source code, an f-string is a literal string, prefixed with 'f', that
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contains expressions inside braces. The expressions are replaced with
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their values. Some examples are::
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>>> import datetime
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>>> name = 'Fred'
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>>> age = 50
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>>> anniversary = datetime.date(1991, 10, 12)
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>>> f'My name is {name}, my age next year is {age+1}, my anniversary is {anniversary:%A, %B %d, %Y}.'
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'My name is Fred, my age next year is 51, my anniversary is Saturday, October 12, 1991.'
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>>> f'He said his name is {name!r}.'
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"He said his name is 'Fred'."
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A similar feature was proposed in PEP 215 [#]_. PEP 215 proposed to
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support a subset of Python expressions, and did not support the
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type-specific string formatting (the __format__ method) which was
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introduced with PEP 3101 [#]_.
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Rationale
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=========
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This PEP is driven by the desire to have a simpler way to format
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strings in Python. The existing ways of formatting are either error
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prone, inflexible, or cumbersome.
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%-formatting is limited as to the types it supports. Only ints, strs,
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and doubles can be formatted. All other types are either not
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supported, or converted to one of these types before formatting. In
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addition, there's a well-known trap where a single value is passed::
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>>> msg = 'disk failure'
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>>> 'error: %s' % msg
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'error: disk failure'
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But if msg were ever to be a tuple, the same code would fail::
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>>> msg = ('disk failure', 32)
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>>> 'error: %s' % msg
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Traceback (most recent call last):
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File "<stdin>", line 1, in <module>
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TypeError: not all arguments converted during string formatting
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To be defensive, the following code should be used::
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>>> 'error: %s' % (msg,)
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"error: ('disk failure', 32)"
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str.format() was added to address some of these problems with
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%-formatting. In particular, it uses normal function call syntax (and
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therefor supports multiple parameters) and it is extensible through
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the __format__() method on the object being converted to a string. See
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PEP-3101 for a detailed rationale. This PEP reuses much of the
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str.format() syntax and machinery, in order to provide continuity with
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an existing Python string formatting mechanism.
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However, str.format() is not without its issues. Chief among them is
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its verbosity. For example, the text 'value' is repeated here::
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>>> value = 4 * 20
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>>> 'The value is {value}.'.format(value=value)
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'The value is 80.'
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Even in its simplest form, there is a bit of boilerplate, and the
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value that's inserted into the placeholder is sometimes far removed
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from where the placeholder is situated::
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>>> 'The value is {}.'.format(value)
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'The value is 80.'
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With an f-string, this becomes::
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>>> f'The value is {value}.'
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'The value is 80.'
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f-strings provide a concise, readable way to include expressions
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inside strings.
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In this sense, string.Template and %-formatting have similar
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shortcomings to str.format(), but also support fewer formatting
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options. In particular, they do not support the __format__ protocol,
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so that there is no way to control how a specific object is converted
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to a string, nor can it be extended to additional types that want to
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control how they are converted to strings (such as Decimal and
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datetime). This example is not possible with string.Template::
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>>> value = 1234
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>>> f'input={value:#0.6x}'
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'input=0x04d2'
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And neither %-formatting nor string.Template can control formatting
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such as::
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>>> date = datetime.date(1991, 10, 12)
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>>> f'{date} was on a {date:%A}'
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'1991-10-12 was on a Saturday'
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No use of globals() or locals()
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-------------------------------
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In the discussions on python-dev [#]_, a number of solutions where
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presented that used locals() and globals() or their equivalents. All
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of these have various problems. Among these are referencing variables
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that are not otherwise used in a closure. Consider::
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>>> def outer(x):
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... def inner():
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... return 'x={x}'.format_map(locals())
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... return inner
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...
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>>> outer(42)()
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Traceback (most recent call last):
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File "<stdin>", line 1, in <module>
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File "<stdin>", line 3, in inner
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KeyError: 'x'
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This returns an error because the compiler has not added a reference
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to x inside the closure. You need to manually add a reference to x in
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order for this to work::
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>>> def outer(x):
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... def inner():
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... x
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... return 'x={x}'.format_map(locals())
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... return inner
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...
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>>> outer(42)()
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'x=42'
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In addition, using locals() or globals() introduces an information
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leak. A called routine that has access to the callers locals() or
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globals() has access to far more information than needed to do the
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string interpolation.
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Guido stated [#]_ that any solution to better string interpolation
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would not use locals() or globals().
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Specification
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=============
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In source code, f-strings are string literals that are prefixed by the
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letter 'f'. 'f' may be combined with 'r', in either order, to produce
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raw f-string literals. 'f' may not be combined with 'b': there are no
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binary f-strings. 'f' may also be combined with 'u', in either order,
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although adding 'u' has no effect.
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f-strings are parsed in to literals and expressions. Expressions
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appear within curly braces '{' and '}. The parts of the string outside
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of braces are literals. The expressions are evaluated, formatted with
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the existing __format__ protocol, then the results are concatenated
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together with the string literals. While scanning the string for
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expressions, any doubled braces '{{' or '}}' are replaced by the
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corresponding single brace. Doubled opening braces do not signify the
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start of an expression.
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Following the expression, an optional type conversion may be
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specified. The allowed conversions are '!s', '!r', or '!a'. These are
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treated the same as in str.format: '!s' calls str() on the expression,
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'!r' calls repr() on the expression, and '!a' calls ascii() on the
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expression. These conversions are applied before the call to
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__format__. The only reason to use '!s' is if you want to specify a
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format specifier that applies to str, not to the type of the
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expression.
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Similar to str.format, optional format specifiers maybe be included
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inside the f-string, separated from the expression (or the type
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conversion, if specified) by a colon. If a format specifier is not
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provied, an empty string is used.
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So, an f-string looks like::
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f ' <text> { <expression> <optional !s, !r, or !a> <optional : format specifier> } text ... '
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The resulting expression's __format__ method is called with the format
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specifier. The resulting value is used when building the value of the
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f-string.
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Expressions cannot contain ':' or '!' outside of strings or parens,
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brackets, or braces. The exception is that the '!=' operator is
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special cased.
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Escape sequences
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----------------
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Scanning an f-string for expressions happens after escape sequences
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are decoded. Because hex(ord('{')) == 0x7b, the f-string
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f'\u007b4*10}' is decoded to f'{4*10}', which evaluates as the integer
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40::
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>>> f'\u007b4*10}'
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'40'
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>>> f'\x7b4*10}'
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'40'
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>>> f'\x7b4*10\N{RIGHT CURLY BRACKET}'
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'40'
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These examples aren't generally useful, they're just to show that
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escape sequences are processed before f-strings are parsed for
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expressions.
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Code equivalence
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----------------
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The exact code used to implement f-strings is not specified. However,
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it is guaranteed that any embedded value that is converted to a string
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will use that value's __format__ method. This is the same mechanism
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that str.format() uses to convert values to strings.
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For example, this code::
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f'abc{expr1:spec1}{expr2!r:spec2}def{expr3:!s}ghi'
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Might be be evaluated as::
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''.join(['abc', expr1.__format__(spec1), repr(expr2).__format__(spec2), 'def', str(spec3).__format__(''), 'ghi'])
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Expression evaluation
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---------------------
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The expressions that are extracted from the string are evaluated in
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the context where the f-string appeared. This means the expression has
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full access to local and global variables. Any valid Python expression
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can be used, including function and method calls.
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Because the f-strings are evaluated where the string appears in the
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source code, there is no additional expressiveness available with
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f-strings. There are also no additional security concerns: you could
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have also just written the same expression, not inside of an
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f-string::
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>>> def foo():
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... return 20
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...
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>>> f'result={foo()}'
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'result=20'
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Is equivalent to::
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>>> 'result=' + str(foo())
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'result=20'
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After stripping leading and trailing whitespace (see below), the
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expression is parsed with the equivalent of ast.parse(expression,
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'<fstring>', 'eval') [#]_. Note that this restricts the expression: it
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cannot contain any newlines, for example::
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>>> x = 0
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>>> f'''{x
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... +1}'''
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File "<fstring>", line 2
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+1
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^
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SyntaxError: invalid syntax
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But note that this works, since the newline is removed from the
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string, and the spaces in front of the '1' are allowed in an
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expression::
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>>> f'{x+\
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... 1}'
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'2'
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Format specifiers
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-----------------
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Format specifiers may also contain evaluated expressions. This allows
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code such as::
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>>> width = 10
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>>> precision = 4
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>>> value = decimal.Decimal('12.34567')
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>>> f'result: {value:{width}.{precision}}'
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'result: 12.35'
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Once expressions in a format specifier are evaluated (if necessary),
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format specifiers are not interpreted by the f-string evaluator. Just as
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in str.format(), they are merely passed in to the __format__() method
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of the object being formatted.
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Concatenating strings
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---------------------
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Adjacent f-strings and regular strings are concatenated. Regular
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strings are concatenated at compile time, and f-strings are
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concatenated at run time. For example, the expression::
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>>> x = 10
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>>> y = 'hi'
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>>> 'a' 'b' f'{x}' '{c}' f'str<{y:^4}>' 'd' 'e'
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yields the value::
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'ab10{c}str< hi >de'
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While the exact method of this run time concatenation is unspecified,
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the above code might evaluate to::
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''.join(['ab', x.__format__(''), '{c}', 'str<', y.__format__('^4'), 'de'])
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Error handling
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--------------
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Either compile time or run time errors can occur when processing
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f-strings. Compile time errors are limited to those errors that can be
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detected when scanning an f-string. These errors all raise
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SyntaxError.
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Unmatched braces::
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>>> f'x={x'
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File "<stdin>", line 1
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SyntaxError: missing '}' in format string expression
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Invalid expressions::
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>>> f'x={!x}'
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File "<fstring>", line 1
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!x
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^
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SyntaxError: invalid syntax
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Run time errors occur when evaluating the expressions inside an
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f-string. Note that an f-string can be evaluated multiple times, and
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work sometimes and raise an error at other times::
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>>> d = {0:10, 1:20}
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>>> for i in range(3):
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... print(f'{i}:{d[i]}')
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...
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0:10
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1:20
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Traceback (most recent call last):
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File "<stdin>", line 2, in <module>
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KeyError: 2
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or::
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>>> for x in (32, 100, 'fifty'):
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... print(f'x = {x:+3}')
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...
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'x = +32'
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'x = +100'
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Traceback (most recent call last):
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File "<stdin>", line 2, in <module>
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ValueError: Sign not allowed in string format specifier
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Leading and trailing whitespace in expressions is skipped
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---------------------------------------------------------
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For ease of readability, leading and trailing whitespace in
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expressions is ignored.
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Evaluation order of expressions
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-------------------------------
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The expressions in an f-string are evaluated in left-to-right
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order. This is detectable only if the expressions have side effects::
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>>> def fn(l, incr):
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... result = l[0]
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... l[0] += incr
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... return result
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...
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>>> lst = [0]
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>>> f'{fn(lst,2)} {fn(lst,3)}'
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'0 2'
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>>> f'{fn(lst,2)} {fn(lst,3)}'
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'5 7'
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>>> lst
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[10]
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Discussion
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==========
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python-ideas discussion
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-----------------------
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Most of the discussions on python-ideas [#]_ focused on three issues:
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- How to denote f-strings,
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- How to specify the location of expressions in f-strings, and
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- Whether to allow full Python expressions.
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How to denote f-strings
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***********************
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Because the compiler must be involved in evaluating the expressions
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contained in the interpolated strings, there must be some way to
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denote to the compiler which strings should be evaluated. This PEP
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chose a leading 'f' character preceeding the string literal. This is
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similar to how 'b' and 'r' prefixes change the meaning of the string
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itself, at compile time. Other prefixes were suggested, such as 'i'. No
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option seemed better than the other, so 'f' was chosen.
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Another option was to support special functions, known to the
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compiler, such as Format(). This seems like too much magic for Python:
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not only is there a chance for collision with existing identifiers,
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the PEP author feels that it's better to signify the magic with a
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string prefix character.
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How to specify the location of expressions in f-strings
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*******************************************************
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This PEP supports the same syntax as str.format() for distinguishing
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replacement text inside strings: expressions are contained inside
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braces. There were other options suggested, such as string.Template's
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$identifier or ${expression}.
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While $identifier is no doubt more familiar to shell scripters and
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users of some other languages, in Python str.format() is heavily
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used. A quick search of Python's standard library shows only a handful
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of uses of string.Template, but hundreds of uses of str.format().
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Another proposed alternative was to have the substituted text between
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\\{ and } or between \\{ and \\}. While this syntax would probably be
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desirable if all string literals were to support interpolation, this
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PEP only supports strings that are already marked with the leading
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'f'. As such, the PEP is using unadorned braces to denoted substituted
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text, in order to leverage end user familiarity with str.format().
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Supporting full Python expressions
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**********************************
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Many people on the python-ideas discussion wanted support for either
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only single identifiers, or a limited subset of Python expressions
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(such as the subset supported by str.format()). This PEP supports full
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Python expressions inside the braces. Without full expressions, some
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desirable usage would be cumbersome. For example::
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>>> f'Column={col_idx+1}'
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>>> f'number of items: {len(items)}'
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would become::
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>>> col_number = col_idx+1
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>>> f'Column={col_number}'
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>>> n_items = len(items)
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>>> f'number of items: {n_items}'
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While it's true that very ugly expressions could be included in the
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f-strings, this PEP takes the position that such uses should be
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addressed in a linter or code review::
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>>> f'mapping is { {a:b for (a, b) in ((1, 2), (3, 4))}}'
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'mapping is {1: 2, 3: 4}'
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Similar support in other languages
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----------------------------------
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Wikipedia has a good discussion of string interpolation in other
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programming languages [#]_. This feature is implemented in many
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languages, with a variety of syntaxes and restrictions.
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Differences between f-string and str.format expressions
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-------------------------------------------------------
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There is one small difference between the limited expressions allowed
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in str.format() and the full expressions allowed inside f-strings. The
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difference is in how index lookups are performed. In str.format(),
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index values that do not look like numbers are converted to strings::
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>>> d = {'a': 10, 'b': 20}
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>>> 'a={d[a]}'.format(d=d)
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'a=10'
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Notice that the index value is converted to the string "a" when it is
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looked up in the dict.
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However, in f-strings, you would need to use a literal for the value
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of 'a'::
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>>> f'a={d["a"]}'
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'a=10'
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This difference is required because otherwise you would not be able to
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use variables as index values::
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>>> a = 'b'
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>>> f'a={d[a]}'
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'a=20'
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See [#]_ for a further discussion. It was this observation that led to
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full Python expressions being supported in f-strings.
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Furthermore, the limited expressions that str.format() understands
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need not be valid Python expressions. For example::
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>>> '{i[";]}'.format(i={'";':4})
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'4'
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For this reason, the str.format() "expression parser" is not suitable
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for use when implementing f-strings.
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Triple-quoted f-strings
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-----------------------
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Triple quoted f-strings are allowed. These strings are parsed just as
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normal triple-quoted strings are. After parsing, the normal f-string
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logic is applied, and __format__() on each value is called.
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Raw f-strings
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-------------
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Raw and f-strings may be combined. For example they could be used to
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build up regular expressions::
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>>> 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}}'
|
||
|
||
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.
|
||
|
||
|
||
..
|
||
Local Variables:
|
||
mode: indented-text
|
||
indent-tabs-mode: nil
|
||
sentence-end-double-space: t
|
||
fill-column: 70
|
||
coding: utf-8
|
||
End:
|