453 lines
14 KiB
Plaintext
453 lines
14 KiB
Plaintext
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.
|
||
|
||
f-strings provide a way to combine string literals with Python
|
||
expressions, using a minimal syntax. It should be noted that an
|
||
f-string is really an expression evaluated at run time, not a constant
|
||
value. An f-string is a 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'."
|
||
|
||
This PEP does not propose to remove or deprecate any of the existing
|
||
string formatting mechanisms.
|
||
|
||
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 mutliple parameters) and it is extensible through
|
||
the __format__() method on the object being converted to a string. See
|
||
PEP-3101 for a detailed rationale.
|
||
|
||
However, str.format() is not without its issues. Chief among them are
|
||
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.
|
||
|
||
string.Template has similar shortcomings to str.format(), but also
|
||
supports fewer formatting options. In particular, it does not support
|
||
__format__.
|
||
|
||
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'
|
||
|
||
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 optionally the type conversion may be
|
||
specified. The allowed conversions are '!s' or '!r'. These are
|
||
treated the same as in str.format: '!s' calls str() on the expression,
|
||
and '!r' calls repr() 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 or !r> <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 that is executed when converting expressions to strings
|
||
is unspecified by this PEP. However, it is specified that once the
|
||
expression is evaluated, the result's __format__() method will be
|
||
called with the given format specifier.
|
||
|
||
For example, this code::
|
||
|
||
f'abc{expr1:spec1}{expr2!r:spec2}def{expr3:!s}ghi'
|
||
|
||
May be evaluated as::
|
||
|
||
''.join(['abc', expr1.__format__('spec1'), repr(expr2).__format__('spec2'), 'def', str(expr3).__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'
|
||
|
||
Format specifiers
|
||
-----------------
|
||
|
||
Format specifiers are not interpreted by the f-string parser. 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::
|
||
|
||
'ab10cstr< hi >de'
|
||
|
||
While the exact code that is executed when evaluating this f-string is
|
||
not specified, one possible strategy is to evaluate::
|
||
|
||
''.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'):
|
||
... 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 whitespace in expressions is skipped
|
||
--------------------------------------------
|
||
|
||
Because expressions may begin with a left brace ('{'), there is a
|
||
problem when parsing such expressions. For example::
|
||
|
||
>>> f'{{k:v for k, v in [(1, 2), (3, 4)}}'
|
||
'{k:v for k, v in [(1, 2), (3, 4)}'
|
||
|
||
In this case, the doubled left braces and doubled right braces are
|
||
interpreted as single braces, and the string becomes just a normal
|
||
string literal. There is no expression evaluation being performed.
|
||
|
||
To account for this, whitespace characters at the beginning of an
|
||
expression are skipped::
|
||
|
||
>>> f'{ {k:v for k, v in [(1, 2), (3, 4)}}'
|
||
'{k:v for k, v in [(1, 2), (3, 4)}'
|
||
|
||
Discussion
|
||
==========
|
||
|
||
Most of the discussions on python-ideas [#]_ focused on a few issues:
|
||
|
||
- Whether to allow full Python expressions.
|
||
- How to designate f-strings, and how specify the locaton of
|
||
expressions in them.
|
||
- How to concatenate adjacent strings and f-strings.
|
||
|
||
XXX: more on the above issues.
|
||
|
||
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.
|
||
|
||
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.
|
||
|
||
!s and !r are redundant
|
||
-----------------------
|
||
|
||
The !s and !r 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().
|
||
|
||
However, !s and !r are supported by this PEP in order to minimize the
|
||
differences with str.format(). !s and !r 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.
|
||
|
||
Lambdas may be used inside of parens::
|
||
|
||
>>> f'{(lambda x: x*2)(3)}'
|
||
'6'
|
||
|
||
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)
|
||
|
||
.. [#] Start of python-ideas discussion
|
||
(https://mail.python.org/pipermail/python-ideas/2015-July/034657.html)
|
||
|
||
.. [#] Differences in str.format() and f-string expressions
|
||
(https://mail.python.org/pipermail/python-ideas/2015-July/034726.html)
|
||
|
||
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:
|