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PEP 498 -- Literal String Interpolation

PEP: 498
Title: Literal String Interpolation
Author: Eric V. Smith <eric at trueblade.com>
Status: Accepted
Type: Standards Track
Created: 01-Aug-2015
Python-Version: 3.6
Post-History: 07-Aug-2015, 30-Aug-2015, 04-Sep-2015, 19-Sep-2015, 06-Nov-2016
Resolution: https://mail.python.org/pipermail/python-dev/2015-September/141526.html

Abstract

Python supports multiple ways to format text strings. These include %-formatting [1] , str.format() [2] , and string.Template [3] . 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 Interpolation. In this PEP, such strings will be referred to as "f-strings", taken from the leading character used to denote such strings, and standing for "formatted 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', which 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 the value of Python 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:#06x}'
'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 [4] , 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 [5] 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' or 'F'. Everywhere this PEP uses 'f', 'F' may also be used. 'f' may be combined with 'r' or 'R', in either order, to produce raw f-string literals. 'f' may not be combined with 'b': this PEP does not propose to add binary f-strings. 'f' may not be combined with 'u'.

When tokenizing source files, f-strings use the same rules as normal strings, raw strings, binary strings, and triple quoted strings. That is, the string must end with the same character that it started with: if it starts with a single quote it must end with a single quote, etc. This implies that any code that currently scans Python code looking for strings should be trivially modifiable to recognize f-strings (parsing within an f-string is another matter, of course).

Once tokenized, f-strings are parsed in to literal strings and expressions. Expressions appear within curly braces '{' and '}' . While scanning the string for expressions, any doubled braces '{{' or '}}' inside literal portions of an f-string are replaced by the corresponding single brace. Doubled literal opening braces do not signify the start of an expression. A single closing curly brace '}' in the literal portion of a string is an error: literal closing curly braces must be doubled '}}' in order to represent a single closing brace.

The parts of the f-string outside of braces are literal strings. These literal portions are then decoded. For non-raw f-strings, this includes converting backslash escapes such as '\n' , '\"' , "\'" , '\xhh' , '\uxxxx' , '\Uxxxxxxxx' , and named unicode characters '\N{name}' into their associated Unicode characters [6] .

Backslashes may not appear anywhere within expressions. Comments, using the '#' character, are not allowed inside an expression.

Following each 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 provided, an empty string is used.

So, an f-string looks like:

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

The expression is then formatted using the __format__ protocol, using the format specifier as an argument. The resulting value is used when building the value of the f-string.

Note that __format__() is not called directly on each value. The actual code uses the equivalent of type(value).__format__(value, format_spec) , or format(value, format_spec) . See the documentation of the builtin format() function for more details.

Expressions cannot contain ':' or '!' outside of strings or parentheses, brackets, or braces. The exception is that the '!=' operator is allowed as a special case.

Escape sequences

Backslashes may not appear inside the expression portions of f-strings, so you cannot use them, for example, to escape quotes inside f-strings:

>>> f'{\'quoted string\'}'
  File "<stdin>", line 1
SyntaxError: f-string expression part cannot include a backslash

You can use a different type of quote inside the expression:

>>> f'{"quoted string"}'
'quoted string'

Backslash escapes may appear inside the string portions of an f-string.

Note that the correct way to have a literal brace appear in the resulting string value is to double the brace:

>>> f'{{ {4*10} }}'
'{ 40 }'
>>> f'{{{4*10}}}'
'{40}'

Like all raw strings in Python, no escape processing is done for raw f-strings:

>>> fr'x={4*10}\n'
'x=40\\n'

Due to Python's string tokenizing rules, the f-string f'abc {a['x']} def' is invalid. The tokenizer parses this as 3 tokens: f'abc {a[' , x , and ']} def' . Just like regular strings, this cannot be fixed by using raw strings. There are a number of correct ways to write this f-string: with a different quote character:

f"abc {a['x']} def"

Or with triple quotes:

f'''abc {a['x']} def'''

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}ghi'

Might be evaluated as:

'abc' + format(expr1, spec1) + format(repr(expr2), spec2) + 'def' + format(expr3) + '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'

Expressions are parsed with the equivalent of ast.parse('(' + expression + ')', '<fstring>', 'eval') [7] .

Note that since the expression is enclosed by implicit parentheses before evaluation, expressions can contain newlines. For example:

>>> x = 0
>>> f'''{x
... +1}'''
'1'

>>> d = {0: 'zero'}
>>> f'''{d[0
... ]}'''
'zero'

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:

'ab' + format(x) + '{c}' + 'str<' + format(y, '^4') + '>de'

Each f-string is entirely evaluated before being concatenated to adjacent f-strings. That means that this:

>>> f'{x' f'}'

Is a syntax error, because the first f-string does not contain a closing brace.

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: f-string: expecting '}'

Invalid expressions:

>>> f'x={!x}'
  File "<stdin>", line 1
SyntaxError: f-string: empty expression not allowed

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 ignored

For ease of readability, leading and trailing whitespace in expressions is ignored. This is a by-product of enclosing the expression in parentheses before evaluation.

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]

Discussion

python-ideas discussion

Most of the discussions on python-ideas [8] 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 preceding 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 [9] . 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 [10] 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 and decoding, the normal f-string logic is applied, and __format__() is called on each value.

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 [11] . 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 conversions 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 parentheses:

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

Can't combine with 'u'

The 'u' prefix was added to Python 3.3 in PEP 414 as a means to ease source compatibility with Python 2.7. Because Python 2.7 will never support f-strings, there is nothing to be gained by being able to combine the 'f' prefix with 'u'.

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)
Source: https://github.com/python/peps/blob/master/pep-0498.txt