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pythonpython-3.xstring-formattingbackslashf-string

Why isn't it possible to use backslashes inside the braces of f-strings? How can I work around the problem?

In Python >=3.6, f-strings can be used as a replacement for the str.format method. As a simple example, these are equivalent:

'{} {}'.format(2+2, "hey")
f'{2+2} {"hey"}'

Disregarding format specifiers, I can basically move the positional arguments of str.format inside braces in an f-string. Note specifically that I am allowed to just put str literals in here, although it may seem a bit unwieldy.

There are however some limitations. Specifically, backslashes in any shape or form are disallowed inside the braces of an f-string:

'{}'.format("new\nline")  # legal
f'{"new\nline"}'          # illegal
f'{"\\"}'                 # illegal

I cannot even use \ to split up a long line if it's inside the braces;

f'{2+\
2}'     # illegal

even though this usage of \ is perfectly allowed inside normal str's;

'{\
}'.format(2+2)  # legal

It seems to me that a hard stop is coded into the parser if it sees the \ character at all inside the braces of an f-string. Why is this limitation implemented? Though the docs specify this behavior, it does not justify why.

Solution

  • You seem to expect

    '{}'.format("new\nline")

    and

    f'{"new\nline"}'

    to be equivalent. That's not what I would expect, and it's not how backslashes in f-strings worked back in the pre-release versions of Python 3.6 where backslashes between the braces were allowed. Back then, you'd get an error because

    "new
line"

    is not a valid Python expression.

    As just demonstrated, backslashes in the braces are confusing and ambiguous, and they were banned to avoid confusion:

    The point of this is to disallow convoluted code like:

    >>> d = {'a': 4}
>>> f'{d[\'a\']}'
'4'

    In addition, I'll disallow escapes to be used for brackets, as in:

    >>> f'\x7bd["a"]}'
'4'

    (where chr(0x7b) == "{").