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pythonpython-3.xf-string

Why can't I use spaces around an f-string expression with a format specifier?

When I use f-strings with a format-specifier in a way like

>>> x = 1
>>> f'{ x:02 }'

It gives me ValueError: Unknown format code '\x20' for object of type 'int'. To fix this, I have to remove the surrounding whitespaces from the f-string expression, like f'{x:02}'.

Why is this so?

Solution

  • Given an f-string expression such as f'{ x:02 }', the format specifier portion of the expression starts and includes all characters after the colon (:). This means that the space to the right of 02 is part of the format specifier. Despite what the error says, as said by Chris Rands, a space is a valid format code. What causes the error is that we used an invalid format specifier for an integer. Adding a space after 02 invalidates the specifier.

    We can fix the problem with

    >>> f'{ x:02}'

    which successfully got me 01. However, I subjectively do not recommend this style for f-string expressions since it looks imbalanced, i.e. the leftmost space has no pairing space at the other end of the expression. I would not use any surrounding spaces instead (e.g. f'{x:02}').

    Recommended Reading

    EDIT: I edited this answer to provide the correct explanation. In a previous edit, I incorrectly stated that a space is not a valid code in a format. Thanks, Chris, for the correction.