Python 3's F-Strings: An Improved String Formatting Syntax

Re:Python 3's F-Strings: An Improved String Formatting Syntax by ploxiln on Wednesday January 20, 2021 @10:40PM (#25855097)

Also, remember that the first argument to the logging functions is considered to be a format string. So using f-strings is not just less efficient, it is also not robust, unless you do:

    logging.debug("%s", f"Unexpected, got {got}")

because otherwise, if the format string is the first/only argument, and `got` unexpectedly contain `%`, an exception will be raised due to a missing format parameter.

(Also, for the logging functions, you don't pass a tuple, just a normal series of arguments.)

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Re:Python 3's F-Strings: An Improved String Formatting Syntax by dagenix on Wednesday January 20, 2021 @11:09PM (#25855280)

As the sibling comment points out, it actually doesn't work that way. The first parameter is only treated as a format string _if_ there are arguments. If not, it is treated as a literal string: https://github.com/python/cpython/blob/dcea78ff53d02733ac598...

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Re:Python 3's F-Strings: An Improved String Formatting Syntax by ploxiln on Thursday January 21, 2021 @12:23AM (#25855754)

I stand corrected. Thanks.

(That's what I get for assuming consistent behavior :P)

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Re:Python 3's F-Strings: An Improved String Formatting Syntax by dagenix on Thursday January 21, 2021 @01:32AM (#25856157)

I only found out about this quirk a few months ago immediately after I had given the same warning you had to someone else. It's extremely unintuitive.

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Re:Python 3's F-Strings: An Improved String Formatting Syntax by altncatchfire on Wednesday January 20, 2021 @10:55PM (#25855204)

I dont think this is true. I just tried with the following program and it runs as expected (Run on Python 3.7.9).

    import logging
    got = "%d"
    logging.warning(f"hi {got}")
    logging.debug(f"hi {got}")

Output

    WARNING:root:hi %d
    DEBUG:root:hi %d

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