Comment by roenxi

3 days ago

This looks like a cheatsheet for writing a hard-to-read Python script. I don't know who gets karmic brownie points for a f"{string:>20}" field, but under most normal cases it'd be better to use rjust() directly and not force people to remember Yet Another DSL.

Once a reader could be reasonably expected to consult reference material while working out what a print() is doing something has gone wrong. This is the programmer equivalent of wearing too much makeup.

4 comments

roenxi

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cloudbonsai  3 days ago

You haven't seen the full depth yet. Suppose that you encountered with this line:

    print(f"{n:.2g}")

What will it print? Here is the official explanation from https://docs.python.org/3.12/library/string.html#formatspec:

    g - General format. For a given precision p >= 1, this rounds the number to p significant digits and then formats the result in either fixed-point format or in scientific notation, depending on its magnitude. A precision of 0 is treated as equivalent to a precision of 1.

    The precise rules are as follows: suppose that the result formatted with presentation type 'e' and precision p-1 would have exponent exp. Then, if m <= exp < p, where m is -4 for floats and -6 for Decimals, the number is formatted with presentation type 'f' and precision p-1-exp. Otherwise, the number is formatted with presentation type 'e' and precision p-1. In both cases insignificant trailing zeros are removed from the significand, and the decimal point is also removed if there are no remaining digits following it, unless the '#' option is used.

    With no precision given, uses a precision of 6 significant digits for float. For Decimal, the coefficient of the result is formed from the coefficient digits of the value; scientific notation is used for values smaller than 1e-6 in absolute value and values where the place value of the least significant digit is larger than 1, and fixed-point notation is used otherwise.

    Positive and negative infinity, positive and negative zero, and nans, are formatted as inf, -inf, 0, -0 and nan respectively, regardless of the precision.

Make sense? You now should be able to see why it's called f-string.

  • jgtrosh  3 days ago

    Yes, it's a generalisation of `%g` in f-string's ancestor printf(3). This is what people expect to find in formatting templates.

  • zahlman  3 days ago

    I don't know why people look at paragraphs of documentation that explain the exact results in strange edge cases (which have to exist because of the underlying complexity; in this example, Python can't change how IEEE-754 works, nor the original C printf specifier they're emulating) and conclude that this proves some strange and unexpected complexity was introduced.

    When documentation isn't this thorough, people complain about that, too.