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pythonf-string

Why do I need another pair of curly braces when using a variable in a format specifier in Python f-strings?

I'm learning how Python f-strings handle formatting and came across this syntax:

a = 5.123
b = 2.456
width = 10
result = f"The result is {(a + b):<{width}.2f}end"
print(result)

This works as expected, however I don't understand why {width} needs its own curly braces within the format specification. Why can't I just use width directly just as "a" and "b"? Isn't width already inside the outer curly braces?

Solution

  • The braces are needed because the part starting with : is the format specification mini-language. That is parsed as its own f-string, where all is literal, except if put in braces.

    That format specification language would bump into ambiguities if those braces were not required: that language gives meaning to certain letters, and it would not be clear whether such letter(s) would need to be taken literally or as an expression to be evaluated dynamically.

    For instance, in Python 3.11+ we have the z option that can occur at the position where you specified the width:

    z = 5
f"The result is {(a + b):<z.2f}end"

    So what would this mean if braces were not needed? Would it be the z option or what the z variable represents?

    There are many more examples that could be constructed to bring the same conclusion home: the braces are needed for anything that needs to be evaluated inside the format specification part.