Text formatting error: '=' alignment not allowed in string format specifier

Question

What does \'=\' alignment mean in the following error message, and why does this code cause it?

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Answer 1

A workaround is to use '>' (right justify) padding, which is with the syntax:

[[fill]align][width]

with align being >, fill being 0 and width being 3.

>>> "{num:0>3}".format(num="1")
'001'

The problem was that there is a different 0 in the format specification:

format_spec     ::=  [[fill]align][sign][#][0][width][grouping_option][.precision][type]
#                                          ^^^ This one

That zero just makes fill default to 0 and align to =.

= alignment is specified as:

Forces the padding to be placed after the sign (if any) but before the digits. This is used for printing fields in the form ‘+000000120’. This alignment option is only valid for numeric types. It becomes the default when ‘0’ immediately precedes the field width.

Source (Python 3 docs)

This expects the argument to be an int, as strings don't have signs. So we just manually set it to the normal default of > (right justify).

Also note that 0 just specifies the default values for fill and align. You can change both or just the align.

>>> # fill defaults to '0', align is '>', `0` is set, width is `3`
>>> "{num:>03}".format(num=-1)
'0-1'
>>> # fill is `x`, align is '>', `0` is set (but does nothing), width is `"3"`
>>> "{num:x>03}".format(num=-1)
'x-1'
>>> # fill is `x`, align is '>', `0` is set (but does nothing), width is `"03"` (3)
>>> "{num:x>003}".format(num=-1)
'x-1'