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For example:
>>> '{:04}'.format('abc')
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
ValueError: '=' alignment not allowed in string format specifier
There is no any '=' in the format string.
>>> '{: }'.format('abc')
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
ValueError: Sign not allowed in string format specifier
There is no any sign ('+' or '-') in the format string.
There also an inconsistency between the wording of "something not allowed in string format specifier" and:
>>> '{:x}'.format('abc')
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
ValueError: Unknown format code 'x' for object of type 'str'
>>> '{:xx}'.format('abc')
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
ValueError: Invalid format specifier