Eric V. Smith <[email protected]> added the comment:
The problem is that type.__format__ doesn't exist, so object.__format__ is
being called, and it throws an error if you provide a format spec. This is done
for future expansion: if we do want to add type.__format__ in the future, we
don't have to worry about existing cases that are using format specs that might
not work with the new type.__format__.
No format spec is the same as calling str() on the argument and returning that,
which is what is happening in your working examples.
If you want to apply a str formatting spec, you should covert the argument to a
str first, using either !s or str():
>>> print('{a!s: >10}'.format(a=type(a)))
<class 'str'>
>>> print('{a: >10}'.format(a=str(type(a))))
<class 'str'>
----------
nosy: +eric.smith
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<https://bugs.python.org/issue33410>
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