Using type in a format with padding causes TypeError

[alexomics]

BPO	33410
Nosy	@ericvsmith, @Vgr255, @alexomics

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2018-05-02.14:33:37.707>
created_at = <Date 2018-05-02.14:22:10.657>
labels = ['type-bug', 'invalid']
title = 'Using type in a format with padding causes TypeError'
updated_at = <Date 2018-05-02.17:06:33.243>
user = 'https://github.com/alexomics'

bugs.python.org fields:

activity = <Date 2018-05-02.17:06:33.243>
actor = 'eric.smith'
assignee = 'none'
closed = True
closed_date = <Date 2018-05-02.14:33:37.707>
closer = 'abarry'
components = []
creation = <Date 2018-05-02.14:22:10.657>
creator = 'alexomics'
dependencies = []
files = []
hgrepos = []
issue_num = 33410
keywords = []
message_count = 3.0
messages = ['316072', '316074', '316081']
nosy_count = 3.0
nosy_names = ['eric.smith', 'abarry', 'alexomics']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue33410'
versions = ['Python 3.6']

[alexomics]

When trying to print a type in a formatted string with padding TypeError is raised. See examples below.

These work:
>>> a = 'abc'
>>> print('{a}'.format(a=type(a)))
<class 'str'>
>>> print('{a}'.format(a=str(type(a))))
<class 'str'>

These don't:
>>> print('{a: >10}'.format(a=type(a)))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported format string passed to type.__format__
>>> t = type(a)
>>> print('{a: >10}'.format(a=t))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported format string passed to type.__format__

[Vgr255]

type has a default __format__ implementation which doesn't accept any formatting options. This is expected behaviour.

[ericvsmith]

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'>