Adding __getitem__ as a class method doesn't work as expected

[samwyse]

BPO	15289
Nosy	@ericsnowcurrently
Files	20120607.py: demonstrates the behavior Issue15289.py: implements the desired behavior

^{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 2012-07-07.20:37:42.567>
created_at = <Date 2012-07-07.19:47:57.289>
labels = ['interpreter-core', 'type-bug', 'invalid']
title = "Adding __getitem__ as a class method doesn't work as expected"
updated_at = <Date 2012-07-10.14:48:09.114>
user = 'https://bugs.python.org/samwyse'

bugs.python.org fields:

activity = <Date 2012-07-10.14:48:09.114>
actor = 'samwyse'
assignee = 'none'
closed = True
closed_date = <Date 2012-07-07.20:37:42.567>
closer = 'eric.snow'
components = ['Interpreter Core']
creation = <Date 2012-07-07.19:47:57.289>
creator = 'samwyse'
dependencies = []
files = ['26309', '26343']
hgrepos = []
issue_num = 15289
keywords = []
message_count = 3.0
messages = ['164926', '164935', '165192']
nosy_count = 2.0
nosy_names = ['samwyse', 'eric.snow']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue15289'
versions = ['Python 2.7']

[samwyse]

I'm using a class as a decorator, and saving information in a class variable. I wanted to access the information via a __getitem__ class method, but using conventional syntax doesn't work on a class. The following exception is thrown when the attached script is run.

Traceback (most recent call last):
  File "/Users/sam/Documents/forth.py", line 34, in <module>
    print Alias["f'"]
TypeError: 'type' object has no attribute '__getitem__'

[ericsnowcurrently]

This appears to be a misunderstanding about special-method lookup which, honestly, isn't super obvious at first. It helps to remember that classes are objects like everything else in Python and thus are instances of some type (their metaclass).

Anyway, here's the key point regarding special-method lookup. As far as Python is concerned, the following are equivalent:

  value = obj[key]

and

  value = type(obj).__getitem__(obj, key)

Thus Alias()["f'"] will give you your answer, but Alias["f'"] tries to call type(Alias).__getitem__(Alias, key). Since Alias is a class, it is an instance of the built-in type class, so type(alias) is type, which does not have a __getitem__() method.

Check out the following resources:

http://docs.python.org/py3k/reference/datamodel.html#special-method-names
http://docs.python.org/py3k/library/inspect.html#fetching-attributes-statically

[samwyse]

Thanks, Eric. Based on what you said, I was able to get the desired behavior by creating a metaclass.