special method lookup docs don't address some important details

[bitdancer]

BPO	11988
Nosy	@birkenfeld, @rhettinger, @bitdancer, @Trundle, @ericsnowcurrently

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

Show more details

GitHub fields:

assignee = 'https://github.com/rhettinger'
closed_at = <Date 2011-05-03.22:01:55.787>
created_at = <Date 2011-05-03.20:47:09.516>
labels = ['type-bug', 'invalid', 'docs']
title = "special method lookup docs don't address some important details"
updated_at = <Date 2011-05-03.22:01:55.786>
user = 'https://github.com/bitdancer'

bugs.python.org fields:

activity = <Date 2011-05-03.22:01:55.786>
actor = 'r.david.murray'
assignee = 'rhettinger'
closed = True
closed_date = <Date 2011-05-03.22:01:55.787>
closer = 'r.david.murray'
components = ['Documentation']
creation = <Date 2011-05-03.20:47:09.516>
creator = 'r.david.murray'
dependencies = []
files = []
hgrepos = []
issue_num = 11988
keywords = []
message_count = 5.0
messages = ['135068', '135074', '135075', '135080', '135082']
nosy_count = 6.0
nosy_names = ['georg.brandl', 'rhettinger', 'r.david.murray', 'Trundle', 'docs@python', 'eric.snow']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue11988'
versions = ['Python 3.1', 'Python 2.7', 'Python 3.2', 'Python 3.3']

[bitdancer]

The following code:

--------------------------------

class X(list):

    def __contains__(self, key):
        print('X contains:', key)


class Y():

    def __init__(self, x):
        self.x = x

    def __getattr__(self, key):
        return getattr(self.x, key)

    def __iter__(self):
        print('Y iter')
        return iter([1,2])

x = X()
y = Y(x)

print('res:', 1 in y)

---

prints True. It has been explained to me that this is because of:

http://docs.python.org/reference/datamodel.html#special-method-lookup-for-new-style-classes

However, there is no way in the world that I would guess the behavior above from the documentation provided (and I find it surprising...I expected x's __contains__ to get called because Y (a class, not an instance) doesn't have a __contains__ method).

Can anyone explain it more clearly and update the documentation?

[ericsnowcurrently]

In 2.7 I get the following (if Y does not inherit from object):

>>> print('res:', 1 in y)
('X contains:', 1)
('res:', False)

This makes sense with old style classes. With Y(object):

>>> print('res:', 1 in y)
Y iter
('res:', True)

[birkenfeld]

Not sure I understand your issue here. How should "1 in y" get at X.__contains__ given the special method lookup rules? The __getattr__ is not called since y.__contains__ isn't looked up.

[bitdancer]

Well, then I suppose my question is why isn't __contains__ looked up? Other special methods that don't exist on Y do cause __getattr__ to be called. Why is __contains__ special? The docs for __getattr__ don't hint at this possibility either.

I think the people that understand this must have a mental model of how the interpreter looks up methods that has some pieces that are missing from mine. I guess I'd like those bits to be made explicit in the special method lookup docs, or if they are already documented somewhere, for there to be a pointer to them in the special method lookup docs.

[bitdancer]

Ah, that's what my problem is. My test example was poorly conceived (I used __del__!) so I *thought* the other special methods were triggering getattr.

I'd have figured it out if I hadn't screwed up my test :(