lrucache should reject maxsize as a function

[jaraco]

BPO	22184
Nosy	@rhettinger, @jaraco, @jwilk, @serhiy-storchaka
Files	lru.diff: Add type check for maxsize

^{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 = 'https://github.com/rhettinger'
closed_at = <Date 2014-08-12.19:46:12.034>
created_at = <Date 2014-08-11.19:53:01.665>
labels = ['type-bug', 'library']
title = 'lrucache should reject maxsize as a function'
updated_at = <Date 2014-08-13.04:59:43.682>
user = 'https://github.com/jaraco'

bugs.python.org fields:

activity = <Date 2014-08-13.04:59:43.682>
actor = 'rhettinger'
assignee = 'rhettinger'
closed = True
closed_date = <Date 2014-08-12.19:46:12.034>
closer = 'rhettinger'
components = ['Library (Lib)']
creation = <Date 2014-08-11.19:53:01.665>
creator = 'jaraco'
dependencies = []
files = ['36355']
hgrepos = []
issue_num = 22184
keywords = ['patch']
message_count = 4.0
messages = ['225202', '225241', '225251', '225254']
nosy_count = 5.0
nosy_names = ['rhettinger', 'jaraco', 'jwilk', 'python-dev', 'serhiy.storchaka']
pr_nums = []
priority = 'low'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue22184'
versions = ['Python 3.4', 'Python 3.5']

[jaraco]

In https://bitbucket.org/jaraco/backports.functools_lru_cache/issue/1/python-2-attributeerror-int-object-has-no, a user was confused when he tried to use the lrucache decorator incorrectly, passing the wrapped function directly to lrucache. Consider:

    @lrucache
    def expensive(param):
        pass

One can even get away with profiling that now decorated function:

    for x in range(10000):
        expensive(x)

The test will run without error, but it's not doing what the user thinks it's doing. In the first section, it's creating a decorator, and in the second section, it's wrapping an int in that decorator, but because the wrapper is never called, an error is never raised.

I propose adding a simple check that if maxsize is callable, raise a TypeError. That would prevent unintentional misuse and subtle non-failure with minimal impact on performance.

[python-dev]

New changeset 780693490c84 by Raymond Hettinger in branch '3.4':
bpo-22184: Early detection and reporting of missing lru_cache parameters
http://hg.python.org/cpython/rev/780693490c84

[jaraco]

Awesome. Thanks for the quick implementation.

[rhettinger]

Awesome. Thanks for the quick implementation.

Happy to do it. Thanks for the clean bug report.