functools.singledispatch: Misleading exception when calling without arguments

[doerwalter]

BPO	33967
Nosy	@doerwalter, @vstinner, @methane, @serhiy-storchaka, @corona10, @ZackerySpytz, @miss-islington
PRs	bpo-33967: singledispatch raises TypeError when no positional arguments #8184 [3.7] bpo-33967: Fix singledispatch raised IndexError when no args (GH-8184) #8220 [3.6] bpo-33967: Fix singledispatch raised IndexError when no args (GH-8184) #8221 bpo-33967: Remove use of deprecated assertRaisesRegexp() #8261 [3.6] bpo-33967: Remove use of deprecated assertRaisesRegexp() (GH-8261) #8295 [3.7] bpo-33967: Remove use of deprecated assertRaisesRegexp() (GH-8261) #8296 bpo-33967: Fix wrong use of assertRaises #8306 bpo-33967: Fix wrong use of assertRaises #8306 [3.7] bpo-33967: Fix wrong use of assertRaises (GH-8306) #8308 [3.6] bpo-33967: Fix wrong use of assertRaises (GH-8306) #8309

^{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 2018-07-17.06:20:39.742>
created_at = <Date 2018-06-26.11:57:10.842>
labels = ['3.7', '3.8', 'type-bug', 'library']
title = 'functools.singledispatch: Misleading exception when calling without arguments'
updated_at = <Date 2018-07-17.06:20:39.741>
user = 'https://github.com/doerwalter'

bugs.python.org fields:

activity = <Date 2018-07-17.06:20:39.741>
actor = 'methane'
assignee = 'none'
closed = True
closed_date = <Date 2018-07-17.06:20:39.742>
closer = 'methane'
components = ['Library (Lib)']
creation = <Date 2018-06-26.11:57:10.842>
creator = 'doerwalter'
dependencies = []
files = []
hgrepos = []
issue_num = 33967
keywords = ['patch']
message_count = 14.0
messages = ['320485', '321259', '321313', '321359', '321363', '321365', '321512', '321514', '321725', '321727', '321743', '321789', '321792', '321794']
nosy_count = 7.0
nosy_names = ['doerwalter', 'vstinner', 'methane', 'serhiy.storchaka', 'corona10', 'ZackerySpytz', 'miss-islington']
pr_nums = ['8184', '8220', '8221', '8261', '8295', '8296', '8306', '8306', '8308', '8309']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue33967'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8']

[doerwalter]

When I call a function decorated with functools.singledispatch without an argument, I get the following:

$ python
Python 3.6.5 (default, Jun 17 2018, 12:13:06)
[GCC 4.2.1 Compatible Apple LLVM 9.1.0 (clang-902.0.39.2)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import functools
>>> @functools.singledispatch
... def f(x):
...     pass
...
>>> f()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/Cellar/python/3.6.5_1/Frameworks/Python.framework/Versions/3.6/lib/python3.6/functools.py", line 803, in wrapper
    return dispatch(args[0].__class__)(*args, **kw)
IndexError: tuple index out of range

I would have expected a TypeError along the lines of

TypeError: f() missing 1 required positional argument: 'x'

[corona10]

@doerwalter

Can I take a look this issue?

[methane]

Signature mismatch should raise TypeError, not IndexError.
I think this should be backported to 3.6 and 3.7.

[methane]

New changeset 445f1b3 by INADA Naoki (Dong-hee Na) in branch 'master':
bpo-33967: Fix singledispatch raised IndexError when no args (GH-8184)
445f1b3

[miss-islington]

New changeset df9f633 by Miss Islington (bot) in branch '3.7':
bpo-33967: Fix singledispatch raised IndexError when no args (GH-8184)
df9f633

[miss-islington]

New changeset 6ceab46 by Miss Islington (bot) in branch '3.6':
bpo-33967: Fix singledispatch raised IndexError when no args (GH-8184)
6ceab46

[ZackerySpytz]

The test added in this change uses the deprecated method assertRaisesRegexp(), so -Werror will cause test_functools to fail. PR 8261 fixes this.

[methane]

New changeset 9e9b2c3 by INADA Naoki (Zackery Spytz) in branch 'master':
bpo-33967: Remove use of deprecated assertRaisesRegexp() (GH-8261)
9e9b2c3

[vstinner]

New changeset 336c953 by Victor Stinner (Miss Islington (bot)) in branch '3.6':
bpo-33967: Remove use of deprecated assertRaisesRegexp() (GH-8261) (GH-8295)
336c953

[vstinner]

New changeset 0a820a2 by Victor Stinner (Miss Islington (bot)) in branch '3.7':
bpo-33967: Remove use of deprecated assertRaisesRegexp() (GH-8261) (GH-8296)
0a820a2

[serhiy-storchaka]

The purpose of using assertRaisesRegexp() was testing the error message. assertRaises() doesn't do this. Needed to restore the appropriate assertion method, but using a non-deprecated alias: assertRaisesRegex().

[methane]

New changeset 56d8f57 by INADA Naoki in branch 'master':
bpo-33967: Fix wrong use of assertRaises (GH-8306)
56d8f57

[miss-islington]

New changeset 892df9d by Miss Islington (bot) in branch '3.7':
bpo-33967: Fix wrong use of assertRaises (GH-8306)
892df9d

[miss-islington]

New changeset 8b5d191 by Miss Islington (bot) in branch '3.6':
bpo-33967: Fix wrong use of assertRaises (GH-8306)
8b5d191