collections.counter update method without argument gives misleading error

[tirkarthi]

BPO	33900
Nosy	@rhettinger, @serhiy-storchaka, @tirkarthi

^{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 2018-06-19.08:46:37.921>
created_at = <Date 2018-06-19.07:54:27.687>
labels = ['3.8', 'library', 'invalid']
title = 'collections.counter update method without argument gives misleading error'
updated_at = <Date 2018-06-19.08:46:37.920>
user = 'https://github.com/tirkarthi'

bugs.python.org fields:

activity = <Date 2018-06-19.08:46:37.920>
actor = 'xtreak'
assignee = 'none'
closed = True
closed_date = <Date 2018-06-19.08:46:37.921>
closer = 'xtreak'
components = ['Library (Lib)']
creation = <Date 2018-06-19.07:54:27.687>
creator = 'xtreak'
dependencies = []
files = []
hgrepos = []
issue_num = 33900
keywords = []
message_count = 3.0
messages = ['319935', '319939', '319940']
nosy_count = 3.0
nosy_names = ['rhettinger', 'serhiy.storchaka', 'xtreak']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue33900'
versions = ['Python 3.8']

[tirkarthi]

There was a change made in collections.Counter that made it accept self as the first parameter and hence while doing *args in the method signature args[0] is always set as the self. Hence the check for not args becomes a no-op during collection_object.update() calls and the type error is not raised. This is only triggered when it's called as a class method like Counter.update and the error message is misleading "descriptor 'update' of 'Counter' object "needs an argument" which says update method of Counter object needs an argument though we are calling the method on class and also calling update on Counter object doesn't need an argument since counter_object.update() works fine as no-op. I think the error message could be improved since argument is not necessary for counter_object.update.

Relevant commit : ae5cb21#diff-f30cb98704c3ccaf71deaf5503f7f4a1R587
No argument to counter object update test case :

cpython/Lib/test/test_collections.py

Line 1077 in ae5cb21

c.update() # test case with no args

Calling update on Counter class test case : 20994f1#diff-b5f669eab2fa576572b6115caa24427fR928

# Python 3

➜  cpython git:(master) ✗ ./python
Python 3.8.0a0 (heads/bpo33830-httplib-docs-fix-dirty:9bf0fb8, Jun 18 2018, 17:09:54)
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from collections import Counter
>>> c = Counter("Hello")


>>> Counter.update()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/user/karthik/cpython/Lib/collections/__init__.py", line 638, in update
    raise TypeError("descriptor 'update' of 'Counter' object "
TypeError: descriptor 'update' of 'Counter' object needs an argument
>>> Counter.update(1) # No error since we check for the first argument to be iterable
>>> c.update() # No args works fine though there was an above error with TypeError: descriptor 'update' of 'Counter' object needs an argument when called on class

>>> Counter.update(1, foo=1) # 1 is considered as self here, a valid counter object works fine and is updated
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/user/karthik/cpython/Lib/collections/__init__.py", line 655, in update
    self.update(kwds)
AttributeError: 'int' object has no attribute 'update'

Thanks

[serhiy-storchaka]

This error is correct. It is the same as for dict.update.

>>> dict.update()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: descriptor 'update' of 'dict' object needs an argument

Perhaps you missed the difference between a descriptor and a method.

[tirkarthi]

Okay, got it. Sorry for the noise. Closing it.

Thanks