Argument names in __annotations__ are not mangled for functions defined inside class scope

[jonasw]

BPO	20625
Nosy	@ncoghlan, @larryhastings, @benjaminp, @merwok, @1st1
Files	issue_20625_01.patch issue_20625_02.patch

^{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/1st1'
closed_at = <Date 2014-02-18.17:56:03.804>
created_at = <Date 2014-02-14.11:57:28.716>
labels = ['interpreter-core', 'type-bug']
title = 'Argument names in __annotations__ are not mangled for functions defined inside class scope'
updated_at = <Date 2014-03-17.06:31:02.344>
user = 'https://bugs.python.org/jonasw'

bugs.python.org fields:

activity = <Date 2014-03-17.06:31:02.344>
actor = 'python-dev'
assignee = 'yselivanov'
closed = True
closed_date = <Date 2014-02-18.17:56:03.804>
closer = 'yselivanov'
components = ['Interpreter Core']
creation = <Date 2014-02-14.11:57:28.716>
creator = 'jonasw'
dependencies = []
files = ['34083', '34085']
hgrepos = []
issue_num = 20625
keywords = ['patch']
message_count = 12.0
messages = ['211215', '211231', '211234', '211237', '211242', '211243', '211244', '211248', '211531', '211556', '211671', '213822']
nosy_count = 9.0
nosy_names = ['nnorwitz', 'ncoghlan', 'larry', 'benjamin.peterson', 'eric.araujo', 'Arfrever', 'python-dev', 'yselivanov', 'jonasw']
pr_nums = []
priority = 'high'
resolution = 'fixed'
stage = None
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue20625'
versions = ['Python 3.3', 'Python 3.4']

[jonasw]

Assume I have this code:

class Spam:
    def eggs(__some_kwarg:int=None):
        print(__some_kwarg)

I can call Spam.bar with keyword arguments as expected from mangling:

>>> Spam.eggs(10)
10
>>> Spam.eggs(_Spam__some_kwarg=10)
10

However, in the __annotations__ field, the argument name is not mangled:

>>> Spam.eggs.__annotations__
{'__some_kwarg': <class 'int'>}

This is an inconsistency which makes it difficult to work with function annotations in this case.

[1st1]

It looks like it's a bug.

Spam.eggs.__code__.co_varnames will have '_Spam__some_kwarg' as a name of the first parameter. So __annotations__ should have '_Spam__some_kwarg' instead of '__some_kwarg'.

[1st1]

Furthermore:

class Foo:
  def bar(self, *, __kw:'anno'='default'):
    pass

>>> Foo.bar.__annotations__
{'__kw': 'anno'}
>>> Foo.bar.__kwdefaults__
{'_Foo__kw': 'default'}

[1st1]

Patch is attached. Please review.

[larryhastings]

Why would we mangle the names of arguments in the first place? What possible use case is there?

[1st1]

Why would we mangle the names of arguments in the first place? What possible use case is there?

The use case is multiple inheritance. You can define your methods with keyword and keyword-only params starting with '__', put in them some cached default for speeding up lookup time, and not being worried that someone can override the arg. The second reason is, I believe, is desire to be consistent with mangling methods and attributes.

Again, using mangling for function arguments is, probably, not the best practice, and that, combined with the fact, that people are just starting to play with annotations, explains why this bug was so long unnoticed.

But the semantics of arg names mangling is already there, implemented everywhere but __annotations__.

The attached patch is fairly simple, and really is just a bug fix. It would be great if we can get this in 3.4 (and even in 3.3 and 3.2).

[benjaminp]

(see review on rietveld)

[benjaminp]

lgtm

[python-dev]

New changeset a63327162063 by Yury Selivanov in branch 'default':
Mangle __parameters in __annotations__ dict properly. Issue bpo-20625.
http://hg.python.org/cpython/rev/a63327162063

[python-dev]

New changeset 5202aca8a673 by Victor Stinner in branch 'default':
Issue bpo-20625: Fix compilation issue
http://hg.python.org/cpython/rev/5202aca8a673

[python-dev]

New changeset e301a515f8f4 by Benjamin Peterson in branch 'default':
update magic number for bpo-20625
http://hg.python.org/cpython/rev/e301a515f8f4

[python-dev]

New changeset 4d7c3cbd8515 by Yury Selivanov in branch '3.4':
Mangle __parameters in __annotations__ dict properly. Issue bpo-20625.
http://hg.python.org/cpython/rev/4d7c3cbd8515

New changeset 3bced76d2706 by Victor Stinner in branch '3.4':
Issue bpo-20625: Fix compilation issue
http://hg.python.org/cpython/rev/3bced76d2706

New changeset e5cde3fd7c74 by Benjamin Peterson in branch '3.4':
update magic number for bpo-20625
http://hg.python.org/cpython/rev/e5cde3fd7c74