MappingProxyType accepts string

[andy-maier]

BPO	43828
Nosy	@rhettinger, @andy-maier

^{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 2021-04-14.03:09:09.323>
created_at = <Date 2021-04-13.08:11:21.863>
labels = ['invalid', 'type-bug', 'library', '3.9']
title = 'MappingProxyType accepts string'
updated_at = <Date 2021-04-14.05:48:35.441>
user = 'https://github.com/andy-maier'

bugs.python.org fields:

activity = <Date 2021-04-14.05:48:35.441>
actor = 'andymaier'
assignee = 'none'
closed = True
closed_date = <Date 2021-04-14.03:09:09.323>
closer = 'rhettinger'
components = ['Library (Lib)']
creation = <Date 2021-04-13.08:11:21.863>
creator = 'andymaier'
dependencies = []
files = []
hgrepos = []
issue_num = 43828
keywords = []
message_count = 3.0
messages = ['390935', '391027', '391036']
nosy_count = 2.0
nosy_names = ['rhettinger', 'andymaier']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue43828'
versions = ['Python 3.9']

[andy-maier]

MappingProxyType objects can currently be initialized from a string object. Given is purpose, I think this is a bug and should result in TypeError being raised.

Example code (on CPython 3.9.1):

>>> from types import MappingProxyType
>>> mp = MappingProxyType('abc')
>>> list(mp)
['a', 'b', 'c']
>>> mp.items()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'str' object has no attribute 'items'

Other invalid input types are properly checked:

>>> mp = MappingProxyType(42)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: mappingproxy() argument must be a mapping, not int

>>> mp = MappingProxyType(['a'])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: mappingproxy() argument must be a mapping, not list

>>> mp = MappingProxyType(('a',))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: mappingproxy() argument must be a mapping, not tuple

[rhettinger]

This isn't a bug because the MappingProxy doesn't promise to do early detection and because it can't really be accomplished cleanly.

Perfect detection of non-mappings isn't possible. Some easy, minimal checks are made for early detection using PyMapping_Check, PyList_Check, and PyTuple_Check. Anything that gets past those checks will just need to be duck-typed downstream when it is used.

We could add a few other specific exclusions but that isn't a general solution and it slows down instantiation. Further it could break code that is intended to work (subclasses of str that mapping methods) or that just happen to work:

    >>> from types import MappingProxyType
    >>> mp = MappingProxyType('abc')
    >>> mp[0]
    'a'

[andy-maier]

I accept that the issue was closed, but wanted to document some things:

1. The dict class manages very well to detect that a string is invalid input:

>>> d = dict('abc')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: dictionary update sequence element #0 has length 1; 2 is required

2. When initialized with strings, it looses some of its dictionary methods, but does a quite reasonable job in pointing that out in the error message:

>>> mp = MappingProxyType('abc')
>>> mp.items()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'str' object has no attribute 'items'