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

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'

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'