Raymond Hettinger added the comment:
> I'm aware of duck typing but I don't think this
> is the right place for it.
The code for dicts is very old, stable, and unlikely to change. Also, the
logic of checking for .keys() is immortalized in the
collections.abc.MutableMapping update() method.
For the most part, consumers of iterables, sequences, and mappings are allowed
to use duct-typing (this is a feature of the language, not a bug).
The docs can be improved in a number of places. For example the docstring on
the dict constructor is out of sync with the dict.update() method:
>>> print(dict.__doc__)
dict() -> new empty dictionary
dict(mapping) -> new dictionary initialized from a mapping object's
(key, value) pairs
dict(iterable) -> new dictionary initialized as if via:
d = {}
for k, v in iterable:
d[k] = v
dict(**kwargs) -> new dictionary initialized with the name=value pairs
in the keyword argument list. For example: dict(one=1, two=2)
>>> print(dict.update.__doc__)
D.update([E, ]**F) -> None. Update D from dict/iterable E and F.
If E is present and has a .keys() method, then does: for k in E: D[k] =
E[k]
If E is present and lacks a .keys() method, then does: for k, v in E: D[k]
= v
In either case, this is followed by: for k in F: D[k] = F[k]
In addition, the glossary entries for iterable, sequence, and mapping need to
be improved to distinguish between their somewhat vague use in a general sense
versus the specific meaning of isinstance(obj, Mapping). Unless the docs
specify a check for the latter, they almost always do some form of duck-typing
or a check for concrete built-in class or subclass.
Terms like "mapping" and "sequence" are often used somewhat generally both
inside and outside the Python world. Sometimes mapping is used in the
mathematic sense (pairing each member of the domain with each member of the
range), http://www.icoachmath.com/math_dictionary/mapping.html, and sometimes
in the sense of a subset of dict capabilities (i.e. has __getitem__ and keys).
The docs for PyMapping_Check() need to be updated to indicate the known
limitations of the check and to disambiguate it from isinstance(obj, Mapping).
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<http://bugs.python.org/issue24659>
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