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Mapping and MutableMapping use the same example argument in docs #7617

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MarcoGorelli opened this issue Oct 3, 2019 · 2 comments · Fixed by #7620
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Mapping and MutableMapping use the same example argument in docs #7617

MarcoGorelli opened this issue Oct 3, 2019 · 2 comments · Fixed by #7620
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@MarcoGorelli
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The cheat sheet from the docs shows the following:

# Mapping describes a dict-like object (with "__getitem__") that we won't
# mutate, and MutableMapping one (with "__setitem__") that we might
def f(my_dict: Mapping[int, str]) -> List[int]:
    return list(my_dict.keys())

f({3: 'yes', 4: 'no'})

def f(my_mapping: MutableMapping[int, str]) -> Set[str]:
    my_mapping[5] = 'maybe'
    return set(my_mapping.values())

f({3: 'yes', 4: 'no'})

In both cases, the argument passed to f is a dict, though in one case the argument is annotated as Mapping and in the other MutableMapping.

From this example, it might not be clear to a beginner when to use Mapping over MutableMapping.
@msullivan
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Good point! Would you like to submit a PR improving it?

@MarcoGorelli
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Sure! I'll get to work

@MarcoGorelli MarcoGorelli mentioned this issue Oct 4, 2019
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Add example of non-mutable mapping to cheat sheet MarcoGorelli/mypy
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@msullivan @MarcoGorelli