Apparent false positive when using reduce with dictionaries

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
45/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
python
Domain
devtools

Research direction

Reproduce the report from the test.py example using the stated Python 3.6.3 and mypy 0.521 versions, then trace how functools.reduce is typed for Iterable[dict]. Add regression coverage for the shown reduce_func call; done means the valid dictionary reduction no longer produces the incompatible Callable error.

Written by the indexing model from the issue text.

Description

bug false-positive priority-1-normal

Python 3.6.3, mypy 0.521.

When using reduce on an Iterable/List of dictionaries, mypy seems to be confused about the signature of the function and seems to be unpacking the dictionary into an iterable of tuples. The following example appears to conform to the reduce function's parameters, so no errors should be appearing.

https://docs.python.org/3.6/library/functools.html#functools.reduce

test.py:14: error: Argument 1 to "reduce" has incompatible type Callable[[Dict[Any, Any], Dict[Any, Any]], Dict[Any, Any]]; expected Callable[[Iterable[Tuple[Any, Any]], Dict[Any, Any]], Iterable[Tuple[Any, Any]]]

#!/usr/bin/env python3
# -*- coding: utf-8 -*-

from functools import reduce

from typing import Iterable

def update(ccc: dict, meta: dict) -> dict:
    foo = dict(ccc)
    foo.update(meta)
    return foo

def reduce_func(configs: Iterable[dict]) -> dict:
    output = dict(reduce(update, configs, {})) # type: dict
    return output

print(reduce_func([{"a": 1}, {"b": 2}]))
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