python / python/mypy

In a function call, if a variable accessed in an iteratable doesn't have an attribute, mypy picks the first overload available, creating spurious further list-item errors.

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Description

Bug Report

If a variable accessed in an iteratable doesn't have an attribute ([attr-defined] error), and that iterable is then passed to a function, mypy seems to pick the first overload available to continue typechecking against that function, creating spurious further list-item errors, even when more information is available to deduce the correct type. The code below should make more clear what I mean.

To Reproduce
mypy-play gist

from typing import Any, Iterable, overload

d = 42 #example value without attributes

@overload
def foo(x: Iterable[str]) -> None: ...
@overload
def foo(x: Iterable[int]) -> None: ...

def foo(x: Any) -> None:
  return None

# Pass the checker, correctly
foo(["ok", "ok"])
foo([1, 1])

# Fail the checker, correctly, with [list-item] errors (although they all assume the str override is the authoritative one)
foo(["ok", 1])
foo([1, "ok"])
foo([None, "ok"])

# Fail the checker, but the int ones generate spurious list-item errors.
foo(["ok", d.x])
foo([2, d.x])
foo([d.x, "ok"])
foo([d.x, 2])

# This doesn't generate an [attr-defined] error, but still produces a spurious list-item error!
foo([d.x, 2]) #type: ignore[attr-defined]

Expected Behavior

If a function is declared to take an Iterable, with multiple types of Iterable specified by overload decorators, then an [attr-defined] error inside the iterable the function is called upon should not prevent mypy from deducing the correct type of that iterable from its other contents. The correct type of the iterable should still be deduced.

Actual Behavior

However, what actually happens is that if an [attr-defined] error occurs inside of the list I am passing to the function, mypy just choses the first overload. In this case, Iterable[str]. Therefore, spurious [list-item] errors are created when the other member of the list is an int. The [list-item] errors in the below output for lines 24, 26, and 29 are spurious:

test.py:18: error: List item 1 has incompatible type "int"; expected "str"  [list-item]
test.py:19: error: List item 0 has incompatible type "int"; expected "str"  [list-item]
test.py:20: error: List item 0 has incompatible type "None"; expected "str"  [list-item]
test.py:23: error: "int" has no attribute "x"  [attr-defined]
test.py:24: error: List item 0 has incompatible type "int"; expected "str"  [list-item]
test.py:24: error: "int" has no attribute "x"  [attr-defined]
test.py:25: error: "int" has no attribute "x"  [attr-defined]
test.py:26: error: "int" has no attribute "x"  [attr-defined]
test.py:26: error: List item 1 has incompatible type "int"; expected "str"  [list-item]
test.py:29: error: List item 1 has incompatible type "int"; expected "str"  [list-item]
test.py:29: note: Error code "list-item" not covered by "type: ignore" comment
Found 10 errors in 1 file (checked 1 source file)

Here is a --pretty listing of the relevant errors:

test.py:24: error: List item 0 has incompatible type "int"; expected "str"  [list-item]
    foo([2, d.x])
         ^
test.py:24: error: "int" has no attribute "x"  [attr-defined]
    foo([2, d.x])
            ^~~
test.py:26: error: "int" has no attribute "x"  [attr-defined]
    foo([d.x, 2])
         ^~~
test.py:26: error: List item 1 has incompatible type "int"; expected "str"  [list-item]
    foo([d.x, 2])
              ^
test.py:29: error: List item 1 has incompatible type "int"; expected "str"  [list-item]
    foo([d.x, 2]) #type: ignore[attr-defined]
test.py:29: note: Error code "list-item" not covered by "type: ignore" comment

29 is especially annoying, because the [attr-defined] ignore does not suppress all warnings that indirectly result from the [attr-defined] error!

You can also switch the order of the overloads in the code to see that mypy now complains that its arguments are str when it needs ints, instead of vice-versa.

Your Environment

  • Mypy version used: 1.9.0
  • Mypy command-line flags: none
  • Mypy configuration options from mypy.ini (and other config files): n/a
  • Python version used: Python 3.11.0 and 3.12

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start by reproducing the issue from the inline example or linked mypy-play gist with mypy 1.9.0, including the overload order and the attr-defined ignore case. Trace overload selection and iterable element inference, then add regression coverage showing that known list items determine the correct overload without spurious list-item errors while the attr-defined error remains.

Written by the indexing model from the issue text.

Assessment

Tech stack
python
Domain
compilers, devtools
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Clearly specified
Newbie friendliness
52/100

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