Type-narrowing does not apply across 2 branches having the same condition within the same block

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#7,928 2 comments 2 reactions 0 assignees View on GitHub

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
42/100
Issue type
Feature
Clarity
Mostly clear
Activity status
Stale
Tech stack
python
Domain
compilers, tooling

Research direction

Start with the a.py reproducer and run mypy with --strict-optional to confirm the current diagnostic. Trace the type narrowing for the two repeated if created: blocks, then add coverage showing that the second use of a is accepted without errors.

Written by the indexing model from the issue text.

Description

feature needs discussion priority-2-low topic-type-narrowing

  • Are you reporting a bug, or opening a feature request?
    Feature request

  • Please insert below the code you are checking with mypy,

from typing import Optional

def my_func() -> bool:
  return True

created: bool = my_func()
a: Optional[int] = None
if created:
  a = 1
  z = a + 1 # This is OK

if created:
  x = a + 1 # Unsupported operand types for + ("None" and "int")
  • What is the actual behavior/output?
$ mypy a.py
a.py:13: error: Unsupported operand types for + ("None" and "int")
a.py:13: note: Left operand is of type "Optional[int]"
Found 1 error in 1 file (checked 1 source file)
  • What is the behavior/output you expect?

No errors. Mypy should be able to narrow the type of a to just int in the second if block, because it narrowed it to that type already the first if block and they have the exact same condition.

You can argue that it's easy to workaround it be merging the if blocks, but in real-life code, there could easily be intervening code that is not trivial to move around.

  • What are the versions of mypy and Python you are using?
    mypy 0.750+dev.5b73e2a5e62d3da723ac517fabacaa2a0c8cdf3c
    Python 3.7.4

  • What are the mypy flags you are using? (For example --strict-optional)
    --strict-optional

Dominant language
Python
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