False positive `unreachable` when variable is set and reset inside loop

[adebrecht661]

Bug Report

mypy warns about the right side of a condition being unreachable when a variable is reset in an outer loop, then set again inside the inner loop.

To Reproduce

from typing import Optional

test: Optional[float] = None
reveal_type(test) # Revealed type: Union[builtins.float, None]

for i in range(10):
 test = None
 reveal_type(test) # Revealed type: None (???)
 for k in range(10):
 if test is None or k > test:
 test = k

Expected Behavior

No error.

Actual Behavior

mypy outputs error: Right operand of "or" is never evaluated. This is clearly because the type of test is re-evaluated at some point, even though it was declared with a type and no statement narrows that type.

This appears related to #7204, #8721, #8865, #13973, #14120, except it can't be solved by declaring the type of the variable before the loop in which it's updated (because that violates no-redef).

Note: I'm not sure why anyone would want to actually do this in practice. But I thought it might provide a useful test case.

Your Environment

• Mypy version used: 1.1.1
• Mypy command-line flags: --disable-error-code name-defined, plus a few others that I don't believe could possibly have any relevance.
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.10.10

[Melebius]

Similar problem here while using mypy==1.3.0.

My example code:

data = """
{foo}
bar >
baz
"""

prompt = None
reponse = None
lines = data.splitlines()
for line in lines:
 if prompt and reponse: # mypy error: Right operand of "and" is never evaluated [unreachable]
 print("Break reached!") # mypy error: Statement is unreachable [unreachable]
 break
 if not prompt and line.endswith(">"):
 prompt = line
 continue
 if not reponse and line.endswith("}"):
 reponse = line
 continue

Run output:

$ python example.py
Break reached!
$

[patrickelectric]

Here looking at this issue after my PR fails on CI.

[grihabor]

+1

Here is a smaller example:

state = None
for x in 'ab':
 if state is not None:
 print('reachable')
 state = 'x'

The script output:

reachable

Mypy error:

main.py:4:9: error: Statement is unreachable [unreachable]
 print('reachable')
 ^~~~~~~~~~~~~~~~~~
Found 1 error in 1 file (checked 1 source file)

✕ mypy failed.

[A5rocks]

@grihabor and @Melebius your issues were fixed by #18433, though the initial example was not.

[grihabor]

Nice, thanks for the ping

[tyralla]

I tend to think the problem underlying the initial example has nothing to do with loops but with promotions, because both float and int are involved.

So, maybe this is a simpler repro:

x: float | None
y: int | None
z: int
reveal_type(x) # float | None
x = y
reveal_type(x) # None !!!!!!!!!
x = z
reveal_type(x) # int

With "normal" subtypes, the same type of type narrowing works:

class A: ...
class B(A): ...

a: A | None
b: B | None
c: B
reveal_type(a) # A | None
a = b
reveal_type(a) # B | None
a = c
reveal_type(a) # B

Likely, an attempt to solve this could start here:

https://github.com/python/mypy/blob/5a0fa556be741270e25c95a923ef8450d28dd448/mypy/meet.py#L130C1-L145C10