Try block handling assumes all statements could have succeeded

Bug Report

When a try block is used to attempt to convert/consolidate the type of a variable, the except block assumes the conversion may or may not have happened. For try blocks with multiple statements, this is the correct behavior for every line but the final one as mypy can't know which of those lines succeeded. However, I would expect mypy to understand that the last/only line can't have executed.

To Reproduce

Minimal example similar to where I encountered the issue:

# file: type_test.py
import sys
from importlib import import_module
from types import ModuleType


def ensure_module(module: str | ModuleType) -> ModuleType:
    if isinstance(module, str):
        try:
            module = import_module(module)
        except ImportError:
            sys.path.append('/path/to/extra/modules')
            # mypy thinks module could be either a string or ModuleType
            module = import_module(module)
    return module

Expected Behavior

mypy shouldn't have any errors:

$ python -m mypy type_test.py
Success: no issues found in 1 source file

Actual Behavior

mypy finds an error in the except block:

$ python -m mypy type_test.py
type_test.py:14: error: Argument 1 to "import_module" has incompatible type "Union[str, Module]"; expected "str"
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: mypy 0.961 (compiled: yes)
• Mypy command-line flags: N/A
• Mypy configuration options from mypy.ini (and other config files): N/A
• Python version used: 3.10.5
• Operating system and version: Arch Linux (5.18.11-zen1-1-zen)

Inizia eseguendo con mypy l’esempio minimo in type_test.py e riproduci l’errore nel blocco except. Analizza il restringimento dei tipi di try/except per l’assegnazione finale, quindi verifica che l’esempio non segnali errori mantenendo il comportamento corretto per più istruzioni.