With v1.4 module-global variables become fixed in type.

This seems to be a regression with v1.4 of mypy.

It looks to me like globals are considered constant if they’re not explicitly typed. For example (see issue https://github.com/Bogdanp/dramatiq/issues/555) a module-global variable like

global_broker = None

seems to be typed now as None. Consequently reading that variable and then asserting its value is always false and therefore raises an exception

broker = some.module.global_broker
assert broker is not None  # Previously this worked; now it's considered None and therefore raises.

and that means that the next statement

broker.enqueue(message, delay=delay)

becomes unreachable.

I therefore changed the module-global variable declaration to

global_broker: Optional["Broker"] = None

which solved the problem. However, that’s in an external module and therefore my own code is now broken after updating to v1.4 of mypy.

Would it make sense to consider the type for such a module-global variable as Any | None (or just Any) instead of just None? (Assuming that a module-global None variable will change.)

To Reproduce

See above.

Expected Behavior

As with v1.3 I would have expected mypy to pass the type check.

Actual Behavior

With v1.4 mypy now flags the statement following the assert as unreachable.

Your Environment

• Mypy version used: mypy 1.4.1
• Mypy configuration options from mypy.ini (and other config files):

   [tool.mypy]
   show_error_codes = true
   show_column_numbers = true
   check_untyped_defs = true
   incremental = false
   strict_equality = true
   warn_return_any = true
   warn_redundant_casts = true
   warn_unreachable = true
   warn_unused_configs = true
   warn_unused_ignores = true
   disallow_untyped_calls = true
   disallow_untyped_defs = true
   disallow_incomplete_defs = true
   disallow_untyped_decorators = true
   disable_error_code = []
   plugins = "sqlalchemy.ext.mypy.plugin"
  

• Python version used: 3.10.12

Start with the minimal module-global reproducer in the issue and compare mypy 1.3 with 1.4.1 using the listed configuration, especially warn_unreachable and strict_equality. Done means the behavior for an unannotated global initialized to None is decided and covered by a regression test, including the assert and subsequent method call.