If it's any consolation, the scope of a variable isn't determined by heuristics, it's just that the rules are (IMO) kinda bad. Basically, if a variable is assigned (in addition to `=` and friends, `import`, `class` and `def` are also assignments in disguise) to in what Python calls a "block" (which is not like a C-style block; rather `class` body, `def` body, or top-level) it is considered to belong to that block (with the notable exception of the name for a caught exception in a `except Exception as e: ...`, see [1]). If you want it to refer to a variable outside the block, you need to use `global` or `nonlocal` as appropriate. The full gory details are in [2]

1: https://docs.python.org/3/reference/compound_stmts.html#the-...

2: https://docs.python.org/3/reference/executionmodel.html#nami...