You must be aware when the Python interpreter evaluates each block of code. The most important thing in interpreter evaluation is "import time" and "runtime".

Import time: At import time, the interpreter goes through the .py module in one pass from top to bottom and generates bytecode, alerting about any syntax errors. If up-to-date .pyc files exist in local __pycache__, those are used instead.

• All top-level code is ran at import time.
• All top-level code of import chain is ran at import time.
• Function bodies are not ran at import time.
• Function decorators are ran at import time.
• Class bodies are ran at import time, even nested classes.
• Class decorators are ran after the wrapped class body.

Source

• Fluent Python, Luciano Ramalho