Python 的 importlib 并检查静态类成员

Question

In a long-running app I need to dynamically modify static class members based on path to the class' module and the class name.

Ex. I have a class pack1.mod1.Person and by definition I know it has a age property. So utilizing the importlib and inspect I try to load the class using the module path and class name and update the age property. It all seems fine until I read the the age property from my naturally imported Person class and find it's not updated.

Here are some more details:

.
├── app.py
└── pack1
    ├── __init__.py
    └── mod1.py

mod1.py

class Person:
    age = 42

app.py

import inspect
import os
from importlib import util

from pack1.mod1 import Person

if __name__ == '__main__':
    Person.age = 3
    print(Person.age)  # => 3

    spec = util.spec_from_file_location('pack1.mod1', os.path.join('pack1', 'mod1.py'))
    module = util.module_from_spec(spec)
    spec.loader.exec_module(module)

    members = inspect.getmembers(module)

    for x, member in inspect.getmembers(module, lambda i: inspect.isclass(i) and i.__name__ == Person.__name__):
        print('Person:', Person.age)  # => Person: 3
        print('Person from inspect:', member.age)  # => Person from inspect: 42
        Person.age = 11
        member.age = 66
        print('Person:', Person.age)  # => Person: 11
        print('Person from inspect:', member.age)  # => Person from inspect: 66

In the app.py I would expect member and Person to be the same thing but as the example shows they aren't.

What am I missing and how to achieve such an update on the static members of a class?

Answer 1

Python has no way of knowing that the regularly imported module and the manually module are "the same": Using util.spec_from_file_location up to spec.loader.exec_module side-steps Python's module registry and explicitly creates a new instance of the module.
Instead, use the native operations of the interpreter ( import , ...) or their programmatic equivalents ( importlib.load_module , ...)

If the module/class are well-known, one can import it regularly and directly inspect it.

import pack1.mod1
pack1.mod1.Person.age = 66

If module and class are only known by name, one can look them up from the existing modules.

import importlib

module_name, qualname, attribute, value = 'pack1.mod1', 'Person', 'age', 66

obj = importlib.import_module(module_name)  # same as `import {module_name}`
for part in qualname.split('.'):
    obj = getattr(obj, part)                # same as `{obj}.{part}

setattr(obj, attribute, value)              # same as `{obj}.{attribute} = {value}`