对于通过包导入的类型和直接从同一模块导入的类型，isinstance 失败

Question

/Project
|-- main.py
|--/lib
|  |--__init__.py
|  |--foo.py
|  |--Types.py

/Project/lib has been added to the PYTHONPATH variables.

Types.py:

class Custom(object):
    def __init__(self):
        a = 1
        b = 2

foo.py:

from Types import Custom
def foo(o):
    assert isinstance(o, Custom)

Finally, from main.py :

from lib.Types import Custom
from lib.foo import foo
a = Custom()
foo(a)

The problem now is, that a is of type lib.foo.Custom , while the isinstance call will check if it equals foo.Custom , which obviously returns false.

How can I avoid this problem, without having to change anything in the library (lib)?

Answer 1

You should not both make lib a package and add it to PYTHONPATH . This makes it possible to import its modules both as lib. and directly, setting yourself up for failure.

As you can see,

lib.Types.Custom != Types.Custom

because of the way Python imports work .

Python searches the import path and parses an appropriate entry that it finds.

• When you import lib.Types , it imports the lib directory as a package, then lib/Types.py as a submodule inside it, creating module objects lib and lib.Types in sys.modules .
• When you import Types , it imports Types.py as a standalone module, creating a module object Types in sys.modules .

So, Types and lib.Types end up as two different module objects. Python doesn't check if they are the same file to keep things simple and to avoid second-guessing you.

(This is actually listed in the Traps for the Unwary in Python's Import System article as the "double import trap".)

---

If you remove lib from PYTHONPATH , the import in lib/foo.py would need to become a relative import:

from .Types import Custom

or an absolute import:

from lib.Types import Custom

Answer 2

When a module is imported thru two different path in the same process - like here with import Types in foo.py and import lib.Types in main.py , it is really imported twice, yielding two distinct module objects, each with it's own distinct functions and class instances (you can check by yourself using id(obj_or_class) ), effectively breaking is and isinstance tests.

The solution here would be to add Project (not Project/lib ) to your pythonpath (fwiw that's what should have been done anyway - pythonpath/sys.path should be a list of directories containing packages and modules, not the packages directories themselves) and use from lib.Type import Custom everywhere, so you only have one single instance of the module.

Answer 3

# For generating a class UUID: uuidgen -n "<MODULE_UUID>" -N <Python class name> -s
# Example: uuidgen -n "dec9b2e9-07c0-4f59-af97-92f171e6fe33" -N Args -s
MODULE_UUID = "dec9b2e9-07c0-4f59-af97-92f171e6fe33"

def get_class_uuid(obj_or_cls):
    if isinstance(obj_or_cls, type):
        # it's a class
        return getattr(obj_or_cls, "CLASS_UUID", None)
    # it's an object
    return getattr(obj_or_cls.__class__, "CLASS_UUID", None)

def same_type(obj, cls):
    return get_class_uuid(obj) == get_class_uuid(cls)

class Foo:
    CLASS_UUID = "340637d8-5cb7-53b1-975e-d3f30bb825cd"

    @staticmethod
    def check_type(obj, accept_none=True):
        if obj is None:
            return accept_none 
        return same_type(obj, Foo)
...

assert Foo.check_type(obj)