Python: What happens when I modify imported class?

Question

I have written the following code to modify the behavior of a method of one class

import mymodule
mymodule.MyClass.f = mydecorator(mymodule.MyClass.f)
mymodule.MyClass.f(x) # call the modified function

This works for my purposes, but: what have I modified exactly? Is mymodule.MyClass a copy of the original class living inside the current module? Does it in any way affect the original class? How does import work exactly?

Answer 1

When you modify imported module, you modify the cached instance. Thus your changes will affect all other modules, which import the modified module.

https://docs.python.org/3/reference/import.html#the-module-cache

UPDATE:

You can test it.

change_sys.py:

import sys

# Let's change a module
sys.t = 3

main.py:

# the order of imported modules doesn't meter
# they both use cached sys
import sys
import change_sys

print(sys.t)

Output for python ./main.py :

3

Answer 2

It depends. In normal uses cases everything should be ok. But one can imagine special cases where it can lead to weird results:

a.py:

import c

x = c.C()

def disp():
    return x.foo()

b.py:

import c

def change():
    c.C.foo = (lambda self: "bar at " + str(self))

c.py:

class C:
    def foo(self):
        return "foo at " + str(self)

Now in top level script (or interactive interpretor) I write:

import a
import b
a.disp()
b.change()
a.disp()

Output will be:

'foo at <c.C object at 0x0000013E4A65D080>'
'bar at <c.C object at 0x0000013E4A65D080>'

It may be what you want, but the change has been done in b module and it does affect a module.