Python import variables from module by value (deep copy) , not by reference

Question

In a directory I have the file dummy_module.py with contents

a = [1]

and

the file main.py with contents

from dummy_module import a
print(f"a = {a}, after first import")
a.append(3)
print(f"a = {a}, after appending an element to list 'a'.")
from dummy_module import a
print(f"a = {a}, after importing 'a' again from 'dummy_module'.")  # NO! I want this to return 'a = 1' again!

This returns

a = [1], after first import
a = [1, 3], after appending an element to list 'a'.
a = [1], after importing 'a' again from 'dummy_module'.

I noticed that the variable a is, by means of import apparently copied by reference instead of by value because if I modify a after the first import it also changes a in dummy_module.

I want to import a by value (deep copy) and not by reference (shallow copy). I could solve this by placing a = deepcopy(a) before the a.append(3), but this becomes cumbersome in applications in which I import multiple variables from a module, so I want to avoid such an approach.

How do I solve this?

Answer 1

In Python, the terms “pass by value” and “pass by reference” are not used because nothing is ever copied when an assignment is made.

Some types of objects have the same behavior as if it were "passed by value" when reassigned because they are immutable, meaning that a reassignment of a name is done by creating a completely new object (leaving the old one unmodified).

What is happening in your case?

An import statement just executes the module code and make some name binding, however the 2nd time you run the import statement, the code a = [1] statement do not get executed again because it has been cached by the module cache.

I could solve this by placing a = deepcopy(a) before the 'a.append(3)'

This works because deepcopy would make a new list object before binding a to it, leaving the previous object unmodified (and cached by the module cache). You would need to do something similar with each name imported.

Quick Hack

All the name from the module can be reinitialized again by making a quick deletion on the module cache before the 2nd import is done (THIS COULD LEAD TO BUGGY CODE). This forces the the import statement to re-execute the module code before making the bindings:

import sys
del sys.modules["dummy_module"]
from dummy_module import a

NOTE: As "user2357112 supports Monica" says, this hack could lead to a number of issues, my purpose is just to further explain what is happening and providing a quick and dirty hack to play with.