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Reference vs. Value

When you pass a value into a function as an argument, one of two things can happen:

It's passed by reference: The function has access to the original value and can change it.
It's passed by value: The function only has access to a copy. Changes to the copy within the function don't affect the original.

There is more nuance to it, but this explanation works for an introduction. In Python, the following types are passed by reference:

Lists
Dictionaries
Sets

These types, on the other hand, are passed by value:

Integers
Floats
Strings
Booleans
Tuples

Most container types are passed by reference (except for tuples!), and most basic types are passed by value.

Example of Pass-By-Reference

Lists are passed by reference and are mutable:

def modify_list(inner_lst: list[int]) -> None:
    inner_lst.append(4)
    # the original "outer_lst" is updated
    # because inner_lst is a reference to the original


outer_lst: list[int] = [1, 2, 3]
modify_list(outer_lst)
# outer_lst = [1, 2, 3, 4]

Example of Pass-By-Value

Integers are passed by value; they can be copied freely but are immutable:

def attempt_to_modify(inner_num: int) -> None:
    inner_num += 1
    # the original "outer_num" is not updated
    # because inner_num is a copy of the original


outer_num: int = 1
attempt_to_modify(outer_num)
# outer_num = 1

Assignment

We have a way for Doc2Doc users to set their supported formats in their settings. In memory, we store those settings as a simple dictionary:

settings: dict[str, bool] = {"docx": True, "pdf": True, "txt": False}

Unfortunately, there's a bug in our code. When a new format is added or removed, it not only updates the new dictionary, but it changes the defaults themselves! That's not good. We want to create a new dictionary with the updates, not change the original.

Fix the bug by making add_format and remove_format pure functions that don't mutate their inputs.

Simply assigning a new variable to an existing dictionary doesn't copy that dictionary; it points to the same dictionary. Instead, use the .copy() method to create a new copy of a dictionary.