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Learn to program with Python

Mutable objects as function arguments #

Mutable objects passed as function arguments can be modified within the function. If we pass a mutable object to a function and that function modifies the object, it can have unintended consequences outside the function scope.

For example:

def add_value(dict_to_add, key, value):
    print(id(dict_to_add)) # 139776005167360
    dict_to_add[key] = value
    return dict_to_add

person = {"name": "James"}
print(id(person)) # 139776005167360
new_person = add_value(person, "age", 200)
print(person) # {'name': 'James', 'age': 200}
print(new_person)# {'name': 'James', 'age': 200}

In the example above, we could see that both person and new_person was changed, even though we didn't want to change the value of person. This is because when person is passed to add_value() function, the variable dict_to_add points to the same object as person does. We could see in lines 2 and 7 that they share the same id.

When a function changes something outside of its scope, it causes side effects, which leads to unintended results and make the program more difficult to debug.

Best practice to follow #

When working with mutable objects, such as dicts and lists, try to avoid modifying them in place whenever possible. Instead, create a new object by calling the method dict.copy() or list.copy(), then modify the new object instead. For example:

def add_value(dict_to_add, key, value):
    dict_to_update = dict_to_add.copy() # <-- Create a copy object
    dict_to_update[key] = value
    return dict_to_update

person = {"name": "James"}
new_person = add_value(person, "age", 200)
print(person) # {'name': 'James'}
print(new_person) # {'name': 'James', 'age': 200}

flowchart
    A[dict_to_add] --> Z["{'name': 'James'}"]
    A -. "dict_to_add.copy()" .-> Y["{'name': 'James'}\n ↓\n{'name': 'James', 'age': 200}"]
    B[dict_to_update] --> Y

In the example above, instead of modifying the dict_to_add directly, we make a new copy and assign it to the variable dict_to_update. Now dict_to_add and dict_to_update are two different objects, and the function add_value() no longer causes side-effects.

Exercise #

Update the body of the function add_to_list() so that it does not modify the list outside its scope.

Tests #

We have created a variable my_list to be [1, 2, 3].

  • add_to_list(my_list, 4) should return [1, 2, 3, 4]
  • add_to_list(my_list, 4) should not modify the value of my_list
Output will be displayed here