4.15 Passing Arguments to Functions: A Deeper Look

• Python arguments are always passed by reference.
• Some people call this pass-by-object-reference, because
• “everything in Python is an object.”
• When a function call provides an argument, Python copies the argument object’s reference—not the object itself—into the corresponding parameter.

Memory Addresses, References and “Pointers”

• After an assignment like the following, the variable x contains a reference to an object containing 7 stored elsewhere in memory.

Built-In Function id and Object Identities

• Every object has a unique identity—an int value which identifies only that object while it remains in memory.
• Built-in id function to obtain an object's identity.

x = 7

id(x)

4311725472

Passing an Object to a Function

def cube(number):
    print('id(number):', id(number))
    return number ** 3

cube(x)

id(number): 4311725472

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• The identity displayed for cube’s parameter number is the same as that displayed for x previously.
• The argument x and the parameter number refer to the same object while cube executes.

Testing Object Identities with the is Operator

• The is operator returns True if its two operands have the same identity:

def cube(number):
    print('number is x:', number is x)  # x is a global variable
    return number ** 3

cube(x)

number is x: True

343

Immutable Objects as Arguments

• When a function receives as an argument a reference to an immutable (unmodifiable) object, even though you have direct access to the original object in the caller, you cannot modify the original immutable object’s value.

def cube(number):
    print('id(number) before modifying number:', id(number))
    number **= 3
    print('id(number) after modifying number:', id(number))
    return number

cube(x)

id(number) before modifying number: 4311725472
id(number) after modifying number: 4348973456

343

print(f'x = {x}; id(x) = {id(x)}')

x = 7; id(x) = 4311725472

Mutable Objects as Arguments

• We’ll show that when a reference to a mutable object like a list is passed to a function, the function can modify the original object in the caller.

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©1992–2020 by Pearson Education, Inc. All Rights Reserved. This content is based on Chapter 4 of the book Intro to Python for Computer Science and Data Science: Learning to Program with AI, Big Data and the Cloud.

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