Python Functions

What Are Functions?

A function is a reusable block of code that performs a specific task.

Functions help you:

• Avoid repetition by writing code once and using it many times
• Organize programs into smaller sections
• Make code easier to debug because each function has a focused job
• Improve readability by giving tasks clear names

Defining and Calling Functions

In Python, functions are created using the def keyword.

A function does not run automatically. You must call the function by writing its name followed by parentheses.

• def starts the function definition
• Function name describes what the function does
• Parentheses can hold parameters
• Indented code belongs to the function body

Parameters and Arguments

A parameter is a variable listed in the function definition.

An argument is the actual value passed into the function when it is called.

You can:

• Create one parameter for one input value
• Create multiple parameters for multiple input values
• Reuse the same function with different arguments

Return Values

The return keyword sends a value back from a function.

This is different from print(). Printing only displays a value, while returning lets you store and reuse it.

You can:

• Return numbers, strings, booleans, lists, dictionaries, or other values
• Store returned values in variables
• Use returned values in calculations or other function calls

Print vs Return

print() displays information to the screen. return gives a value back to the program.

Use return when you need the result later.

Default Parameters

A default parameter gives a parameter a backup value.

If the function is called without an argument, Python uses the default value.

You can:

• Make arguments optional
• Provide fallback values
• Still override defaults by passing an argument

Keyword Arguments

Keyword arguments let you pass values using parameter names.

This makes function calls easier to understand, especially when there are multiple parameters.

You can:

• Pass arguments out of order
• Make function calls more readable
• Avoid confusion when parameters have similar data types

*args

*args lets a function accept multiple positional arguments.

Python stores these extra arguments inside a tuple.

You can:

• Accept an unknown number of values
• Loop through the values
• Use it for flexible calculations

**kwargs

**kwargs lets a function accept multiple keyword arguments.

Python stores these arguments inside a dictionary.

You can:

• Accept flexible named values
• Work with dictionary-style data
• Create reusable functions for profiles, settings, or records

Scope

Scope describes where a variable can be used.

A variable created inside a function is called a local variable. A variable created outside a function is called a global variable.

You can:

• Use local variables only inside the function
• Use global variables outside functions
• Avoid naming conflicts by understanding scope

Practice: Grade Average Function

Prompt: Create a function called calculate_average that takes a list of grades and returns the average. Then, use the function to print each student's name and average grade.

Target Output:

Alice: 87.5
Bob: 91.5

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