Functions in Python

Functions

• A function is a group of related statements performing a specific task.
• Functions break programs into smaller, manageable parts.
• They enhance code organization and reusability, avoiding repetition.

Function Example

• $f(x) = 15x^2 + 4x + 2$
• $f(3)$ returns 149.
• f: Function name.
• Task: Solves the expression.
• Reusable: Evaluates the expression each time it's called.
• Actual parameter: The value passed to the function (e.g., 3).
• Formal parameter: The parameter that receives the actual parameter (e.g., x).
• Return value: The result of the function (e.g., 149), with a specific data type (e.g., int).

Types of Functions

• Built-in functions: Pre-defined functions in Python.
• User-defined functions: Functions created by users.

Python Built-in Functions

• Always available for use.
• Examples:
  • print(): Prints to the screen.
  • input(): Reads a line of string.
  • len(): Returns the length of an object.
  • pow(x,y): Returns $x$ to the power of $y$.

User-defined Functions

• Defined by the users.
• Examples:
  • isEven(): Checks if a number is even.
  • greatestOfThree(): Finds the greatest of three numbers.

Syntax of a Python Function

def function_name(parameters):
    """docstring"""
    statement(s)
    return [expression_list]

• def: Keyword marking the start of the function header.
• function_name: Uniquely identifies the function.
• parameters: Values passed to the function (optional).
• :: Marks the end of the function header.
• docstring: Optional documentation string describing the function.
• Statements: Python statements forming the function body (same indentation level).
• return: Optional statement to return a value from the function.

Function Call

• Call a function from:
  • Another function.
  • The program.
  • The Python prompt.
• To call: type the function name with appropriate parameters.
• Example: greet(“Amna”)

Return Statement

• return [expression_list]
• A function will always return a value.
• Can contain an expression to be evaluated and returned.
• If no expression or return statement, the function returns None.
• Exits the function and returns to the caller.

Scope and Lifetime of Variables

• Scope: Portion of the program where the variable is recognized.
  • Variables defined inside a function are not visible outside (local scope).
• Lifetime: Duration the variable exists in memory.
  • Variables inside a function exist as long as the function executes.
  • Variables are destroyed after the function returns.
  • Functions do not remember variable values from previous calls.

Function Arguments

• A function can have multiple arguments.
• Example:

def greet(name, msg):
    print("Hello", name + ', ' + msg)

• name & msg are the formal arguments.