6.1 User-Defined Functions

Functions (General)

• A program may perform the same operation repeatedly, causing a large and confusing program due to redundancy.
  • Program redundancy can be reduced by creating a grouping of predefined statements for repeatedly used operations, known as a function.
  • Even without redundancy, functions can prevent a main program from becoming large and confusing.

Function basics

• A function is a named series of statements.
• A function definition consists of the new function's name and a block of statements. Ex: def calc_pizza_area(): An indented block of statements follows the definition.
• A function call is an invocation of the function's name, causing the function's statements to execute.
  • Python comes with a number of built-in functions, such as input(), int(), len(), etc. The def keyword is used to create new functions.
• Good practice is to follow the convention of naming functions with lowercase letters and underscores, such as get_name or calc_area.
• The function call calc_pizza_area() in the animation below causes execution to jump to the function's statements. Execution returns to the original location after executing the function's last statement.

Return Statements

• A function can only return one item, not two or more (though a list or a tuple with multiple elements could be returned)
• A function with no return statement, or a return statement with no following expression, returns the value None.
  • None is a special keyword that indicates no value.
  • A function may also contain multiple return statements in different locations.

Parameters

• A parameter is a function input specified in a function definition. Ex: A pizza area function might have diameter as an input.
• An argument is a value provided to a function's parameter during a function call. Ex: A pizza area function might be called as calc_pizza_area(12.0) or as calc_pizza_area(16.0)
  • A parameter is like a variable definition.
  • A parameter cannot be an expression:
    • NOT THIS: def my_fct(userNum + 5):

Multiple or no parameters

• A function may have multiple parameters, which are separated by commas.
• A function definition with no parameters must still have the parentheses, as in: def calc_something():
  • The call to such a function must include parentheses, and they must be empty, as in: calc_something()

Hierarchical function calls

• A function's statements may include function calls, known as hierarchical function calls or nested function calls.