Python uses simple syntax to use or call a preexisting function. A function can be called by writing the name of it, followed by parentheses. For example, the code above would call the doHomework() method.

A developer can create or define his or her own function in Python. To do so, the keyword def is followed by the name of the function, parentheses, and a colon. The body of the function, or the code for what the function will actually do, comes after the colon on an indented line.

Python uses indentation to identify blocks of code. Code within the same block should be indented at the same level. A Python function is one type of code block. All code under a function declaration should be indented to identify it as part of the function. There can be additional indentation within a function to handle other statements such as for and if so long as the lines are not indented less than the first line of the function code.

Sometimes functions require input to provide data for their code. This input is defined using parameters.

Parameters are variables that are defined in the function definition. They are assigned the values which were passed as arguments when the function was called, elsewhere in the code.

For example, the function definition defines parameters for a character, a setting, and a skill, which are used as inputs to write the first sentence of a book.

Python functions can have multiple parameters. Just as you wouldn’t go to school without both a backpack and a pencil case, functions may also need more than one input to carry out their operations.

To define a function with multiple parameters, parameter names are placed in a comma-separated list within the parentheses of the function definition.

Parameters in python are variables— placeholders for the actual values the function needs. When the function is called, these values are passed in as arguments. For example, the arguments passed into the function .sales() are the Farmer’s Market, toothpaste, and $1 which correspond to the parameters grocery_store, item_on_sale, and cost.

Python functions can be defined with named arguments which may have default values provided. The arguments of a Python function are available inside the function as variables. They are also available when the function is called elsewhere in a program. When function arguments are passed using their names, they are referred to as keyword arguments. The use of keyword arguments when calling a function allows the arguments to be passed in any order - NOT just the order that they were defined in the function.

A return keyword is used to return a value from a Python function. The value returned from a function can be assigned to a variable which can then be used in the program. In the example, the function check_leap_year returns a string which indicates if the passed parameter is a leap year or not.

Python functions are able to return multiple values using one return statement. All values that should be returned are listed after the return keyword and are separated by commas.

In the example, the function square_point returns x_squared, y_squared, and z_squared by listing them after the return statement and separating them by commas.

Function parameters behave identically to a function’s local variables. They are initialized with the values passed into the function when it was called.

Like local variables, parameters cannot be referenced from outside the scope of the function.

In the example, the parameter value is defined as part of the definition of my_function, and therefore can only be accessed within my_function. Attempting to print the contents of value from outside the function causes an error.

A variable that is defined outside of a function is called a global variable. It can usually be accessed inside a function. While a global variable can be modified on the outside of a function, it cannot be modified within a function. In the example, the variable a is a global variable because it is defined outside of the function f1. It is therefore accessible to f1, which will print the value of a.

Not all the variables used by a function need to be included in the function definition. For instance, this function uses a variable defined on a previous line to create a greeting customized with a name.

In Python, a variable defined inside a function is called a local variable. It cannot be used outside of the scope of the function, and attempting to do so without defining the variable outside of the function will cause an error. In the example, the variable a is defined both inside and outside of the function. When the function f1 is implemented, a is printed as 2 because it is locally defined to be so. However, when printing a outside of the function, a is printed as 5 because it is implemented outside of the scope of the function.