Function Syntax

Here’s the syntax for writing functions:


Just like all variables, functions have names. We establish a function’s name after the def keyword.


Parameters are what the function takes in. They go inside the parentheses ( ) following the function name, and we must give them types. Parameters are local to the function definition, so they can only be used within the function.

  • located within the () of a function definition
  • refer to variables that the user passes in when the function is called
  • are local to the function definition (can only be used after the : of the function definition)
  • function can have multiple parameters seperated by commas
  • if there are no parameters the parentheses are left empty

Return Type

The return type of a function indicates the type of data the function will return. The function must return a variable or expression that simplifies to the same type as the return type.

  • can be int, float, bool, str, or None
  • function must return an object or expression that simplifies to the same type as the return type
  • procedures do not contain a return statement

Function Body

Everything after the colon in the first line (:) makes up the function body. Statements in the body of the function will only be executed when the function is called.

  • gives ‘functionality’ to the function
  • code will only be executed when the function is called elsewhere in the code
  • if not void, MUST contain a return statement

Return Statement

The return statement is located inside the function body and indicates when the function call

  • type must match the return type of its function
  • every function that returns a value must have at least one return statement
  • as soon as any return statement is reached the function call is complete, even if within an if-then-else block!

Example function:

We can see the parts of the function in the following way:

Parameters: Person1 and Person2 (both strings)

Return type: string

Function body: couple: str = “Congratulations” + person1 + " " + person2

Return statement: return couple;

Although in the example above, we only have one type in our parameters, you can have multiple

For example, we could modify the matchMaker function in the following way:

If we wanted to use the function, we would just call it using the function name followed by ( ) where we would include arguments to match the parameters the function requires.

Arguments are what we pass into parameters. For example, calling matchMaker as it appears in the example above:

Arguments don’t have to be literals though, we can also (and much more commonly) pass variables into functions.

For example using the matchMaker function defined above:

A helpful way to think of a function is to compare it to a recipe:

  • parameters are the ingredients we need (can’t make the recipe without the correct quantity and type of ingredients!)
  • the function body is the instructions
  • the return type would be whatever we want to make!

The Main Function

The main function, by convention, is the function that starts your program! It is formatted similarly to any other function definition (see below), and is used to call the other functions in the program! You can think of calling main as being similar to turning your key in the ignition - it is what starts the engine of your program to get things running!

The main function does look a little different than the functions we are used to writing, but don’t overthink it - it works essentially the same! We have a function definition and function body just as outlined above, however we are lacking parameters and a return statement. This is ok! When main() is called, the code in the function body executes, and then thats it! The only difference is that no value is returned.

Below is an example of how the main function can be used:

Arguments vs Parameters

Extra pieces of information required by the function definition are called parameters. These are like empty variables.

The values provided in the function call are called arguments. These are like the variables’ values.

Here is an example of how we could call an arbitrary function f.

In the example above using function f - Parameters: a: str, b: int, c: bool - located in the function definition! - Arguments: “aardvark”, 57, false - located in the function call! - “aardvark” matches with a, 57 with b, false with c

Arguments and parameters must line up via type and purpose 1:1…Remember, functions are like recipes that require specific ‘ingredients’ (aka arguments) in to produce a final result. If you don’t give it the correct ingredients (or not enough/too many ingredients) you will get an error!

Examples of an incorrect call of f:

Calling Functions

If a function is like a written recipe, a function call is like opening the recipe book to follow the recipe.

  • Function calls in Python have parentheses () after the function’s name.
  • Arguments are the values provided in the function call. They are found within the parentheses.
  • Arguments get matched one-to-one with parameters. This means that each argument must be the same type as its corresponding parameter. Read more on the difference between arguments and parameters above.

Here is an example function:

The parameters are required in the order they are specified, and all parameters must be given an argument. Examples of incorrect calls to myFunc:

Important notes: - Function calls are expressions! They evaluate to a single value. - Just as (3+1) * 2 simplifies to 8 (PEMDAS), myFunc(“Hola”, 1000) simplifies to 1000! - Other actions may also be taken in the function body so watch out - but ultimately the original function call can be replaced/simplified to the lone return value. - Parameters are usually used within the function– but they aren’t required to be. (In the myFunc function, parameter b is used, but parameter a is not. Though this isn’t generally useful, it is allowed!)

Function call tracing example:

  1. The call to main starts the program and we jump into the main function.

  2. The code inside the main function definition begins executing.

  3. initial is declared and assigned the value of 5.

  4. increment is then assigned the value of the function call addOne(initial). This is valid because function calls are also expressions and can evaluate to a single value. However, the return type of the function must match the type of the variable.

  5. We then jump into the addOne function. The parameter, n, takes the value of the argument passed to the function, initial.

  6. The value of the expression n + 1 is returned. In this case, it’s 6.

  7. The result of the call addOne(initial) is stored in increment.

  8. The value of increment is printed out and the program is done executing!

Functions with non-None Return Types

In functions that have non-None return types (dont worry, we’ll cover “None” later), a value is returned by the function when we call it. We know a function has a non-void return type by looking for a return type in the first line of the function definition or a return statement in the body of the function. Here’s an example:

In our function declaration after the parentheses and the arrow we see the word int, which is the return type.

Another clue that this is a non-None function is that in the body, the statement return 110; means that this function will return the integer value 110. None functions will not include a return statement.

Key points about return

  • When a function is evaluated, once the computer hits a return statement, execution of the function ends and the specified value is returned. Any code within the function after the return statement will not be evaluated.
  • Only one return statement is evaluated during any one function call.
  • The return type declared in the first line of the function must be the same as the data type of what is actually being returned from within the body of the function.

Examples of non-void functions and their return statements used incorrectly:

How it Works

When we call a function, the computer drops a bookmark there and jumps into the definition of the function it called. It does whatever we tell it in the function body and reaches the return statement. Once the computer hits a return it takes the value from the return statement and goes back, or returns, to the place of the bookmark. Here, it substitutes the function call with the value it brought back! Seeing this in action with an example will help it make more sense:

We’ve got a lot going on here! This little program starts out with the import statement and the main function.

After that, we see the definition for the makeCourse function. In this function, we declare the return type to be str. Within the function body, we see the statement return course. The type of what we’re returning (the value stored in course) matches the return type we specified since course is the result of concatenating name and level so its type is also string.

Note: until a function is called (even main()), we only need to acknowledge the function definitions…we dont really care about the function body until a function is called and that code is actually executed!

When this program runs, the computer sees the definition of the main function first, and then it sees the definition of the makeCourse function.

Next, the computer calls main(). This sends it into main’s function body, which includes a call to the makeCourse function using arguments “COMP” and 110. At this function call, the computer drops a bookmark and heads on to the makeCourse function.

There, the computer uses the arguments to make the string for the course variable, which it then returns. The computer brings the value of course, which is “COMP110”, back with it into main where the bookmark was left, and drops this value there. So, myFavCourse is initialized with “COMP110”. The last step in the program is to use this string in the print statement, then we’re done!


Functions that return are great for when we need to go through a lot of steps to get a value that we want to use in our code somewhere. Calling a non-void function tells the computer to head over to the function definition, do everything, and come back with the result once it’s done. This is nice if we’re doing complicated math or trying to combine a bunch of data in some way. We won’t have to worry about all the little steps each time we need the value. Instead, we can tell the computer to go do the hard work for us and come back with the result, which we can then use as we keep going in our program.


Sometimes a function returns nothing, called procedures! Procedures are super helpful when we need to modify variables or objects in our program, especially when we may need to make these same changes over and over again. Procedures allow us to make use of code that may not produce a return value but is still nonetheless pivotal to our program’s execution.

Let’s take a look at an example:

The resulting output to the screen will be:

hello hello

world world world

If we didn’t make use of this procedure, we would’ve had to rewrite the same print statement 2 or 3 times. This example highlights how void functions reduce redundancy in our code!

Our main function is also a procedure, although unlike other procedure we only call main once.