# 5.3. Defining Procedures - How¶

How does Python know what to do when we call functions like abs or procedures like alex.forward(50)? Someone had to define them. Defining a new procedures or function, associates a name with a name with a sequence of steps.

In Python, we define a new procedure or function with the keyword def. To use def, we also need to specify: a name for the procedure, a list of its inputs, and the instructions the procedure will perform in this format:

1def ProcedureName(Input1, Input2, Input3, ...):
2    Instruction1
3    Instruction2
4    ...


Key things to pay attention to:

• The procedure name should obey the same rules as other names. Both syntax rules (no spaces or odd characters) and style conventions (camelCase or snake_case). Like other names, it should be meaningful - it should describe what task it performs.

• The inputs (called parameters) are always inside ()

• There may be multiple inputs - if there are, they are separated by commas. There may also be just one input or even no inputs.

• There is a : after the input list.

• The instructions that are part of the procedure or function are indented. These instructions are known as the function’s body.

Here is a definition for the square procedure we tried to use on the last page. The square procedure takes one input (called turtleName). It has 8 instructions in its body. Try running this code sample:

If you are wondering why the Run button didn’t seem to do anything, all that the program did was define the procedure square which takes a turtleName as input. The code never actually told Python to actually do the square procedure!

Warning

Remember - defining a procedure or function tells Python HOW to do a particular job. It does NOT tell Python to actually do the job. To do the procdeure or function, we must call the procedure.

We call a procedure by using its name, and then giving it the right number of inputs. Our square function requires one input - the name of the turtle that should make a square - so when we call sqaure we must provide the name of a turtle.

This code sample defines our function, creates a turtle, and then calls the square function and gives it our turtle as input: