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:
- square(lilly)
- Correct.
- lilly.square(turtleName)
- Square is a standalone function. We do not call it using dot notation.
- lilly.square()
- Square is a standalone function. We do not call it using dot notation.
- square(turtleName)
- When we call this function, we need to give it a particular turtle as its input. lilly is the name of the turtle we want to use.
Given the definition of the square function above, which is the correct way to make
a square using a turtle lilly?