4.5: Generalization

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The next step is to add a length parameter to square. Here is a solution:

def square(t, length):
    for i in range(4):
        t.fd(length)
        t.lt(90)

square(bob, 100)

Adding a parameter to a function is called generalization because it makes the function more general: in the previous version, the square is always the same size; in this version it can be any size.

The next step is also a generalization. Instead of drawing squares, polygon draws regular polygons with any number of sides. Here is a solution:

def polygon(t, n, length):
    angle = 360 / n
    for i in range(n):
        t.fd(length)
        t.lt(angle)

polygon(bob, 7, 70)

This example draws a 7-sided polygon with side length 70.

If you are using Python 2, the value of angle might be off because of integer division. A simple solution is to compute angle = 360.0 / n. Because the numerator is a floating-point number, the result is floating point.

When a function has more than a few numeric arguments, it is easy to forget what they are, or what order they should be in. In that case it is often a good idea to include the names of the parameters in the argument list:

polygon(bob, n=7, length=70)

These are called keyword arguments because they include the parameter names as “keywords” (not to be confused with Python keywords like while and def).

This syntax makes the program more readable. It is also a reminder about how arguments and parameters work: when you call a function, the arguments are assigned to the parameters.