Python Functions

A function is a named block of code that you can run whenever you need it. The keyword def introduces a function definition. return sends a value back to the caller.

def mean(values):
    return sum(values) / len(values)

print(mean([72, 88, 95, 61]))
print(mean([1.5, 2.5]))

Parameters and default values

A parameter can have a default value. The caller may then leave that argument out.

def normalize(values, scale=1.0):
    highest = max(values)
    return [v / highest * scale for v in values]

print(normalize([10, 20, 40]))
print(normalize([10, 20, 40], scale=100))

Writing scale=100 in the call is a keyword argument. It makes the call easier to read.

Docstrings

A string on the first line of a function body is its documentation string, or docstring. help() displays it.

def accuracy(correct, total):
    """Return the share of correct predictions, from 0 to 1."""
    return correct / total

help(accuracy)
print(accuracy(46, 50))

Returning more than one value

Separate the values with a comma. Python packs them into a tuple, which you can unpack into separate names.

def min_max(values):
    return min(values), max(values)

low, high = min_max([72, 88, 95, 61])
print(low, high)

Try it

Write count_above(values, threshold) that returns how many values are greater than the threshold. Test it with count_above([72, 88, 95, 61], 70). The answer is 3.