19.1: Conditional expressions

We saw conditional statements in Section 5.4. Conditional statements are often used to choose one of two values; for example:

if x > 0:
    y = math.log(x)
else:
    y = float('nan')

This statement checks whether x is positive. If so, it computes math.log. If not, math.log would raise a ValueError. To avoid stopping the program, we generate a “NaN”, which is a special floating-point value that represents “Not a Number”.

We can write this statement more concisely using a conditional expression:

y = math.log(x) if x > 0 else float('nan')

You can almost read this line like English: “y gets log-x if x is greater than 0; otherwise it gets NaN”.

Recursive functions can sometimes be rewritten using conditional expressions. For example, here is a recursive version of factorial:

def factorial(n):
    if n == 0:
        return 1
    else:
        return n * factorial(n-1)

We can rewrite it like this:

def factorial(n):
    return 1 if n == 0 else n * factorial(n-1)

Another use of conditional expressions is handling optional arguments. For example, here is the init method from GoodKangaroo (see Exercise 17.13.2):

    def __init__(self, name, contents=None):
        self.name = name
        if contents == None:
            contents = []
        self.pouch_contents = contents

We can rewrite this one like this:

    def __init__(self, name, contents=None):
        self.name = name
        self.pouch_contents = [] if contents == None else contents 

In general, you can replace a conditional statement with a conditional expression if both branches contain simple expressions that are either returned or assigned to the same variable.