# 19.1: Conditional expressions

- Page ID
- 40843

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.