19.1: Conditional expressions
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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: “
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
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.