Editing Default argument (section)

==Evaluation==

For every function call default argument values must be passed to the called function. If a default argument value contains side-effects, it is significant when those side effects are evaluated – once for the entire program (at parse time, compile time, or load time), or once per function call, at call time.

[[Python (programming language)|Python]] is a notable language that evaluates expressions in default arguments once, at the time the function declaration is evaluated. If evaluation per function call is desired, it can be replicated by having the default argument be a [[sentinel value]], such as <code>None</code>, and then having the body of the function evaluate the default value's side effects only if the sentinel value was passed in.

For example:
<syntaxhighlight lang="python">
import random

def eager(a=random.random()):
    return a

x = eager()
y = eager()
assert x == y

def lazy(a=None):
    if a is None:
        a = random.random()
    return a

x = lazy()
y = lazy()
assert x != y
</syntaxhighlight>