Editing Side effect (computer science) (section)

== Idempotence ==
{{Main|Idempotence#Computer science meaning}}
A [[subroutine]] with side effects is idempotent if multiple applications of the subroutine have the same effect on the system state as a single application, in other words if the function from the system state space to itself associated with the subroutine is idempotent in the [[Idempotence#Definition|mathematical sense]]. For instance, consider the following [[Python (programming language)|Python]] program:

<syntaxhighlight lang="python">
x = 0

def setx(n):
    global x
    x = n

setx(3)
assert x == 3
setx(3)
assert x == 3
</syntaxhighlight>

<code>setx</code> is idempotent because the second application of <code>setx</code> to 3 has the same effect on the system state as the first application: <code>x</code> was already set to 3 after the first application, and it is still set to 3 after the second application.

A [[pure function]] is idempotent if it is idempotent in the [[idempotence#Definition|mathematical sense]]. For instance, consider the following Python program:

<syntaxhighlight lang="python">
def abs(n):
    return -n if n < 0 else n

assert abs(abs(-3)) == abs(-3)
</syntaxhighlight>

<code>abs</code> is idempotent because the second application of <code>abs</code> to the return value of the first application to -3 returns the same value as the first application to -3.