
If the denominator to a division or modulo operation is zero, a
`ZeroDivisionError` is raised.

include::../introduction.adoc[]

== Why is this an issue?

A division (`/`) or modulo operation (`%`) by zero indicates a bug or logical
error.
This is because in Python, a division or modulo operation where the denominator
is zero always results in a `ZeroDivisionError` being raised.

include::../impact.adoc[]

== How to fix it

The goal is to ensure that a zero value never reaches the denominator.

=== Code examples

==== Noncompliant code example

[source,python,diff-id=1,diff-type=noncompliant]
----
def foo():
    z = 0
    if (unknown()):
        z = 4
        ...
    else:
        # (z is not reassigned to a non-zero value here)
        ...
    z = 1 / z # Noncompliant: divisor can be 0
----

==== Compliant solution

[source,python,diff-id=1,diff-type=compliant]
----
def foo():
    z = 0
    if (unknown()):
        z = 4
        ...
    else:
        z = 1
        ...
    z = 1 / z
----

==== Noncompliant code example

[source,python,diff-id=2,diff-type=noncompliant]
----
def report_average():
    sum = compute_sum()
    denominator = population()
    print(sum / denominator) # Noncompliant: divisor can be 0

def population():
    size = read_size()
    return size if size is not None else 0
----

==== Compliant solution

[source,python,diff-id=2,diff-type=compliant]
----
def report_average():
    sum = compute_sum()
    denominator = population()
    if denominator == 0:
      print("We have no data.")
    else:
      print(sum / denominator)

def population():
    size = read_size()
    return size if size is not None else 0
----

=== How does this work?

One approach is to inspect every possible execution path that affects the
denominator and adjust them such that none of them lead to a zero value.
The first example illustrates this by ensuring that for all the paths that can
define the variable ++z++, none assign it zero.
Thus, we are sure that the issue is fixed.

In other cases it might be necessary and safer to check and explicitly handle
the case of a zero denominator.
For instance, in the second example, an error message is displayed to the user
if the denominator is zero.

//=== Pitfalls

//=== Going the extra mile

== Resources

=== Documentation

* https://docs.python.org/3/reference/expressions.html#binary-arithmetic-operations[Binary arithmetic operations]
* https://docs.python.org/3/library/exceptions.html#ZeroDivisionError[ZeroDivisionError]

=== Standards

* CWE - https://cwe.mitre.org/data/definitions/369[CWE-369 - Divide by zero]
* STIG Viewer - https://stigviewer.com/stig/application_security_and_development/2023-06-08/finding/V-222612[Application Security and Development: V-222612] - The application must not be vulnerable to overflow attacks.

