Python: do not use empty array as function parameters

What is a default parameter in Python?

A default parameter in Python is a default value assigned to a function parameter. If I define the function foo with a parameter bar = 3, it ensures that if no value is passed for this parameter, the default value 3 will be used.

def foo(bar=3):
  return bar + 2

n = foo()   # returns 5
n = foo(10) # return 12

The major caveat is that default parameters are only evaluated once and may cause problems when a function is called more than once.

Why does an empty array as a default parameter in Python may cause a problem?

As parameters are evaluated only ONCE, returning or using an array with a default parameter may create an issue, as the value from a previous call may be reused in a later call.

Let's look at a concrete example.

import typing


def foo(bar: typing.List[int] = []):
    bar.append(42)
    return bar


l1 = foo()
print(l1)
l2 = foo()
print(l2)

When executing the code above, at the second call, the value of the first call was reused, which is (almost) never what we want.

$ python pythoncode.py
[42]
[42, 42]

This is a common issue in Python code already documented (see the Common Gotchas for Python).

How to avoid having issues with an empty array as default parameter in Python?

The idea is not to use an array as a default parameter and instead is None. Then, inside the function, we check if a value has been passed and assign it.

There is the corrected version of the code shown above.

import typing


def foo(bar: typing.Optional[typing.List[int]]=None):
    if bar is None:
        res = []
    else:
        res = bar
    res.append(42)
    return res


l1 = foo()
print(l1)
l2 = foo()
print(l2)

When executing the code, we have the correct result.

$ python pythoncode.py
[42]
[42]

Automatically empty array as default parameter in Python

Codiga provides IDE plugins and integrations with GitHub, GitLab, or Bitbucket to detect empty arrays as default parameters. The Codiga static code analysis detects this issue directly in your IDE or code reviews.

In particular, this analysis rule produces a warning each time there is an empty array as a default parameter in a function definition.

Avoid empty array parameter

To use this rule consistently, all you need to do is to install the integration in your IDE (for VS Code or JetBrains) or code management system and add a codiga.yml file at the root of your profile with the following content:

rulesets:
  - python-security

It will then check all your Python code against 100+ rules that detect unsafe and insecure code and suggests fixes for each of them.

Explore Code Analysis Rules for Python

More resources

Common Gotchas in Python