Default Values as Way to Make a Parameter Optional in Python — and Why Assigning Mutables as Defaults is a Bad Idea.

Pavol Kutaj

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Feb 25, 2021

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• The aim of this page📝is to cover the capabilities of function arguments in python by defining its three rules, extracting from the Core Python Course: Getting Started | Pluralsight

1. Default argument values come last — and make a parameter optional

• rule: an argument with default values must come after those without default values
• parameters specified in a function definition with def are a comma-separated list
• they can be made optional by assigning default values to the parameter
• you get a syntax error if you don’t sign function call with all arguments that don’t have an assigned default value
• consider the following one-liner

def printMessage(message="Hello World"): print(message)>>> printMessage()
Hello World

2. Default values only on immutables

• rule: do not use mutable default values; use only immutable default values

1. ✅ ints
2. ✅ float
3. ✅ string
4. ✅ tupple

• lots of newcomers to Python are affected by this

• remember that def itself is a statement executed at runtime, typically upon module import
• i.e. this is dynamic and no matter how many times you call that the function object is created only once with def
• this can also have other negative consequences when using e.g. lists as default values

• the fix is to use immutable None