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01. Define a function that adds two numbersE02. Write a function that returns a greetingE03. Implement a function to find the maximum of threeE04. Create a function to join words with a separatorE05. Write a function that returns the length of a sequenceE06. Implement a function with a default argumentE07. Create a function that accepts any number of argsE08. Write a function that handles keyword argumentsE09. Implement a function that returns multiple valuesE10. Write a function that modifies a passed listE11. Define a function that reads a global variableE12. Write a function with a simple docstringE13. Write a function that demonstrates nonlocal usageM14. Create a function factory returning customized functionsM15. Implement argument unpacking with *args and **kwargsM16. Write a function that avoids mutable default argsM17. Create a function with keyword-only parametersM18. Implement a simple recursive function with a base caseM19. Implement a decorator that logs function callsH20. Write a decorator that memoizes function resultsH

Problem No 16

Write a function that avoids mutable default args

Medium

≈ 13 minute session

Back to curriculumNext lesson

Summary

Learn how to write functions that use immutable defaults to avoid shared mutable state between calls.

Problem statement

In Python, using a mutable object (like a list) as a default argument is a common pitfall because the same object is reused across calls. Your task is to implement append_to(element, lst=None) that appends element to lst and returns the list. If lst is not provided (None), create and use a new list for that call so calls don't share state.

Task

Implement a function that safely appends an element to a list without using a mutable default argument.

Examples

Basic usage

Input

append_to(1)

Output

[1]

Explanation

When called without a list, append_to should create a new list and return [1].

Input format

A function call append_to(element, lst=None). element may be any value; lst is optional.

Output format

Return the list after appending the element. The returned value will be shown as a Python list string representation.

Constraints

Do not use a mutable object as a default value. If lst is None, create a new list inside the function.

Samples

Sample input 0

append_to('a', ['b'])

Sample output 0

['b', 'a']

Explanation 0

When a list is provided, append the element to it and return the same list.

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