---
title: "Why Does My Python Function Keep List Values From Earlier Calls?"  
description: "Why Does My Python Function Keep List Values From Earlier Calls?"  
author: "Waldemar Thiessen"  
published: 2026-10-19  
canonical: https://answers.mindstick.com/qa/117912/why-does-my-python-function-keep-list-values-from-earlier-calls  
category: "Mix Category"  
tags: ["Python", "functions", "Mutable Objects", "Debugging"]  
reading_time: 1 minute  

---

# Why Does My Python Function Keep List Values From Earlier Calls?

I’m writing a small helper that collects validation messages. If I don’t pass in a list, I want each call to start with an empty one. Instead, messages from an earlier call appear in the next result.

```python
def add_warning(message, warnings=[]):
    # Append to the supplied list, or the default list if none was passed.
    warnings.append(message)
    return warnings

first = add_warning("Missing email")
print(first)  # Prints ['Missing email'] at this point.

second = add_warning("Invalid postcode")
print(second)  # Includes 'Missing email' from the previous call.

print(first is second)  # True: both names refer to the same list.
```

I expected the second result to contain only `"Invalid postcode"`. I’m also surprised that inspecting `first` after the second call shows both messages, even though I never explicitly updated `first`.

How are **mutable default arguments in Python** evaluated? Is the list created when the function is defined or each time it runs?

What is the usual way to make an omitted argument produce a fresh list while still allowing a caller to pass an existing list that the function should modify?


---

Original Source: https://answers.mindstick.com/qa/117912/why-does-my-python-function-keep-list-values-from-earlier-calls

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