---
title: "Why Does My Python Function Keep Values From an Earlier Call?"  
description: "Why Does My Python Function Keep Values From an Earlier Call?"  
author: "Björn Ólafsson"  
published: 2026-10-18  
canonical: https://answers.mindstick.com/qa/117819/why-does-my-python-function-keep-values-from-an-earlier-call  
category: "Mix Category"  
tags: ["Python", "Debugging", "functions"]  
reading_time: 1 minute  

---

# Why Does My Python Function Keep Values From an Earlier Call?

I wrote a small helper to collect tags. If I don’t pass an existing list, I want it to start a new one. Instead, the second call includes the tag from the first call:

```python
def add_tag(tag, tags=[]):
    # Add the requested tag and return the collection.
    tags.append(tag)
    return tags

# Make two calls without supplying a list.
first = add_tag("python")
second = add_tag("seo")

print(first)   # Actual output: ['python', 'seo']
print(second)  # Actual output: ['python', 'seo']
```

I expected `first` to contain only `"python"` and `second` to contain only `"seo"`. What confuses me most is that `first` changes even though I never pass it into the second call.

How are **Python default arguments** evaluated, and how should I write this function if it needs to support both a fresh list and a caller-supplied list? In the second case, modifying the supplied list is intentional.


---

Original Source: https://answers.mindstick.com/qa/117819/why-does-my-python-function-keep-values-from-an-earlier-call

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