Why Your Python Default Arguments Keep Changing (And How to Fix It)

A subtle and dangerous Python bug many developers encounter is this:

Your function works once, then starts behaving strangely on later calls.

Data keeps growing.

Values persist unexpectedly.

And you never intended that.

This happens a lot in:

• API handlers
• Utility functions
• Data processing scripts
• Django / Flask apps
• Caching logic
• Logging helpers

It feels random, but it is not.

Let’s fix it step by step.

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The Problem

Suppose you write this:

def add_item(item, items=[]):
    items.append(item)
    return items

print(add_item("apple"))
print(add_item("banana"))

Expected Output

["apple"]
["banana"]

Actual Output

["apple"]
["apple", "banana"]

Very confusing.

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Why This Happens

In Python, default arguments are evaluated only once.

This line:

items=[]

runs only when the function is defined—not each time it is called.

So the same list is reused across all calls.

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What’s Really Happening

This code:

def add_item(item, items=[]):

behaves like:

shared_list = []

def add_item(item, items=shared_list):

Every call uses the same list.

That is why values keep accumulating.

---

Step 1: Fix Using None

Correct solution:

def add_item(item, items=None):
    if items is None:
        items = []

    items.append(item)
    return items

---

Output Now

["apple"]
["banana"]

Now each call gets a fresh list.

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Why This Fix Works

Because None is safe and immutable.

Each function call creates a new list only when needed.

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Real Example

Bad:

def create_user(name, roles=[]):
    roles.append("user")
    return {"name": name, "roles": roles}

Correct:

def create_user(name, roles=None):
    if roles is None:
        roles = []

    roles.append("user")
    return {"name": name, "roles": roles}

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Another Case

Bad:

def update_config(config={}):
    config["debug"] = True
    return config

Correct:

def update_config(config=None):
    if config is None:
        config = {}

    config["debug"] = True
    return config

---

Quick Debug Rule

If data keeps "remembering" old values:

Ask yourself:

Am I using a mutable default argument?

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Final Thoughts

This is not a bug—it is Python behavior.

Remember:

• Default arguments run once
• Lists and dicts are mutable
• Mutable defaults create shared state
• Always use None as default

This small fix prevents big bugs.

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Your Turn

Have you seen this bug before?

Most Python developers remember the first time it happened.

Peace,
Emily Idioms