MediumExperience level: 3-5 years · 8-15 years1 min read

Why is a mutable default argument (like def f(x, items=[])) dangerous?

Reviewed by Gurusankar M.

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#default argument#mutable#gotcha#functions#bug
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Short Answer

Default argument values are evaluated once when the function is defined, not on each call. So a mutable default like `items=[]` is shared across all calls — it accumulates data between calls. The fix is `items=None` and create a fresh list inside the function.

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Concept Made Simple

Coffee Chat Question

“Why is a mutable default argument (like def f(x, items=[])) dangerous?”

Remember It Faster

Mind Map Answer

Cause
default evaluated once at def time
Symptom
list/dict persists across calls
Fix
default None, create inside

Learn by Doing

Hands-on Keyboard

python
def bad(x, items=[]):     # shared default!
    items.append(x)
    return items

print(bad(1))   # [1]
print(bad(2))   # [1, 2]  <- surprise

def good(x, items=None):
    items = items or []
    items.append(x)
    return items
print(good(1)); print(good(2))   # [1]  then  [1]
Output
[1]
[1, 2]
[1]
[1]

Think Beyond the Expected

What If?

Why does the second call to `bad(2)` return [1, 2] instead of [2]?

The default list is created once, when `def bad` runs — every call that doesn't pass `items` reuses that same list object, so appends accumulate. Using `items=None` and building a new list inside gives each call its own fresh list.

Real World

This is one of the most-asked Python 'gotcha' interview questions and a real source of state-leak bugs in caches, accumulators, and config defaults — the None sentinel pattern is the standard fix.

Interviewer's Expectation

Keywords they're listening for:

  • defaults evaluated once at def
  • mutable default is shared
  • state leaks across calls
  • fix with None sentinel

Common Mistakes

  • Using [] or {} as a default value
  • Assuming defaults are recreated each call
  • Mutating a shared default and blaming the caller

Best Practices

  • Default to None; build the mutable inside
  • Keep defaults immutable
  • Document any intentional shared default

Follow-up Questions

  1. Does the same problem apply to dict/set defaults?
  2. When is a mutable default sometimes used deliberately (caching)?
  3. How do keyword-only args help API clarity?

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Plain-language foundations

I'm preparing for a software engineering interview and want to understand this from scratch, as a beginner.

Topic: Functions (Python)
Interview question: "Why is a mutable default argument (like def f(x, items=[])) dangerous?"

Please:
1. Explain the core idea in simple, plain language, using an everyday analogy.
2. Define any technical terms you use.
3. Walk through one small, concrete example.
4. Finish with a single sentence I can easily remember.

Keep the tone friendly and assume I'm new to this topic.
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