Day 4: Using Mutable Default Arguments

 

Day 4: Using Mutable Default Arguments

Using mutable objects as default arguments is one of the most common and dangerous Python mistakes.

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❌ The Mistake

def fun(x=[]): 
x.append(1)
return x

print(fun())
print(fun())

❗ Unexpected Output

[1]
[1, 1]

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❌ Why this fails?

Because default arguments are evaluated only once, not every time the function is called.

The list [] is created a single time and then shared across all function calls.
Each call modifies the same list.

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✅ The Correct Way

Use None as the default value and create the list inside the function.

def fun(x=None):
if x is None:
      x = []
  x.append(1) 
  return x

print(fun())
print(fun())

✔ Correct Output

[1]
[1]

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🧠 Simple Rule to Remember

• ❌ Never use mutable objects (list, dict, set) as default arguments

• ✅ Use None and initialize inside the function

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✅ Key Takeaway

Default arguments in Python are shared, not recreated.
This can cause unexpected behavior if you’re not careful.