7 Common Python Dry Run Mistakes

Python dry-running is an essential skill for debugging and interviews. Most candidates who struggle do so because of a few repeatable habits. Here are the top mistakes and how to fix them.

Mistake 1: Evaluating Expressions Right-to-Left

x = 5
y = x + (x := 10)
print(x, y)

Python evaluates expressions strictly from left to right. Here, the first x is read as 5, then the walrus operator sets x to 10. So y = 5 + 10 = 15. The output is 10 15.

Mistake 2: Forgetting Default Mutable Argument Persistence

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

print(add_item(1))
print(add_item(2))

Default mutable parameters are created once when defined. Both calls share the same list object. The output is [1] followed by [1, 2].

Mistake 3: Confusing Shallow Copy with Reference Aliasing

Writing b = a does not copy the list; both names point to the same memory address. Use b = a.copy() or b = list(a) if you need an independent list.