7 Common Python Dry Run Mistakes (And How to Avoid Them)

Python dry-running (code tracing without executing) is the most underrated interview skill. Most candidates fail not because they don't know Python, but because they make systematic tracing mistakes. Practice fixing these at /practice/.

Mistake 1: Evaluating Right-to-Left Instead of Left-to-Right

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

Python evaluates left-to-right within an expression. Here, `x` is read as 5 first, then the walrus operator assigns x=10. So y = 5 + 10 = 15. Output: 10 15.

Mistake 2: Forgetting Mutable Default Arguments

def append_item(item, lst=[]):
    lst.append(item)
    return lst

print(append_item(1))
print(append_item(2))

Default mutable arguments are created once at function definition, not per call. Both calls share the same list object. Output: [1] then [1, 2] - not [1] and [2].

Mistake 3: Misreading Shallow vs Deep Copy

When you trace list assignment like `b = a`, you must mark both `a` and `b` pointing to the SAME memory object. Mutating through either name affects both. Practice list tracing at /practice/lists-foundations/.