How to Read Unfamiliar Python Code Quickly

Step 1 — Spot the Inputs and Outputs

Scan for input(), function parameters, and print(). This tells you what the code consumes and produces.

Step 2 — Find the Loops

Loops contain most of the logic. Ask: how many times does it run? What variable changes each iteration?

Step 3 — Identify the State

Which variables accumulate information (counters, totals, result lists)? These are where output usually comes from.

Step 4 — Trace One Iteration

Run through the first iteration with concrete values. Write down the state after each line. If you can do iteration 1 and 2, you understand the code.

Example

def mystery(lst):
    best, best_i = lst[0], 0
    for i, x in enumerate(lst):
        if x > best:
            best, best_i = x, i
    return best_i

Inputs: a list. Output: an index. Loop: over each element with index. State: best, best_i. Conclusion: returns the index of the maximum.