Python Fundamentals for Interviews: Data Structures, Comprehensions, and Gotchas

Common Mistakes Worth Remembering

• A mutable default argument (def f(x=[])) or mutable class attribute — one of Python's most-asked "spot the bug" interview questions.
• Using == where is None was intended, or vice versa.
• Modifying a list while iterating over it directly, which silently skips elements — iterate over a copy (for x in list(original):) if the loop body needs to mutate the original.
• Reaching for a list when a set would make a membership check O(1) instead of O(n).
• Building a large string with repeated += in a loop instead of str.join, turning an O(n) operation into O(n²).
• Late-binding closures in a loop ([lambda: i for i in range(3)]) — every lambda sees the loop variable's final value, not the value at the time it was created.
• A bare except: swallowing KeyboardInterrupt and SystemExit along with the bug you actually meant to catch.
• Confusing a shallow copy with a deep copy on nested lists/dicts, then being surprised that mutating the "copy" affects the original too.

Practice these exact ideas — including the mutable-default gotcha and the closure-in-a-loop trap — as real "predict the output" questions in the code lab. Python's web-framework side is covered separately in Django Fundamentals.