Python debugging & code review interview questions

Question 1

add_tag() should return a fresh tag list for each post unless one is passed in. Why do tags from one post show up on another?

def add_tag(tag: str, tags: list[str] = []) -> list[str]:
    """Append a tag and return the list of tags."""
    if tag not in tags:
        tags.append(tag)
    return tags

first = add_tag("python")   # ["python"]
second = add_tag("sql")     # expected ["sql"]

Accepted Answer

Line 1 (Wrong API usage): Default values are evaluated once, when the function is defined, so every call without `tags` shares the same list. `second` comes back as ["python", "sql"], and the list keeps growing for the life of the process.

Question 2

This looks up a user by email for the login form. What's the problem?

import sqlite3

def find_user(conn: sqlite3.Connection, email: str):
    cur = conn.cursor()
    cur.execute(f"SELECT id, email, password_hash FROM users WHERE email = '{email}'")
    return cur.fetchone()

Accepted Answer

Line 6 (Security / injection): The email is pasted into the SQL text, so input like `' OR '1'='1` changes the query and returns the first user, letting an attacker log in as someone else. Let the driver bind the value instead.

Question 3

Several worker processes call ensure_dir() at startup. Occasionally one crashes with FileExistsError. Why?

import os

def ensure_dir(path: str) -> str:
    """Create the directory if it doesn't exist yet."""
    if not os.path.exists(path):
        os.makedirs(path)
    return path

Accepted Answer

Line 7 (Race condition): This is a time-of-check to time-of-use race. Two processes can both see that the path doesn't exist, then both call makedirs, and the slower one raises FileExistsError. Make the operation itself idempotent instead of checking first.

Question 4

last_n() powers the "recent activity" widget. A user set their widget size to 0. What do they see?

def last_n(items: list, n: int) -> list:
    """Return the last n items (n >= 0)."""
    if n < 0:
        raise ValueError("n must be non-negative")
    return items[-n:]

Accepted Answer

Line 5 (Wrong edge case): When n is 0, `-n` is also 0, and `items[0:]` is the whole list. Asking for zero items returns everything.

Question 5

This middleware was added to debug a production issue. What should a reviewer flag?

import logging

logger = logging.getLogger(__name__)

def log_request(request) -> None:
    logger.info("%s %s", request.method, request.path)
    logger.info("headers: %s", dict(request.headers))

Accepted Answer

Line 8 (Leaked secret): Request headers include `Authorization` bearer tokens, session cookies and API keys. Writing them to logs leaks live credentials to everyone with log access and to every log pipeline they pass through.

Python debugging & code review interview questions

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