The Python Developer Trait Every Hiring Manager Secretly Wants

Hi, my code is not working. I am getting some error
in the API. Can you please check?

(No traceback, no input, no steps tried.)

What the senior has to do next:
- ask which endpoint
- ask for the error
- ask what input was sent
- reproduce it themselves

Hi, POST /orders returns 500 when quantity is 0.

Traceback ends with:
ZeroDivisionError: division by zero
  File "services/pricing.py", line 42, in unit_price

Repro: curl -X POST localhost:8000/orders \
  -H 'Content-Type: application/json' \
  -d '{"sku": "A1", "quantity": 0}'

I think we should reject quantity < 1 in the Pydantic
model. Does that match how we handle it elsewhere?

Traceback (most recent call last):
  File "app/main.py", line 18, in run
    report = build_report(users)
  File "app/report.py", line 9, in build_report
    domains = [u["email"].split("@")[1] for u in users]
IndexError: list index out of range

Reading order:
1. Last line: IndexError, list index out of range.
2. Our frame: report.py line 9, split("@")[1].
3. Meaning: some email has no "@", so split()
   returns one item and [1] does not exist.

import logging

logger = logging.getLogger(__name__)


def build_report(users):
    domains = []
    for u in users:
        email = u.get("email") or ""
        if "@" not in email:
            logger.warning("skipping user %s: bad email %r",
                           u.get("id"), email)
            continue
        domains.append(email.rsplit("@", 1)[1].lower())
    return domains

def average_scores(scores, history=[]):
    history.extend(scores)
    return sum(history) / len(history)

print(average_scores([80, 90]))  # 85.0
print(average_scores([70]))      # 80.0, expected 70.0

Strong spoken trace:
'The second call should give 70, but it gives 80.
So something remembers the first call. The only shared
thing is history. A default list is created once when
the function is defined, so both calls share it.'

Fix:
def average_scores(scores, history=None):
    history = [] if history is None else history
    history.extend(scores)
    return sum(history) / len(history)

# Pause execution at a line and inspect values
def unit_price(total, quantity):
    breakpoint()          # Python 3.7+, opens pdb here
    return total / quantity

# Useful pdb commands
#   p quantity     print a variable
#   pp locals()    pretty-print all local variables
#   n              run the next line
#   s              step into a function call
#   c              continue until the next breakpoint
#   q              quit the debugger

BREAK-AND-FIX DRILL (20 minutes)

1. Take one of your own working projects.
2. Ask a friend to change one line without telling you.
   (Wrong key, off-by-one, missing return, wrong type.)
3. Run it. Start a timer.
4. Out loud: read the last line, find your frame,
   reproduce, isolate, fix.
5. Write a test that would have caught it.

Swap roles. Breaking code well teaches you
where bugs like to hide.

BUG JOURNAL ENTRY

Date: ________
Symptom (what I saw): ____________________
Exception and line: ______________________
How I reproduced it: _____________________
What I tried first (and why it failed): ___
Actual cause: ____________________________
Fix: _____________________________________
Test added: ______________________________

After 10 entries, you have 10 real interview
stories about debugging.

WHEN PYTHON THROWS AN ERROR

[ ] Read the LAST line: exception type and message
[ ] Find the first frame in MY code
[ ] Can I reproduce it with the same input?
[ ] Print or breakpoint() just before the failing line
[ ] Check type() and value of each variable there
[ ] Change ONE thing, run again
[ ] Add a test for this case
[ ] Note it in the bug journal