PART 1 · PYTHON FROM ZERO · LESSON 17

Reading errors without panic

FreeLesson 19 of 78

Errors are Python telling you exactly what went wrong and where. Learning to read them is faster than any amount of guessing.

A REAL TRACEBACKPYTHON 3.12

def average(values):
    return sum(values) / len(values)

orders = []
print(average(orders))

Output

ZeroDivisionError: division by zero

Error	What it means	Typical cause	Fix
`SyntaxError`	Python cannot read the line	Missing colon, bracket or quote	Look at the line and the one above
`IndentationError`	Spacing is wrong	Mixed tabs and spaces, missing indent after :	Use 4 spaces everywhere
`NameError`	Unknown name	Typo, wrong capital, used before created	Check spelling and order
`TypeError`	Wrong kind of value	"5" + 5, calling a non-function	Convert types: int(), str()
`ValueError`	Right type, bad value	int("abc"), float("")	Validate or try/except
`IndexError`	Position does not exist	xs[10] on a 5-item list	Check len() first
`KeyError`	Key not in the dict	d["phone"] when there is none	Use d.get("phone")
`ZeroDivisionError`	Dividing by zero	Average of an empty list	Check for empty first
`FileNotFoundError`	No such file	Wrong name or folder	Check the path with pathlib

TRY / EXCEPT: HANDLE THE ERROR, KEEP GOINGPYTHON 3.12

rows = ["1198", "796", "abc", "", "1499"]

total, bad = 0, []
for r in rows:
    try:
        total += float(r)          # might fail
    except ValueError:
        bad.append(r)              # note it and keep going
print("total:", total, "| could not read:", bad)

Output

total: 3493.0 | could not read: ['abc', '']

DEBUGGING: PRINT WHAT YOU HAVE, THEN ASSERT WHAT YOU EXPECTPYTHON 3.12

orders = [{"id": 101, "amount": "1198"}, {"id": 102, "amount": 796}]

total = 0
for o in orders:
    kind = type(o["amount"]).__name__
    print("DEBUG", o["id"], repr(o["amount"]), kind)
    total += float(o["amount"])
print("total:", total)

assert total == 1994.0, f"expected 1994, got {total}"
print("check passed")

Output

DEBUG 101 '1198' str
DEBUG 102 796 int
total: 1994.0
check passed

Try it yourself before moving on

Cause a KeyError on purpose and read the last line of the message.
Return 0 instead of crashing when averaging an empty list.
Fix "Total: " + 5 in two different ways.