Reading Tracebacks and Finding Bugs

45 min · Intermediate

A debugging round gives you code that fails or prints the wrong thing. Python tells you a lot when it fails. Read the last line first, then the line number above it. For wrong output, trace a small input by hand and then try the edges.

Read a traceback from the bottom

A traceback is the report Python prints when an exception is not handled. It lists the chain of calls with file and line numbers, and ends with the exception type and message.

The last line has the exception type and message. The lines above it show the calls that led there, with the most recent call last. The line marked closest to the end is where Python stopped.

Java

1def avg(nums):2    return sum(nums) / len(nums)3
4def report(scores):5    return avg(scores)6
7try:8    report([])9except ZeroDivisionError as e:10    print(type(e).__name__, "-", e)11    print("raised on line", e.__traceback__.tb_next.tb_next.tb_lineno)

This prints ZeroDivisionError - division by zero, then raised on line 2. The cause is the empty list, but the failing line is inside avg.

Common exceptions and their causes

Each exception has a usual cause. TypeError means a wrong type for an operation. ValueError means the right type with a bad value. KeyError and IndexError mean a missing key or position. AttributeError means the object has no such attribute, often because it is None.

Java

1def kind(action):2    try:3        action()4    except Exception as e:5        return type(e).__name__6
7print(kind(lambda: int("3.5")))8print(kind(lambda: "age" + 30))9print(kind(lambda: {"a": 1}["b"]))10print(kind(lambda: [1, 2][5]))11print(kind(lambda: None.upper()))

This prints ValueError, TypeError, KeyError, IndexError and AttributeError, one per line.

Bugs that print, not crash

An edge case is an input at the boundary of what the code should handle, such as an empty list, one item, duplicates, zero or negative numbers. Most bugs show up there first.

Many bugs give wrong output without any error. The usual suspects are a missing return (the function gives None), a return inside the loop that ends it early, an off-by-one range, if x == 1 or 2, and a bare except that hides the real error.

Java

1def total(prices):2    s = 03    for p in prices:4        s += p5
6code = 57if code == 1 or 2:8    print("fast")9print(total([5, 10]))

Example 1 · A return inside the loop

Python

1def first_even(nums):2    for n in nums:3        if n % 2 == 0:4            return n5        else:6            return None7
8print(first_even([1, 2, 4]))9
10def first_even_fixed(nums):11    for n in nums:12        if n % 2 == 0:13            return n14    return None15
16print(first_even_fixed([1, 2, 4]))

Output

None
2

What this code does

The else returns on the first odd number, so the loop never reaches 2.
Fix: move return None after the loop, so it runs only when nothing matched.
Test: [1, 2, 4] must give 2, and [1, 3] must give None.

Example 2 · A bare except hides the cause

Python

1def parse(text):2    try:3        return int(text)4    except:5        return -16
7print(parse("abc"), parse(None))8
9def parse_fixed(text):10    try:11        return int(text)12    except ValueError:13        return -114
15print(parse_fixed("abc"))

Output

-1 -1
-1

What this code does

The bare except also catches the TypeError for None, hiding a real mistake.
Fix: catch only ValueError, so other errors still show.
Test: parse_fixed(None) must raise TypeError, not return -1.

Example 3 · A function that changes its argument

Python

1def top2(nums):2    nums.sort(reverse=True)3    return nums[:2]4
5scores = [70, 95, 82]6print(top2(scores), scores)7
8def top2_fixed(nums):9    return sorted(nums, reverse=True)[:2]10
11scores = [70, 95, 82]12print(top2_fixed(scores), scores)

Output

[95, 82] [95, 82, 70]
[95, 82] [70, 95, 82]

What this code does

sort changes the caller's list, a side effect nobody asked for.
Fix: sorted returns a new list and leaves the input alone.
Test: check the input list after the call.

Example 4 · finally always runs

Python

1def f():2    try:3        return "try"4    finally:5        print("cleanup", end=" ")6
7print(f())

Output

cleanup try

What this code does

The return value is prepared, then finally runs, then the function returns.
So cleanup is printed before the result.

This prints fast and then None. 2 is always truthy, so the if is always true, and total has no return.

Explain the fix: cause, fix, test

In a live round, say three things for each bug: the cause in one sentence, the smallest change that fixes it, and one test that would have caught it. Do not rewrite the function.

Next step

Check your understanding

12 quick questions

Read a traceback from the bottom

A traceback is the report Python prints when an exception is not handled. It lists the chain of calls with file and line numbers, and ends with the exception type and message.

The last line has the exception type and message. The lines above it show the calls that led there, with the most recent call last. The line marked closest to the end is where Python stopped.

1def avg(nums):2 return sum(nums) / len(nums)3 4def report(scores):5 return avg(scores)6 7try:8 report([])9except ZeroDivisionError as e:10 print(type(e).__name__, "-", e)11 print("raised on line", e.__traceback__.tb_next.tb_next.tb_lineno)

1def kind(action):2 try:3 action()4 except Exception as e:5 return type(e).__name__6 7print(kind(lambda: int("3.5")))8print(kind(lambda: "age" + 30))9print(kind(lambda: {"a": 1}["b"]))10print(kind(lambda: [1, 2][5]))11print(kind(lambda: None.upper()))

Bugs that print, not crash

An edge case is an input at the boundary of what the code should handle, such as an empty list, one item, duplicates, zero or negative numbers. Most bugs show up there first.

1def first_even(nums):2 for n in nums:3 if n % 2 == 0:4 return n5 else:6 return None7 8print(first_even([1, 2, 4]))9 10def first_even_fixed(nums):11 for n in nums:12 if n % 2 == 0:13 return n14 return None15 16print(first_even_fixed([1, 2, 4]))

1def parse(text):2 try:3 return int(text)4 except:5 return -16 7print(parse("abc"), parse(None))8 9def parse_fixed(text):10 try:11 return int(text)12 except ValueError:13 return -114 15print(parse_fixed("abc"))

1def top2(nums):2 nums.sort(reverse=True)3 return nums[:2]4 5scores = [70, 95, 82]6print(top2(scores), scores)7 8def top2_fixed(nums):9 return sorted(nums, reverse=True)[:2]10 11scores = [70, 95, 82]12print(top2_fixed(scores), scores)