Debugging Tracebacks

A repeatable debugging process

When you see an exception:

Read the last line (exception type + message)
Go to the line number where it occurred
Check the types/values of variables
Create a minimal reproduction
Fix and re-run

Example traceback

bug.py

def avg(nums):
    return sum(nums) / len(nums)
 
print(avg([]))

bug.py

def avg(nums):
    return sum(nums) / len(nums)
 
print(avg([]))

Traceback

Traceback (most recent call last):
  File "bug.py", line 4, in <module>
    print(avg([]))
  File "bug.py", line 2, in avg
    return sum(nums) / len(nums)
ZeroDivisionError: division by zero

Traceback

Traceback (most recent call last):
  File "bug.py", line 4, in <module>
    print(avg([]))
  File "bug.py", line 2, in avg
    return sum(nums) / len(nums)
ZeroDivisionError: division by zero

Fix approach

The list is empty, so len(nums)len(nums) is 0.
Decide policy: return NoneNone, raise a clear error, or handle with default.

fix.py

def avg(nums):
    if len(nums) == 0:
        raise ValueError("avg() requires at least one number")
    return sum(nums) / len(nums)
 
print(avg([10, 20]))

fix.py

def avg(nums):
    if len(nums) == 0:
        raise ValueError("avg() requires at least one number")
    return sum(nums) / len(nums)
 
print(avg([10, 20]))

Quick tip: print types

type_debug.py

value = "10"
print(type(value), value)
print(int(value))

type_debug.py

value = "10"
print(type(value), value)
print(int(value))

Optional: use the debugger

Python has a built-in debugger.

pdb_example.py

import pdb
 
x = 10
pdb.set_trace()
print(x)

pdb_example.py

import pdb
 
x = 10
pdb.set_trace()
print(x)

Useful commands inside the debugger:

nn next
ss step into
p varp var print variable
cc continue

🧪 Try It Yourself

Exercise 1 – Read a Traceback

Exercise 2 – Traceback Module

Exercise 3 – Get Traceback as String

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