Python Debugging: Read Errors, Inspect Values, and Fix Bugs

Python Debugging

Debugging means finding the difference between what the code does and what you expected it to do.

Good debugging starts with the exact error message, the line number, and the smallest input that reproduces the problem.

Use prints, assertions, and breakpoints to inspect values before changing code blindly.

Traceback Reading

A traceback shows the call path that led to the error. The most useful line is usually the last frame that points to your own file.

Isolate a Bad Value

prices = ["10", "20", "free"]

for price in prices:
    print("checking:", price)
    print(int(price))

Output

checking: 10
10
checking: 20
20
checking: free
ValueError: invalid literal for int() with base 10: 'free'

Breakpoints

breakpoint() pauses the program so you can inspect variables step by step in the debugger.

Pause Before a Calculation

quantity = 3
price = "25"

breakpoint()
total = quantity * int(price)
print(total)

Before you move on

Can You Use Python Debugging?

5 checks

You can identify the user-code line in a traceback.
You can reproduce a bug with a small input.
You can inspect values before the failing line.
You can use breakpoint() for local debugging.
You can explain the bug before applying a fix.

Debugging Decisions

0 of 2 checked

Q1. Where should you usually start reading a traceback?

The last frame that points to your code The browser toolbar A random import Only the first word

Q2. What makes a bug easier to fix?

A small reproduction with known input Changing many lines at once Deleting every function Ignoring the error type

Debugging Habits That Mislead

Changing too many lines
Change one cause at a time and rerun the smallest reproduction.
Ignoring the first bad value
Print or inspect inputs near the failing line before rewriting logic.
Catching every exception
Do not hide the traceback while learning. Catch only expected problems.

Try this next

Debug a Broken Script

0 of 3 completed

Debug a conversion
Create a list with one invalid number string and print the value before int().
Use an assertion
Add assert total >= 0 before returning a calculated total.
Read one traceback
Copy a small error and mark the exact file line you should inspect first.

Questions About Python Debugging

No. Small print checks are useful. Remove or replace them with logging when the code becomes a real program.

Use it when you need to inspect several variables or step through code locally.

Start from the last frame that points to your code, then use earlier frames only if the cause is not obvious.

Python Debugging: Read Errors, Inspect Values, and Fix Bugs