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14. First Steps in Debugging

1. Introduction​

Debugging is the process of finding and fixing errors (bugs) in your code.
Every programmer encounters bugs — they are a normal part of coding.
In this chapter, we’ll learn some basic debugging techniques in Python.

2. Types of Errors​

  1. Syntax Errors → Mistakes in code structure.
    Example:

    print("Hello"

    Output:

    SyntaxError: unexpected EOF while parsing

  2. Runtime Errors → Errors that occur while the program is running.
    Example:

    x = 10 / 0

    Output:

    ZeroDivisionError: division by zero

  3. Logic Errors → Code runs but produces the wrong result.
    Example:

    def add(a, b):
        return a - b   # Bug: should be +

3. Using Print Statements​

The simplest debugging method: insert print() statements.

Example:

x = 5
y = 0
print("Before division:", x, y)
z = x / y

Output shows the values before the crash.

4. Using the Built-in pdb Debugger​

Python includes a command-line debugger called pdb.

Run your script with:

python3 -m pdb script.py

Commands inside pdb:

  • n → next line
  • s → step into function
  • c → continue execution
  • p variable → print variable value
  • q → quit debugger

5. Debugging in IDLE​

  • Open your script in IDLE.
  • Go to Debug → Debugger.
  • This opens a panel where you can step through code and inspect variables.

6. Debugging in VS Code​

  1. Open your script in VS Code.
  2. Click in the margin next to a line to add a breakpoint.
  3. Press F5 or go to Run → Start Debugging.
  4. The program will pause at the breakpoint.
  5. Inspect variable values in the Debug Console.

7. Debugging in PyCharm​

  1. Open your script in PyCharm.
  2. Right-click and choose Debug instead of Run.
  3. Use breakpoints (red dots) to pause execution.
  4. Step through code with the toolbar (Step Over, Step Into).

8. Common Debugging Tips​

  • Read error messages carefully — they tell you the file, line, and type of error.
  • Start by fixing the first error in the traceback.
  • Use small test cases to isolate problems.
  • Don’t be afraid of bugs — they are opportunities to learn.

9. Next Steps​

✅ You now know the basics of debugging in Python.
In the next chapter, we’ll learn about pip, Python’s package manager, which lets you install and use external libraries.