Data Structures and Algorithms

by Albert Major

Types of Errors & How to Fix Them

1. Indentation Errors (Python-specific)
   • If indentation is inconsistent, Python throws an IndentationError.
   • Fix: Ensure all lines have the correct indentation.
2. Syntax Errors (Detected Before Running the Code)
   • Python first analyzes the code for structural mistakes before execution.
   • Example: Missing colon (:) at the end of a function or loop declaration.
   • Fix: Add the missing syntax (e.g., the colon).
3. Runtime Errors (Detected While Running the Code)
   • Even if the syntax is correct, errors can occur during execution.
   • Example: TypeError: unsupported operand types for +=: int and str
   • This means that Python is trying to add a string (str) to an integer (int), which is not allowed.
   • Fix: Identify where the wrong types are being used and convert them appropriately.
4. Debugging a TypeError Example
   • The error occurs when a string is multiplied by an integer, creating unexpected results.
   • Investigating the problem:
     • Print statements can help but are inefficient.
     • Instead, use breakpoints to pause execution and inspect variables step-by-step.

Using Breakpoints for Debugging

• What is a breakpoint?
  • A marker in the code that pauses execution at a specific line.
  • Allows you to inspect variable values before proceeding.
• Setting up breakpoints in VS Code:
  • Click next to a line number to set a red dot (breakpoint).
  • Run the program in debug mode instead of normal execution.
  • The program stops at the breakpoint, letting you analyze data.
• Debugging Step-by-Step in VS Code:
  1. Pausing at a breakpoint: Execution stops at the marked line.
  2. Inspecting variables: Check variable values in the Locals and Globals panels.
  3. Stepping through code:
     • Step Into: Move inside a function call.
     • Step Over: Execute a line but stay in the current function.
     • Continue: Resume execution until the next breakpoint.

Fixing a New Error: UnboundLocalError

• Error message: UnboundLocalError: local variable 'result' referenced before assignment
• Why it happens:
  • A variable was used before it was assigned a value.
  • In Python, when using +=, Python first reads the variable before modifying it.
  • If the variable doesn’t exist yet, it causes an error.
• Fix: Initialize the variable before using += to ensure it has a starting value.