Monitoring Scripts for Errors

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Debugging and Fixing Errors

• 3 categories:
  • Syntax errors
    • occur when the Python parser is unable to understand a line of code
    • usually the result of typos or misunderstandings about the Python language syntax
    • e.g., forgetting parenthesis
  • Runtime errors
    • occur during the execution of a program
    • often caused by
      • operations that are mathematically illegal
        • e.g., division by 0
      • or attempting to access a resource that isn’t available
  • Semantic errors
    • program runs without crashing, but doesn’t produce the expected output
    • could be due to error in program’s logic

Debugging is the process of identifying and correcting these bugs.

• various tools help with debugging:
  • debuggers
  • linters
  • integrated development environments

Python Debugging

• basic steps:
  • Understanding the Problem
    • involves reproducing the error and analyzing the error message or incorrect output
  • Isolating the Problem
    • isolate the section of code causing the error
    • done by commenting out sections of code or using print statements to check the values of variables at different stages of the program
  • Using a Debugger
    • Python comes with a built-in debugger called pdb
      • allows stepping through the code line by line, inspecting variables, and setting breakpoints at specific lines of code
  • Fixing the Error
    • could involve correcting a typo, changing a variable, or rewriting a section of code
  • Testing the Solution
    • test the solution under different scenarios to ensure the error has been completely resolved

Common Debugging Techniques

• Print statements
  • use print statements to display the values of variables at certain points in the program
  • can help identify unexpected values or behavior
• Using a Debugger
  • Python’s built-in debugger, pdb, allows stepping through the code line by line, inspecting variables, and setting breakpoints
  • powerful tool for understanding the flow of the program and identifying where things go wrong
• Code Review
  • can be done individually or as part of a pair programming or code review session
• Unit Testing
  • can help catch errors and prevent regressions
  • Python’s unittest module provides a framework for creating and running tests
• Logging
  • Python’s logging module can provide valuable insights into the behavior of the program over time
• Profilling
  • For performance issues, Python’s cProfile module can help identify bottlenecks in the code

IDE Code Errors

• are issues that arise when writing code in an IDE
  • classified as:
    • syntax errors
    • runtime errors
    • semantic errors
• IDEs can flag linting errors
  • not a bug per se, but violation of code conventions or best practices
  • resolving makes code more readable and maintainable
• each IDE may have different ways of indicating these errors

Debugging Tools

• common debugging tools in Python:
  • PDB
    • built-in Python debugger
  • PyCharm Debugger
    • PyCharm, a popular Python IDE, comes with a powerful debugger
  • Visual Studio Code Debugger
    • popular IDE that includes a versatile debugger with support for remote debugging, multi-threaded debugging, and conditional breakpoints
  • Logging
    • built-in logging module can be used to record the flow of the program and help identify issues
  • Unit Testing Tools
    • unittest, pytest, and doctest can help catch errors and prevent regressions
  • Linters
    • pylint and flake8 can catch potential issues in the code that might lead to errors
  • Profiling Tools
    • cProfile and memory_profiler can help identify performance bottlenecks
• The Best Tools to Debug Python

Input Validation

try/except Blocks

try/except blocks are used for exception handling.

• allow the program to continue running even if an error or exception occurs

try:
    # Code that might raise an exception
except ExceptionType:
    # Code to handle the exception

• The try block contains the code that might raise an exception
  • Python will attempt to execute this code
• If an exception is raised in the try block, the execution immediately moves to the except block
  • The ExceptionType is the type of exception that the except block can handle
  • If the exception type matches ExceptionType, then the code within the except block is executed
• If no exception is raised in the try block, the except block is skipped