How to Debug Common Python Errors

Python debugging involves identifying and fixing errors in your code using tools like tracebacks, print() calls, breakpoints, and tests. In this tutorial, you’ll learn how to interpret error messages, use print() to track variable values, and set breakpoints to pause execution and inspect your code’s behavior. You’ll also explore how writing tests can help prevent errors and ensure your code runs as expected.

By the end of this tutorial, you’ll understand that:

• Debugging means identifying, analyzing, and resolving issues in your Python code using systematic approaches.
• Tracebacks are messages that help you pinpoint where errors occur in your code, allowing you to resolve them effectively.
• Using print() helps you track variable values and understand code flow, aiding in error identification.
• Breakpoints let you pause code execution to inspect and debug specific parts, improving error detection.
• Writing and running tests before or during development aids in catching errors early and ensures code reliability.

Understanding these debugging techniques will empower you to handle Python errors confidently and maintain efficient code.

Take the Quiz: Test your knowledge with our interactive “How to Debug Common Python Errors” quiz. You’ll receive a score upon completion to help you track your learning progress:

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Interactive Quiz

How to Debug Common Python Errors

Take this quiz to review core Python debugging techniques like reading tracebacks, using print(), and setting breakpoints to find and fix errors.

How to Get Started With Debugging in Python

Debugging means to unravel what is sometimes hidden. It’s the process of identifying, analyzing, and resolving issues, errors, or bugs in your code.

At its core, debugging involves systematically examining code to determine the root cause of a problem and implementing fixes to ensure the program functions as intended. Debugging is an essential skill for you to develop.

Debugging often involves using tools and techniques such as breakpoints, logging, and tests to achieve error-free and optimized performance of your code. In simpler terms, to debug is to dig through your code and error messages in an attempt to find the source of the problem, and then come up with a solution to the problem.

Say you have the following code:

print(cat)

The code that prints the variable cat is saved in a file called cat.py. If you try to run the file, then you’ll get a traceback error saying that it can’t find the definition for the variable named cat:

$ python cat.py
Traceback (most recent call last):
  File "/path_to_your_file/cat.py", line 1, in <module>
    print(cat)
          ^^^
NameError: name 'cat' is not defined

When Python encounters an error during execution, it prints a traceback, which is a detailed message that shows where the problem occurred in your code. In this example, the variable named cat can’t be found because it hasn’t been defined.

Here’s what each part of this Python traceback means:

In this example, the Python interpreter can’t find any prior definition of the variable cat and therefore can’t provide a value when you call print(cat). This is a common Python error that can happen when you forget to define variables with initial values.

To fix this error, you’ll need to take a step-by-step approach by reading the error message, identifying the problem, and testing solutions until you find one that works.

In this case, the solution would be to assign a value to the variable cat before the print call. Here’s an example:

cat = "Siamese"
print(cat)

Notice that the error message disappears when you rerun your program, and the following output is printed:

$ python cat.py
Siamese

The text string stored in cat is printed as the code output. With this error resolved, you’re well on your way to quickly debugging errors in Python.

In the next sections, you’ll explore other approaches to debugging, but first, you’ll take a closer look at using tracebacks.