Python Errors

It’s common to get error messages when working in Python. These messages can be a little intimidating at first, but once you understand how they work, you’ll find them very helpful.

Many of the errors you’ll make in Python fall into two categories: syntax errors and traceback errors.

Syntax errors

When you ask Python to run a program or execute a line in the REPL, it will first check to see if the program is valid Python code—that is, if the code properly follows the grammar of Python, its syntax. If it doesn’t, before the program even runs, you’ll see a syntax error message printed out to the screen.

In the example below, the syntax error is a common one—mismatched single and double quotes, which is not allowed in Python. You can replicate the error for yourself by opening the REPL (type python in the command line) and entering the line after the >>> prompt.

>>> print('This string has mismatched quotes. But Python will help us figure out this bug.")
  File "<stdin>", line 1
    print('This string has mismatched quotes. But Python will help us figure out this bug.")
                                                                                          ^
SyntaxError: EOL while scanning string literal

Note the caret (^) underneath the mismatched quote, helpfully pointing out where the error lies. If this error happened in the course of running a script, Python would tell us the filename and the number of the line where the error occurs.

Traceback errors

Traceback errors occur during the execution of a Python program when the program finds itself in an untenable state and must stop. Traceback errors are often logical inconsistencies in a program that is syntactically valid Python code. A common traceback error is referring to a variable that hasn’t been defined, as below.

>>> print(not_yet_a_variable)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'not_yet_a_variable' is not defined

Traceback error messages try to tell you a bit about what happened in the program that caused the problem, including the category of error, such as NameError or TypeError.

Debugging

Debugging is simply the (not always simple) process of fixing problems with a program. It takes patience. Here are some common strategies for debugging a program when first learning Python:

• If the error is a syntax error:
  • Look at where the caret is pointing.
  • Pay attention to syntax features such as quotes, parentheses, and indentation.
  • Consider reading the program, or the offending line, backward. It’s surprising, but this often helps to detect the issue.
• If the error is a traceback error:
  • First, look at the line where the error occured, then consider the general category of error. What could have gone wrong?
  • If the error is a name error (NameError), check your spelling.
  • Try copying the last line of the error and pasting it into a search engine such as Google or Duck Duck Go. You’ll often find a quick solution this way.
• If you changed the program and expect a different output, but are getting old output, you may not have saved the file. Go back and make sure the file has been correctly saved.