How to Debug Most Common Errors and Exceptions in Python (with Examples)

When getting started with Python, you will likely face exceptions. Exceptions in Python refers to the problems in your code.

When facing an exception in Python, something has gone wrong, which means you the programmer, have to debug what problem may exist in your code.

In this tutorial, we will learn to most common things that can go wrong with your code. We will also learn how to read and debug Python errors (traceback).

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Reading the Python Error (Traceback)

When you have an Exception in your Python code, you will get a traceback. A traceback is a message telling you what exception happened and where it happened.

If you are interested to learn more, your can read my tutorial on the reading the Python traceback.

Most Common Exceptions in Python

SyntaxError
IndentationError
ValueError

SyntaxError

A SyntaxError in Python is an exception that is raised when there is an error with the syntax of the programming language. Similar to how missing a dot at the end of a sentence would be an error in the English language, missing an colon could trigger a SyntaxError in Python.

Here is are simple SyntaxError examples in Python:

# Missing Colon
if condition  # Missing colon
    # do something

# Unclosed String
my_string = "This is an unclosed string # Missing closing quote
# SyntaxError: unterminated string literal (detected at line 1)

For more information on how to fix SyntaxError in Python.

IndentationError

The IndentationError exception generally happens in Python when creating if…else blocks, defining a function, using loops or creating other blocks such as try… except statements. It most often occurs when there is no indentation, or when there is uneven indentation.

# Not indented if block
if True:
print('not indented')
# IndentationError: expected an indented block after 'if' statement

# Uneven indentation
def funct():
     print("Properly indented")
   print("Uneven indentation")
# IndentationError: unindent does not match any outer indentation level

For more information on how to fix IndentationError in Python.

ValueError

In Python, a ValueError most often occur when Python fails to make the conversion of one data type to the other.

# Conversion Errors
int("hello") # ValueError: invalid literal for int() with base 10: 'hello'
float("3.14.159") # ValueError: could not convert string to float: '3.14.159'

ModuleNotFoundError

The ModuleNotFoundError exception happens when a library was imported but was not installed, or when the module was incorrectly cased or misspelled.

# importing a package that was not installed
import advertools

# misspelling import
import pandu

# wrong casing
import Pandas as pd

IndexError

my_list = [1, 2, 3]
print(my_list[3])  # IndexError: list index out of range

AttributeError

my_list = [1, 2, 3]
my_list.append(4)
my_list.add(5)  # AttributeError: 'list' object has no attribute 'add'

TypeError

A `TypeError` occurs when an operation or function is applied to an object of inappropriate type.

Mismatched types in arithmetic operations
Function arguments of the wrong type
Indexing a non-indexable object
Calling a non-callable object
Using None in operations
Incompatible types in comparisons
Incorrect dictionary key type
Type mismatch in built-in functions

# Mismatched types in arithmetic operations
# Adding a string to an integer
result = "Hello" + 5

# Indexing a Non-Indexable Object
# Indexing a data type that does not support it
my_number = 123
print(my_number[0]) 
# TypeError: 'int' object is not subscriptable

# Using None in Operations
total = None
total += 5

# Incompatible Types in Comparisons
result = "apple" < 5

# Type Mismatch in Built-in Functions
int("3.14")

NameError

IOError

KeyError

# import without alias
import numpy
numpy.ones(4)
# array([1., 1., 1., 1.])

# import without alias
import numpy as np
np.ones(4)
# array([1., 1., 1., 1.])

If a library is not installed, you will get the ModuleNotFoundError.

Preventing RunTime Errors with Try… Except

To prevent runtime errors in Python, you can use the try… except statements, which will work for exceptions that are not syntax related (SyntaxError, IndentationError).

try:
    print(1 / 0)
except Exception as e:
    print(e)

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Jean-Christophe Chouinard

SEO Strategist at Tripadvisor, ex- Seek (Melbourne, Australia). Specialized in technical SEO. Writer in Python, Information Retrieval, SEO and machine learning. Guest author at SearchEngineJournal, SearchEngineLand and OnCrawl.