Python Errors

When an error arises, there will be an error message with the type of error and the line the error occured on. This notebook goes over how to handle the common types of errors and exceptions in Python.

I recommend looking at the Python tutorial page for more information on errors. Searching for the error message directly on Google can help the debugging process if there is an error not discussed in this page.

Syntax Error

A SyntaxError occurs when the syntax of your code is incorrect.

if True 
print("Hello World")

SyntaxError: expected ':' (975521850.py, line 1)

A colon is expected after the if statement, which arises the syntax error. The error goes away after adding the colon.

if True:
    print("Hello World")

Hello World

Name Error

A NameError occurs when a variable, function, or module used does not exist. When this happens, it is usually because of a spelling error.

add

NameError: name 'add' is not defined

string(9)

NameError: name 'string' is not defined

Type Error

A TypeError occurs when you input an incorrect data type for an operation or function.

"abc" + 9

TypeError: can only concatenate str (not "int") to str

In python, you cannot add strings to integers. You can add, however, an integer to an integer or a string to a string with a +.

9 + 9

"abc" + "def"

'abcdef'

Zero Division Error

A ZeroDivisionError occurs when you try to divide by zero. To fix this, recheck your computation.

2 / (9 * 0)

ZeroDivisionError: division by zero

#code corrected to no longer divide by zero
(2 / 9) * 0

0.0

Value Error

A ValueError occurs when an input for a function is the correct data type but is invalid in regards to the domain of the function. This is most common with mathematical operations.

import math

math.sqrt(-10)

ValueError: math domain error

In the example above, you must input a positive number into the sqrt() function. The negative number is still an integer, but it is not in the function’s domain.

math.sqrt(10)

3.1622776601683795

Index Error

An IndexError occurs when you try to access an item in a list with an index out of bounds.

list = [1,2,3,4,5]
list[5]

IndexError: list index out of range

The range of a list is [0, n-1], where “n” is the length of the list. So, the list [1,2,3,4,5] has index elements in the range 0-4.

list[4]

Module Not Found Error

A ModuleNotFoundError occurs when you try to import a module that does not exist. It is a type of ImportError. To fix this error, check if you have installed the module in your python environment from the terminal command-line.

import pillow

ModuleNotFoundError: No module named 'pillow'

Catching Exceptions with Try Statements

You can use a try statement to catch errors. A try clause includes the code you want to run that might cause an error. If no error occurs, the try clause runs successfully. If an error does occur, the except clause runs after the line in the try clause that caused an error.

try:
    "abc" + 9
    print("Success")
except:
    print("Failure to execute")

Failure to execute

The except clause above can catch any type of error. However, an except clause can also catch a specific type of error. There can be mulptile except clauses in a try statement to catch the different types of errors.

try:
    hello
    "abc" + 9
    print("Success")
except TypeError:
    print("TypeError failure to execute")
except NameError:
    print("NameError failure to execute")

NameError failure to execute

try:
    list = [1,2,3,4,5]
    list[5]
    print("Success")
except TypeError:
    print("TypeError failure to execute")
except NameError:
    print("NameError failure to execute")
except IndexError:
    print("IndexError failure to execute")

IndexError failure to execute

Next Steps

These are not all the errors that might come up in your coding. If another type of error occurs, you can search the error type on Google to learn more about what has caused it. As always, remember to look at the line resulting in the error for hints on what could have gone wrong!