Python Errors and Built-in Exceptions

Learn about different types of errors and exceptions that are built-in to Python. They are raised whenever the Python interpreter encounters errors.

We can make certain mistakes while writing a program that lead to errors when we try to run it. A python program terminates as soon as it encounters an unhandled error. These errors can be broadly classified into two classes:

1. Syntax errors

2. Logical errors (Exceptions)

Python Syntax Errors

Error caused by not following the proper structure (syntax) of the language is called syntax error or parsing error.

# Example 1:

if a < 3

  File "<ipython-input-1-a8dfd56d4d8e>", line 3
    if a < 3
            ^
SyntaxError: invalid syntax

Exaplanation:

As shown in the example, an arrow indicates where the parser ran into the syntax error.

We can notice here that a colon : is missing in the if statement.

# Example 2:

print 'hello world'

  File "<ipython-input-3-cb8ac02fbcd3>", line 3
    print 'hello world'
          ^
SyntaxError: Missing parentheses in call to 'print'. Did you mean print('hello world')?

Explanation:

As you can see we made a syntax error because we forgot to enclose the string with parenthesis () and Python already suggests the solution. Let us fix it.

Python Logical Errors (Exceptions)

Errors that occur at runtime (after passing the syntax test) are called exceptions or logical errors.

For instance, they occur when we try to open a file(for reading) that does not exist (FileNotFoundError), try to divide a number by zero (ZeroDivisionError), or try to import a module that does not exist (ImportError).

Whenever these types of runtime errors occur, Python creates an exception object. If not handled properly, it prints a traceback to that error along with some details about why that error occurred.

Let’s look at how Python treats these errors:

# Example 1: ZeroDivisionError

1 / 0

---------------------------------------------------------------------------
ZeroDivisionError                         Traceback (most recent call last)
<ipython-input-4-9b9247988596> in <module>
      1 # Example 1: ZeroDivisionError
      2 
----> 3 1 / 0

ZeroDivisionError: division by zero

# Example 2: FileNotFoundError

open("imaginary.txt")

---------------------------------------------------------------------------
FileNotFoundError                         Traceback (most recent call last)
<ipython-input-5-6ac8e81b6f0c> in <module>
      1 # Example 2: FileNotFoundError
      2 
----> 3 open("imaginary.txt")

FileNotFoundError: [Errno 2] No such file or directory: 'imaginary.txt'

# Example 3: ImportError

from math import power

---------------------------------------------------------------------------
ImportError                               Traceback (most recent call last)
<ipython-input-6-0ef33cc3a94d> in <module>
      1 # Example 3: ImportError
      2 
----> 3 from math import power

ImportError: cannot import name 'power' from 'math' (unknown location)

Exaplanation:

There is no function called power in the math module, it goes with a different name: pow.

Python NameError

We debugged the error by defining the variable name.

# Example 1:

print(age)

---------------------------------------------------------------------------
NameError                                 Traceback (most recent call last)
<ipython-input-7-119e50d96cfb> in <module>
      1 # Example 1:
      2 
----> 3 print(age)

NameError: name 'age' is not defined

Exaplanation:

As you can see from the message above, name age is not defined. Yes, it is true that we did not define an age variable but we were trying to print it out as if we had had declared it. Now, lets fix this by declaring it and assigning with a value.

Python IndexError

We debugged the error by defining the variable name.

# Example 1:

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

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-8-c00e5d7d3c3e> in <module>
      2 
      3 numbers = [1, 2, 3, 4, 5]
----> 4 numbers[5]

IndexError: list index out of range

Exaplanation:

In the example above, Python raised an IndexError, because the list has only indexes from 0 to 4 , so it was out of range.

Python ModuleNotFoundError

# Example 1:

import maths

---------------------------------------------------------------------------
ModuleNotFoundError                       Traceback (most recent call last)
<ipython-input-9-e2e604bf92de> in <module>
      1 # Example 1:
      2 
----> 3 import maths

ModuleNotFoundError: No module named 'maths'

Exaplanation:

In the example above, I added an extra s to math deliberately and ModuleNotFoundError was raised. Lets fix it by removing the extra s from math.

Python AttributeError

# Example 1:

import math
math.PI

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-10-df6ce7ede1dc> in <module>
      2 
      3 import math
----> 4 math.PI

AttributeError: module 'math' has no attribute 'PI'

Exaplanation:

As you can see, I made a mistake again! Instead of pi, I tried to call a PI function from math module. It raised an AttributeError, it means, that the function does not exist in the module. Lets fix it by changing from PI to pi.

Python KeyError

# Example 1:

users = {'name':'Milaan', 'age':96, 'country':'England'}
users['name']

'Milaan'

users['county']

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-12-fbf7cddd4b2a> in <module>
----> 1 users['county']

KeyError: 'county'

Exaplanation:

As you can see, there was a typo in the key used to get the dictionary value. so, this is a KeyError and the fix is quite straight forward. Let’s do this!

Python TypeError

# Example 1:

6 + '3'

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-13-f7ffd6347533> in <module>
      1 # Example 1:
      2 
----> 3 6 + '3'

TypeError: unsupported operand type(s) for +: 'int' and 'str'

Exaplanation:

In the example above, a TypeError is raised because we cannot add a number to a string. First solution would be to convert the string to int or float. Another solution would be converting the number to a string (the result then would be ‘63’). Let us follow the first fix.

Python ValueError

# Example 1:

int('19a')

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-14-ee4e8ee8c904> in <module>
      1 # Example 1:
      2 
----> 3 int('19a')

ValueError: invalid literal for int() with base 10: '19a'

Exaplanation:

In this case we cannot change the given string to a number, because of the a letter in it.

Python Built-in Exceptions

Illegal operations can raise exceptions. There are plenty of built-in exceptions in Python that are raised when corresponding errors occur. We can view all the built-in exceptions using the built-in local() function as follows:

print(dir(locals()['__builtins__']))

locals()['__builtins__'] will return a module of built-in exceptions, functions, and attributes. dir allows us to list these attributes as strings.

Some of the common built-in exceptions in Python programming along with the error that cause them are listed below:

Exception	Cause of Error
AssertionError	Raised when an assert statement fails.
AttributeError	Raised when attribute assignment or reference fails.
EOFError	Raised when the input() function hits end-of-file condition.
FloatingPointError	Raised when a floating point operation fails.
GeneratorExit	Raise when a generator’s close() method is called.
ImportError	Raised when the imported module is not found.
IndexError	Raised when the index of a sequence is out of range.
KeyError	Raised when a key is not found in a dictionary.
KeyboardInterrupt	Raised when the user hits the interrupt key (Ctrl+C or Delete).
MemoryError	Raised when an operation runs out of memory.
NameError	Raised when a variable is not found in local or global scope.
NotImplementedError	Raised by abstract methods.
OSError	Raised when system operation causes system related error.
OverflowError	Raised when the result of an arithmetic operation is too large to be represented.
ReferenceError	Raised when a weak reference proxy is used to access a garbage collected referent.
RuntimeError	Raised when an error does not fall under any other category.
StopIteration	Raised by next() function to indicate that there is no further item to be returned by iterator.
SyntaxError	Raised by parser when syntax error is encountered.
IndentationError	Raised when there is incorrect indentation.
TabError	Raised when indentation consists of inconsistent tabs and spaces.
SystemError	Raised when interpreter detects internal error.
SystemExit	Raised by sys.exit() function.
TypeError	Raised when a function or operation is applied to an object of incorrect type.
UnboundLocalError	Raised when a reference is made to a local variable in a function or method, but no value has been bound to that variable.
UnicodeError	Raised when a Unicode-related encoding or decoding error occurs.
UnicodeEncodeError	Raised when a Unicode-related error occurs during encoding.
UnicodeDecodeError	Raised when a Unicode-related error occurs during decoding.
UnicodeTranslateError	Raised when a Unicode-related error occurs during translating.
ValueError	Raised when a function gets an argument of correct type but improper value.
ZeroDivisionError	Raised when the second operand of division or modulo operation is zero.

If required, we can also define our own exceptions in Python. To learn more about them, visit Python User-defined Exceptions.

We can handle these built-in and user-defined exceptions in Python using try, except and finally statements. To learn more about them, visit Python try, except and finally statements.