Python – Error Types

The most common reason of an error in a Python program is when a certain statement is not in accordance with the prescribed usage.
Such an error is called a syntax error. The Python interpreter immediately reports it, usually along with the reason.

>>> print “hello”
SyntaxError: Missing parentheses in call to ‘print’. Did you mean print(“hello”)?

In Python 3.x, print is a built-in function and requires parentheses. The statement above violates this usage and hence syntax error is displayed.

Many times though, a program results in an error after it is run even if it doesn’t have any syntax error. Such an error is a runtime error, called an exception.
A number of built-in exceptions are defined in the Python library.
Let’s see some common error types.

IndexError is thrown when trying to access an item at an invalid index.

>>> L1=[1,2,3]
>>> L1[3]
Traceback (most recent call last):
File “<pyshell#18>”, line 1, in <module>

L1[3]
IndexError: list index out of range

ModuleNotFoundError is thrown when a module could not be found.

>>> import notamodule
Traceback (most recent call last):
File “<pyshell#10>”, line 1, in <module>

import notamodule
ModuleNotFoundError: No module named ‘notamodule’

KeyError is thrown when a key is not found.

>>> D1={‘1’:”aa”, ‘2’:”bb”, ‘3’:”cc”}
>>> D1[‘4’]
Traceback (most recent call last):
File “<pyshell#15>”, line 1, in <module>

D1[‘4’]
KeyError: ‘4’

ImportError is thrown when a specified function can not be found.

>>> from math import cube
Traceback (most recent call last):
File “<pyshell#16>”, line 1, in <module>

from math import cube
ImportError: cannot import name ‘cube’

StopIteration is thrown when the next() function goes beyond the iterator items.

>>> it=iter([1,2,3])
>>> next(it)
1
>>> next(it)
2
>>> next(it)
3
>>> next(it)
Traceback (most recent call last):
File “<pyshell#23>”, line 1, in <module>

next(it)
StopIteration

TypeError is thrown when an operation or function is applied to an object of an inappropriate type.

>>> ‘2’+2
Traceback (most recent call last):
File “<pyshell#23>”, line 1, in <module>

‘2’+2
TypeError: must be str, not int

ValueError is thrown when a function’s argument is of an inappropriate type.

>>> int(‘xyz’)
Traceback (most recent call last):
File “<pyshell#14>”, line 1, in <module>

int(‘xyz’)
ValueError: invalid literal for int() with base 10: ‘xyz’

NameError is thrown when an object could not be found.

>>> age
Traceback (most recent call last):
File “<pyshell#6>”, line 1, in <module>

age
NameError: name ‘age’ is not defined

ZeroDivisionError is thrown when the second operator in the division is zero.

>>> x=100/0
Traceback (most recent call last):
File “<pyshell#8>”, line 1, in <module>

x=100/0
ZeroDivisionError: division by zero

KeyboardInterrupt is thrown when the user hits the interrupt key (normally Control-C) during the execution of the program.

>>> name=input(‘enter your name’)
enter your name^c
Traceback (most recent call last):
File “<pyshell#9>”, line 1, in <module>

name=input(‘enter your name’)
KeyboardInterrupt

The following table lists important built-in exceptions in Python.

Visit Python Docs for more information.

Learn how to handle exceptions in the next chapter.