Python Errors and Built-in Exceptions

Python Errors and Built-in Exceptions

Python (interpreter) raises exceptions when it encounter errors. For example: divided by zero. In this article, you will learn about different exceptions that are built-in in Python.

When writing a program, we, more often than not, will encounter errors.

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

>>> if a < 3
  File "<interactive input>", line 1
    if a < 3
SyntaxError: invalid syntax

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

Errors can also occur at runtime and these are called exceptions. They occur, for example, when a file we try to open does not exist (FileNotFoundError), dividing a number by zero (ZeroDivisionError), module we try to import is not found (ImportError) etc.

Whenever these type of runtime error 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.

>>> 1 / 0
Traceback (most recent call last):
 File "<string>", line 301, in runcode
 File "<interactive input>", line 1, in <module>
ZeroDivisionError: division by zero

>>> open("imaginary.txt")
Traceback (most recent call last):
 File "<string>", line 301, in runcode
 File "<interactive input>", line 1, in <module>
FileNotFoundError: [Errno 2] No such file or directory: 'imaginary.txt'

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 local() built-in functions as follows.

>>> locals()['__builtins__']

This will return us a dictionary of built-in exceptions, functions and attributes.

Some of the common built-in exceptions in Python programming along with the error that cause then are tabulated below.

Python Built-in Exceptions
Exception Cause of Error
AssertionError Raised when assert statement fails.
AttributeError Raised when attribute assignment or reference fails.
EOFError Raised when the input() functions 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 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 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 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 argument of correct type but improper value.
ZeroDivisionError Raised when second operand of division or modulo operation is zero.

We can also define our own exception in Python (if required). Visit this page to learn more about user-defined exceptions

We can handle these built-in and user-defined exceptions in Python using try, except and finally statements