Built-in Exceptions in Python

All instances in Python must be instances of a class that derives from BaseException. Two exception classes that are not related via subclassing are never equivalent, even if they have the same name. The built-in exceptions can be generated by the interpreter or built-in functions.

There are several built-in exceptions in Python that are raised when errors occur. These built-in exceptions can be viewed using the local() built-in functions as follows :

>>> locals()['__builtins__']

This returns a dictionary of built-in exceptions, functions and attributes.

Base Classes

The following exceptions are used mostly as base classes for other exceptions.

1. exception BaseException
   This is the base class for all built-in exceptions. It is not meant to be directly inherited by user-defined classes. For, user-defined classes, Exception is used. This class is responsible for creating a string representation of the exception using str() using the arguments passed. An empty string is returned if there are no arguments.
   • args : The args are the tuple of arguments given to the exception constructor.
   • with_traceback(tb) : This method is usually used in exception handling. This method sets tb as the new traceback for the exception and returns the exception object.

     Code :

     try:
         ...
     except SomeException:
         tb = sys.exc_info()[2]
         raise OtherException(...).with_traceback(tb)
     

2. exception Exception
   This is the base class for all built-in non-system-exiting exceptions. All user-defined exceptions should also be derived from this class.
3. exception ArithmeticError
   This class is the base class for those built-in exceptions that are raised for various arithmetic errors such as :
   • OverflowError
   • ZeroDivisionError
   • FloatingPointError

   Example :

   
   

   
   
   

   try:       a = 10/0       print (a) except ArithmeticError:           print ("This statement is raising an arithmetic exception.") else:       print ("Success.")

   Output :

   This statement is raising an arithmetic exception.
   

4. exception BufferError
   This exception is raised when buffer related operations cannot be performed.
5. exception LookupError
   This is the base class for those exceptions that are raised when a key or index used on a mapping or sequence is invalid or not found. The exceptions raised are :
   • KeyError
   • IndexError

   Example :

   
   

   
   
   

   try:      a = [1, 2, 3]      print (a[3])  except LookupError:      print ("Index out of bound error.") else:      print ("Success")

   Output :

   Index out of bound error.
   

Concrete exceptions

The following exceptions are the exceptions that are usually raised.

1. exception AssertionError
   An AssertionError is raised when an assert statement fails.

   Example :

   assert False, 'The assertion failed'
   

   Output :

   Traceback (most recent call last):
     File "exceptions_AssertionError.py", line 12, in 
       assert False, 'The assertion failed'
   AssertionError: The assertion failed
   

2. exception AttributeError
   An AttributeError is raised when an attribute reference or assignment fails such as when a non-existent attribute is referenced.

   Example :

   
   

   
   
   

   class Attributes(object):     pass    object = Attributes() print (object.attribute)

   Output :

   Traceback (most recent call last):
     File "d912bae549a2b42953bc62da114ae7a7.py", line 5, in 
       print object.attribute
   AttributeError: 'Attributes' object has no attribute 'attribute'
   

3. exception EOFError
   An EOFError is raised when built-in functions like input() hits an end-of-file condition (EOF) without reading any data. The file methods like readline() return an empty string when they hit EOF.

   Example :

   
   

   
   
   

   while True:     data = input('Enter name : ')     print ('Hello  ', data)

   Output :

   Enter Name :Hello Aditi
   Enter Name :Traceback (most recent call last):
     File "exceptions_EOFError.py", line 13, in 
       data = raw_input('Enter name :')
   EOFError: EOF when reading a line
   

4. exception FloatingPointError
   A FloatingPointError is raised when a floating point operation fails. This exception is always defined, but can only be raised when Python is configured with the–with-fpectl option, or the WANT_SIGFPE_HANDLER symbol is defined in the pyconfig.h file.

   Example :

   
   

   
   
   

   import math    print (math.exp(1000))

   Output :

   Traceback (most recent call last):
     File "", line 1, in 
   FloatingPointError: in math_1
   

5. exception GeneratorExit
   This exception directly inherits from BaseException instead of Exception since it is technically not an error. A GeneratorExit exception is raised when a generator or coroutine is closed.

   Example :

   
   

   
   
   

   def my_generator():     try:         for i in range(5):             print ('Yielding', i)             yield i     except GeneratorExit:         print ('Exiting early')    g = my_generator() print (g.next()) g.close()

   Output :

   Yielding 0
   0
   Exiting early
   

6. exception ImportError
   An ImportError is raised when the import statement is unable to load a module or when the “from list” in from … import has a name that cannot be found.

   Example :

   
   

   
   
   

   import module_does_not_exist

   Output :

   Traceback (most recent call last):
     File "exceptions_ImportError_nomodule.py", line 12, in 
       import module_does_not_exist
   ImportError: No module named module_does_not_exist
   
   

   Example :

   
   

   
   
   

   from exceptions import Userexception

   Output :

   Traceback (most recent call last):
     File "exceptions_ImportError_missingname.py", line 12, in 
       from exceptions import Userexception
   ImportError: cannot import name Userexception
   
   

7. exception ModuleNotFoundError
   This is the subclass of ImportError which is raised by import when a module could not be found. It is also raised when None is found in sys.modules.
8. exception IndexError
   An IndexError is raised when a sequence is referenced which is out of range.

   Example :

   
   

   
   
   

   array = [ 0, 1, 2 ] print (array[3])

   Output :

   Traceback (most recent call last):
     File "exceptions_IndexError.py", line 13, in 
       print array[3]
   IndexError: list index out of range
   

9. exception KeyError
   A KeyError is raised when a mapping key is not found in the set of existing keys.

   Example :

   
   

   
   
   

   array = { 'a':1, 'b':2 } print (array['c'])

   Output :

   Traceback (most recent call last):
     File "exceptions_KeyError.py", line 13, in 
       print array['c']
   KeyError: 'c'
   

10. exception KeyboardInterrupt
    This error is raised when the user hits the interrupt key such as Control-C or Delete.

    Example :

    
    

    
    
    

    try:     print ('Press Return or Ctrl-C:',)     ignored = input() except Exception, err:     print ('Caught exception:', err) except KeyboardInterrupt, err:     print ('Caught KeyboardInterrupt') else:     print ('No exception')

    Output :

    Press Return or Ctrl-C: ^CCaught KeyboardInterrupt
    

11. exception MemoryError
    This error is raised when an operation runs out of memory.

    Example :

    
    

    
    
    

    def fact(a):     factors = []     for i in range(1, a+1):         if a%i == 0:             factors.append(i)     return factors     num = 600851475143 print (fact(num))

    Output :

    Traceback (most recent call last):
      File "4af5c316c749aff128df20714536b8f3.py", line 9, in 
        print fact(num)
      File "4af5c316c749aff128df20714536b8f3.py", line 3, in fact
        for i in range(1, a+1):
    MemoryError
    

12. exception NameError
    This error is raised when a local or global name is not found. For example, an unqualified variable name.

    Example :

    
    

    
    
    

    def func():     print ans    func()

    Output :

    Traceback (most recent call last):
      File "cfba0a5196b05397e0a23b1b5b8c7e19.py", line 4, in 
        func()
      File "cfba0a5196b05397e0a23b1b5b8c7e19.py", line 2, in func
        print ans
    NameError: global name 'ans' is not defined
    

13. exception NotImplementedError
    This exception is derived from RuntimeError. Abstract methods in user defined classed should raise this exception when the derived classes override the method.

    Example :

    
    

    
    
    

    class BaseClass(object):     """Defines the interface"""     def __init__(self):         super(BaseClass, self).__init__()     def do_something(self):         """The interface, not implemented"""         raise NotImplementedError(self.__class__.__name__ + '.do_something')    class SubClass(BaseClass):     """Implements the interface"""     def do_something(self):         """really does something"""         print (self.__class__.__name__ + ' doing something!')    SubClass().do_something() BaseClass().do_something()

    Output :

    Traceback (most recent call last):
      File "b32fc445850cbc23cd2f081ba1c1d60b.py", line 16, in 
        BaseClass().do_something()
      File "b32fc445850cbc23cd2f081ba1c1d60b.py", line 7, in do_something
        raise NotImplementedError(self.__class__.__name__ + '.do_something')
    NotImplementedError: BaseClass.do_something
    

14. exception OSError([arg])
    The OSError exception is raised when a system function returns a system-related error, including I/O failures such as “file not found” or “disk full” errors.

    Example :

    
    

    
    
    

    def func():     print (ans)    func()

    Output :

    Traceback (most recent call last):
      File "442eccd7535a2704adbe372cb731fc0f.py", line 4, in 
        print i, os.ttyname(i)
    OSError: [Errno 25] Inappropriate ioctl for device
    

15. exception OverflowError
    The OverflowError is raised when the result of an arithmetic operation is out of range. Integers raise MemoryError instead of OverflowError. OverflowError is sometimes raised for integers that are outside a required range. Floating point operations are not checked because of the lack of standardization of floating point exception handling in C.

    Example :

    
    

    
    
    

    import sys    print ('Regular integer: (maxint=%s)' % sys.maxint) try:     i = sys.maxint * 3     print ('No overflow for ', type(i), 'i =', i) except OverflowError, err:     print ('Overflowed at ', i, err)    print() print ('Long integer:') for i in range(0, 100, 10):     print ('%2d' % i, 2L ** i)    print() print ('Floating point values:') try:     f = 2.0**i     for i in range(100):         print (i, f)         f = f ** 2 except OverflowError, err:     print ('Overflowed after ', f, err)

    Output :

    Regular integer: (maxint=9223372036854775807)
    No overflow for   i = 27670116110564327421
    
    Long integer:
     0 1
    10 1024
    20 1048576
    30 1073741824
    40 1099511627776
    50 1125899906842624
    60 1152921504606846976
    70 1180591620717411303424
    80 1208925819614629174706176
    90 1237940039285380274899124224
    
    Floating point values:
    0 1.23794003929e+27
    1 1.53249554087e+54
    2 2.34854258277e+108
    3 5.5156522631e+216
    Overflowed after  5.5156522631e+216 (34, 'Numerical result out of range')
    
    

16. exception RecursionError
    The RecursionError is derived from the RuntimeError. This exception is raised when the interpreter detects that the maximum recursion depth is exceeded.
17. exception ReferenceError
    The ReferenceError is raised when a weak reference proxy is used to access an attribute of the referent after the garbage collection.

    Example :

    
    

    
    
    

    import gc import weakref    class Foo(object):        def __init__(self, name):         self.name = name            def __del__(self):         print ('(Deleting %s)' % self)    obj = Foo('obj') p = weakref.proxy(obj)    print ('BEFORE:', p.name) obj = None print ('AFTER:', p.name)

    Output :

    BEFORE: obj
    (Deleting )
    AFTER:
    
    Traceback (most recent call last):
      File "49d0c29d8fe607b862c02f4e1cb6c756.py", line 17, in 
        print 'AFTER:', p.name
    ReferenceError: weakly-referenced object no longer exists
    

18. exception RuntimeError
    The RuntimeError is raised when no other exception applies. It returns a string indicating what precisely went wrong.
19. exception StopIteration
    The StopIteration error is raised by built-in function next() and an iterator‘s __next__() method to signal that all items are produced by the iterator.

    Example :

    
    

    
    
    

    Arr = [3, 1, 2] i=iter(Arr)    print (i) print (i.next()) print (i.next()) print (i.next()) print (i.next())

    Output :

    
    3
    1
    2
    
    Traceback (most recent call last):
      File "2136fa9a620e14f8436bb60d5395cc5b.py", line 8, in 
        print i.next()
    StopIteration
    

20. exception SyntaxError
    The SyntaxError is raised when the parser encounters a syntax error. A syntax error may occur in an import statement or while calling the built-in functions exec() or eval(), or when reading the initial script or standard input.

    Example :

    
    

    
    
    

    try:     print (eval('Lazyroar for Lazyroar')) except SyntaxError, err:     print ('Syntax error %s (%s-%s): %s' % \         (err.filename, err.lineno, err.offset, err.text))     print (err)

    Output :

    Syntax error  (1-9): Lazyroar for Lazyroar
    invalid syntax (, line 1)
    

21. exception SystemError
    The SystemError is raised when the interpreter finds an internal error. The associated value is a string indicating what went wrong.
22. exception SystemExit
    The SystemExit is raised when sys.exit() function is called. A call to sys.exit() is translated into an exception to execute clean-up handlers (finally clauses of try statements) and to debug a script without running the risk of losing control.
23. exception TypeError
    TypeError is raised when an operation or function is applied to an object of inappropriate type. This exception returns a string giving details about the type mismatch.

    Example :

    
    

    
    
    

    arr = ('tuple', ) + 'string' print (arr)

    Output :

    Traceback (most recent call last):
      File "30238c120c0868eba7e13a06c0b1b1e4.py", line 1, in 
        arr = ('tuple', ) + 'string'
    TypeError: can only concatenate tuple (not "str") to tuple
    

24. exception UnboundLocalError
    UnboundLocalError is a subclass of NameError which is raised when a reference is made to a local variable in a function or method, but no value has been assigned to that variable.

    Example :

    
    

    
    
    

    def global_name_error():     print (unknown_global_name)    def unbound_local():     local_val = local_val + 1     print (local_val)    try:     global_name_error() except NameError, err:     print ('Global name error:', err)    try:     unbound_local() except UnboundLocalError, err:     print ('Local name error:', err)

    Output :

    Global name error: global name 'unknown_global_name' is not defined
    Local name error: local variable 'local_val' referenced before assignment
    

25. exception UnicodeError
    This exception is a subclass of ValueError. UnicodeError is raised when a Unicode-related encoding or decoding error occurs.
26. exception ValueError
    A ValueError is raised when a built-in operation or function receives an argument that has the right type but an invalid value.

    Example :

    
    

    
    
    

    print (int('a'))

    Output :

    Traceback (most recent call last):
      File "44f00efda935715a3c5468d899080381.py", line 1, in 
        print int('a')
    ValueError: invalid literal for int() with base 10: 'a'
    

27. exception ZeroDivisionError
    A ZeroDivisionError is raised when the second argument of a division or modulo operation is zero. This exception returns a string indicating the type of the operands and the operation.

    Example :

    
    

    
    
    

    print (1/0)

    Output :

    Traceback (most recent call last):
      File "c31d9626b41e53d170a78eac7d98cb85.py", line 1, in 
        print 1/0
    ZeroDivisionError: integer division or modulo by zero
    

This article is contributed by Aditi Gupta. If you like Lazyroar and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the Lazyroar main page and help other Geeks.

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