When Python interpreter executes a program and encounters an error or exception, it usually stops and throws error message. Handling such situation is termed as error handling or exception handling and Python has three keywords for this purpose:
In the below example, x is not defined anywhere in the program, which raises an error. As the error occurred in try block of statement, it will be handled by its except block of statement. Without try block, the program will stop immediately after an error and throw error message.
try: statements except: statements
try: x = x + 1 print(x) except: print("An error occurred.")
An error occurred.
A program can have multiple except blocks to handle each type of exception differently.
try: statements except errortype_1: statements except errortype_2: statements ... ... except: statements
try: x = 1 y = 0 x = x / y print(x) except ZeroDivisionError: print("Zero Division Error") except (ValueError, TypeError): print("Value/Type Error") except: print("An error occurred.")
Zero Division Error
else block statement can also be added with try-except blocks which is executed when no error has occurred.
try: statements except: statements else: statements
try: x = 10 y = 2 x = x / y print(x) except ZeroDivisionError: print("Zero Division Error") except: print("An error occurred.") else: print("No error has occurred.")
5.0 No error has occurred.
In the below example, finally block of statement is executed regardless of try-except block results, which facilitates the program to close the file before proceeding further.
try: statements except: statements else: statements finally: statements
MyFile = None try: MyFile = open("math_example.txt") x = MyFile.readline() y = MyFile.readline() x = x/y print(x) except: print("An error occurred.") finally: if MyFile == None: print("MyFile does not exist.") else: print("MyFile has been closed.") MyFile.close()
An error occurred. MyFile does not exist.
|AssertionError||Raised when the assert statement fails.|
|AttributeError||Raised on the attribute assignment or reference fails.|
|EOFError||Raised when the input() function hits the end-of-file condition.|
|FloatingPointError||Raised when a floating point operation fails.|
|GeneratorExit||Raised 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 the local or global scope.|
|NotImplementedError||Raised by abstract methods.|
|OSError||Raised when a system operation causes a 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 the next() function to indicate that there is no further item to be returned by the iterator.|
|SyntaxError||Raised by the parser when a syntax error is encountered.|
|IndentationError||Raised when there is an incorrect indentation.|
|TabError||Raised when the indentation consists of inconsistent tabs and spaces.|
|SystemError||Raised when the interpreter detects internal error.|
|SystemExit||Raised by the sys.exit() function.|
|TypeError||Raised when a function or operation is applied to an object of an 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 translation.|
|ValueError||Raised when a function gets an argument of correct type but improper value.|
|ZeroDivisionError||Raised when the second operand of a division or module operation is zero.|