2. Standard Exceptions¶

Good exception handling involves both creating exceptions for special cases unique to your program and then catching and handling these with try: except: blocks and catching and handling standard exceptions that you think may be triggered by your program. For example, if you know it is possible that a user may enter a zero as input for a variable used as a divisor, and you want to change the flow of control in response to this, then you’ll want to make a try: except: block that handles a ZeroDivisionError. Note that with standard exceptions, you do not need to use raise. E.g., you would never write code to raise a ZeroDivisionError; it will be raised automatically in the matching situation.

Standard exceptions are exceptions that are built into Python; most of them are listed below. They are organized into related groups based on the types of issues they deal with. Some of these may not yet be familiar to you, but this will be a handy reference as you continue learning Python.

Language Exceptions Description
StandardError Base class for all built-in exceptions except StopIteration and SystemExit.
ImportError Raised when an import statement fails.
SyntaxError Raised when there is an error in Python syntax.
IndentationError Raised when indentation is not specified properly.
NameError Raised when an identifier is not found in the local or global namespace.
UnboundLocalError Raised when trying to access a local variable in a function or method but no value has been assigned to it.
TypeError Raised when an operation or function is attempted that is invalid for the specified data type.
LookupError Base class for all lookup errors.
KeyError Raised when the specified key is not found in the dictionary.
ValueError Raised when the built-in function for a data type has the valid type of arguments, but the arguments have invalid values specified.
RuntimeError Raised when a generated error does not fall into any category.
MemoryError Raised when a operation runs our of memory.
RecursionError Raised when the maximum recursion depth has been exceeded.
SystemError Raised when the interpreter finds an internal problem, but when this error is encountered the Python interpreter does not exit.
Math Exceptions Description
ArithmeticError Base class for all errors that occur for numeric calculation. You know a math error occurred, but you don’t know the specific error.
OverflowError Raised when a calculation exceeds maximum limit for a numeric type.
FloatingPointError Raised when a floating point calculation fails.
ZeroDivisionError Raised when division or modulo by zero takes place for all numeric types.
I/O Exceptions Description
FileNotFoundError Raised when a file or directory is requested but doesn’t exist.
IOError Raised when an input/ output operation fails, such as the print statement or the open() function when trying to open a file that does not exist. Also raised for operating system-related errors.
PermissionError Raised when trying to run an operation without the adequate access rights.
EOFError Raised when there is no input from either the raw_input() or input() function and the end of file is reached.
KeyboardInterrupt Raised when the user interrupts program execution, usually by pressing Ctrl+c.
Other Exceptions Description
Exception Base class for all exceptions. This catches most exception messages.
StopIteration Raised when the next() method of an iterator does not point to any object.
AssertionError Raised in case of failure of the Assert statement.
SystemExit Raised when Python interpreter is quit by using the sys.exit() function. If not handled in the code, it causes the interpreter to exit.
OSError Raises for operating system related errors.
EnvironmentError Base class for all exceptions that occur outside the Python environment.
AttributeError Raised in case of failure of an attribute reference or assignment.
NotImplementedError Raised when an abstract method that needs to be implemented in an inherited class is not actually implemented.

All exceptions are objects. The classes that define the objects are organized in a hierarchy, which is shown below. This is important because the parent class of a set of related exceptions will catch all exception messages for itself and its child exceptions. For example, an ArithmeticError exception will catch itself and all FloatingPointError, OverflowError, and ZeroDivisionError exceptions.

BaseException
+-- SystemExit
+-- KeyboardInterrupt
+-- GeneratorExit
+-- Exception
+-- StopIteration
+-- StopAsyncIteration
+-- ArithmeticError
|    +-- FloatingPointError
|    +-- OverflowError
|    +-- ZeroDivisionError
+-- AssertionError
+-- AttributeError
+-- BufferError
+-- EOFError
+-- ImportError
+-- LookupError
|    +-- IndexError
|    +-- KeyError
+-- MemoryError
+-- NameError
|    +-- UnboundLocalError
+-- OSError
|    +-- BlockingIOError
|    +-- ChildProcessError
|    +-- ConnectionError
|    |    +-- BrokenPipeError
|    |    +-- ConnectionAbortedError
|    |    +-- ConnectionRefusedError
|    |    +-- ConnectionResetError
|    +-- FileExistsError
|    +-- FileNotFoundError
|    +-- InterruptedError
|    +-- PermissionError
|    +-- ProcessLookupError
|    +-- TimeoutError
+-- ReferenceError
+-- RuntimeError
|    +-- NotImplementedError
|    +-- RecursionError
+-- SyntaxError
|    +-- IndentationError
|         +-- TabError
+-- SystemError
+-- TypeError
+-- ValueError
|    +-- UnicodeError
|         +-- UnicodeDecodeError
|         +-- UnicodeEncodeError
|         +-- UnicodeTranslateError
+-- Warning
+-- DeprecationWarning
+-- PendingDeprecationWarning
+-- RuntimeWarning
+-- SyntaxWarning
+-- UserWarning
+-- FutureWarning
+-- ImportWarning
+-- UnicodeWarning
+-- BytesWarning
+-- ResourceWarning