Exceptions

• Exceptions are errors detected during execution.
• They are objects that are raised (or thrown) by code that encounters an unexpected circumstance.
• Most exceptions are not handled by programs.
• The last line of the error message indicates what happened.
• All exceptions must be instances of a class that derives from BaseException. try-except handles any exception classes derived from that class.

BaseException

+– SystemExit is raised by sys.exit(). +– KeyboardInterrupt is raised when the user hits the interrupt key (normally ctrl+c or del). During execution, a check for interrupts is made regularly. +– GeneratorExit is raised when a generator or coroutine is closed. +– Exception +– StopIteration is raised by next() and iterator’s next() to signal there are no further items produced by the iterator. +– StopAsyncIteration is raised by anext() method of an asynchronous iterator object to stop the iteration. +– ArithmeticError | +– FloatingPointError is not used currently. | +– OverflowError is raised when the result of an arithmetic operation is too large to be represented. | +– ZeroDivisionError is raised when the second argument of a division or modulo operation is zero. +– AssertionError is raised when an assert statement fails. +– AttributeError is raised when an attribute reference or assignment fails. +– BufferError is raised when a buffer-related operation cannot be performed. +– EOFError is raised when the input() function hits an EOF condition without reading any data. +– ImportError is raised when the import statement has trouble trying to load a module. | +– ModuleNotFoundError is raised when a module could not be located. +– LookupError is raised when a key or index used on a mapping or sequence is invalid. | +– IndexError is raised when a sequence subscript is out of range. | +– KeyError is raised when a mapping key is not found in the set of existing keys. +– MemoryError is raised when an operation runs out of memory. +– NameError is raised when a local or global name is not found. | +– UnboundLocalError is raised when a reference is made to a local variable in a function or method, but no value has been bound to that variable. +– OSError is raised when a system function returns a system-related error. | +– BlockingIOError is raised when an operation would block an object set for a non-blocking operation. | +– ChildProcessError is raised when an operation on a child process failed. | +– ConnectionError | | +– BrokenPipeError is raised when trying to write on a pipe while the other end has been closed, or trying to write on a socket which has been shut down for writing. | | +– ConnectionAbortedError is raised when a connection attempt is aborted by the peer. | | +– ConnectionRefusedError is raised when a connection attempt is refused by the peer. | | +– ConnectionResetError is raised when a connection is reset by the peer. | +– FileExistsError is raised when trying to create a file or directory which already exists. | +– FileNotFoundError is raised when a file or directory is requested but doesn’t exist. | +– InterruptedError is raised when a system call is interrupted by an incoming signal. | +– IsADirectoryError is raised when a file operation is requested on a directory. | +– NotADirectoryError is raised when a directory operation is requested on something which is not a directory. | +– PermissionError is raised when trying to run an operation without adequate access rights. | +– ProcessLookupError is raised when a given process doesn’t exist. | +– TimeoutError is raised when a system function timed out at the system level. +– ReferenceError is raised when a weak reference proxy is used to access an attribute of the referent after it has been garbage collected. +– RuntimeError is raised when an error is detected that doesn’t fall in any of the other categories. | +– NotImplementedError should be raised in user-defined base classes by abstract methods when they require derived classes to override the method, or while the class is being developed to indicate that the real implementation still needs to be added. | +– RecursionError is raised when the interpreter detects the maximum recursion depth. +– SyntaxError is raised when the parser encounters a syntax error. | +– IndentationError is raised for incorrect indentation. | +– TabError is raised for inconsistent use of tabs and spaces. +– SystemError is raised when the interpreter finds an internal error, but the situation doesn’t look so serious to cause it to abandon all hope. +– TypeError is raised when an operation or function is applied to an object of an inappropriate type. +– ValueError is raised when an operation or function receives an argument that has the right type but an inappropriate value. | +– UnicodeError is raised when a Unicode-related encoding or decoding error occurs. | +– UnicodeDecodeError is raised when a Unicode-related error occurs during decoding. | +– UnicodeEncodeError is raised when a Unicode-related error occurs during encoding. | +– UnicodeTranslateError is raised when a Unicode-related error occurs during translating. +– Warning +– DeprecationWarning +– PendingDeprecationWarning +– RuntimeWarning +– SyntaxWarning +– UserWarning +– FutureWarning +– ImportWarning +– UnicodeWarning +– BytesWarning +– ResourceWarning

Ref 1 , Ref 2 , Ref 3

Exception handling

• If an exception occurs which does not match the exception named in the except clause, it is passed on to outer try statements; if no handler is found, it is an unhandled exception and execution stops with a message.

• A try statement may have more than one except clause to specify handlers for different exceptions.

  • i.e.,
    • except OSError as err:
    • except (RuntimeError, TypeError, NameError):

• except: may omit the exception name to serve as a wildcard.

• else is optional for try…except statement. try: … except OSError: … else:

Ref

Exception chaining

try:
  ...
except IOerror as exc:
  raise RuntimeError('...') from exc

Ref

Raising exceptions

• It allows us to force a specified exception to occur.
• i.e., raise NameError(‘HiThere’)

Ref

User-defined exceptions

• Exceptions should typically be derived from the Exception class.

Ref

Broken Pipe Error

• A subclass of ConnectionError, raised when trying to write on a pipe while the other end has been closed, or trying to write on a socket which has been shut down for writing.

Errors

There are (at least) two distinguishable kinds of errors:

1. Syntax errors
2. Exceptions

Ref1: colorful terminal , Ref2

Syntax errors

• a.k.a. parsing errors
• File name and line number are printed so you know where to look in case the input came from a script.