Common Built-in Exceptions

Книга: Practical Programming, Fourth Edition

Python provides a rich set of built-in exception types (71 at the time of writing; see ). All of them are subclasses of the built-in class BaseException (subclasses are explained in the section ), but using except BaseException itself is generally a bad idea. If necessary, go for except Exception if you need a catch-all and need access to the details.

To retrieve the list of built-in exceptions for your current Python version, start by importing the built-in module builtins. The module has a dictionary of its contents, accessible as builtins.__dict__, where keys are item names and values are the corresponding objects. Iterate through the dictionary and select only those items that are instances of the built-in class type and subclasses of BaseException, using the built-in functions isinstance and issubclass, respectively.

	>>> import builtins
	>>> exceptions = sorted(name for name, value in builtins.__dict__.items()
	... if isinstance(value, type) and issubclass(value, BaseException))
	>>> len(exceptions)
	71

The following table lists the most frequently used built-in exceptions. An asterisk (*) next to an exception name indicates that the exception has not appeared in examples earlier in the book.

Exception Name	What Happened?
AssertionError	An assertion you made failed.
*FileExistsError	The file or directory you tried to create already exists.
FileNotFoundError	The file you tried to access does not exist.
*ImportError	The module you attempted to import was not imported successfully.
IndentationError	Your code is incorrectly indented (for example, you mixed spaces and tabs).
IndexError	You tried to access a list or tuple element with a nonexistent index.
KeyError	You tried to access a dictionary item with a nonexistent key.
*KeyboardInterrupt	You pressed Ctrl-C (or a similar key chord) to interrupt the program in the console window.
*ModuleNotFoundError	The module you tried to import does not exist in the system path (sys.path, a list of directories to search).
NameError	You referred to an undefined variable or function.
*OSError	The operating system failed to complete the operation you requested (for example, file handling, networking, or process management).
PermissionError	You attempted to access a file or resource without sufficient permissions.
StopIteration	Your generator has yielded all of its values.
SyntaxError	Your program contains a syntax error.
TypeError	You provided a value of the wrong type (for example, adding a string to a number).
ValueError	You passed an argument of the correct type but with an inappropriate value.
ZeroDivisionError	You attempted to divide by zero.

       Raise the most appropriate exception and include helpful context       to make errors easier to understand and fix     

A well-designed function should raise appropriate exceptions to signal different error conditions. A well-written program, in turn, should handle these exceptions gracefully.

In the following sections, you’ll learn how to raise your own exceptions and define custom exception types to make your programs more robust and expressive.

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