Chapter 6. Exceptions

Python uses exceptions to communicate errors and anomalies. An exception is an object that indicates an error or anomalous condition. When Python detects an error, it raises an exceptionthat is, Python signals the occurrence of an anomalous condition by passing an exception object to the exception-propagation mechanism. Your code can explicitly raise an exception by executing a raise statement.

Handling an exception means receiving the exception object from the propagation mechanism and performing whatever actions are needed to deal with the anomalous situation. If a program does not handle an exception, the program terminates with an error traceback message. However, a program can handle exceptions and keep running despite errors or other abnormal conditions.

Python also uses exceptions to indicate some special situations that are not errors, and are not even abnormal. For example, as covered in "Iterators" on page 65, an iterator's next method raises the exception StopIteration when the iterator has no more items. This is not an error, and it is not even an anomaly since most iterators run out of items eventually. The optimal strategies for checking and handling errors and other special situations in Python are therefore different from what might be best in other languages, and I cover such considerations in "Error-Checking Strategies" on page 134. This chapter also covers the logging module of the Python standard library, in "Logging Errors" on page 136, and the assert Python statement, in "The assert Statement" on page 138.