Python in a Nutshell, 2nd Edition

Exception Propagation

When an exception is raised, the exception-propagation mechanism takes control. The normal control flow of the program stops, and Python looks for a suitable exception handler. Python’s try statement establishes exception handlers via its except clauses. The handlers deal with exceptions raised in the body of the try clause, as well as exceptions propagating from any of the functions called by that code, directly or indirectly. If an exception is raised within a try clause that has an applicable except handler, the try clause terminates and the handler executes. When the handler finishes, execution continues with the statement after the try statement.

If the statement raising the exception is not within a try clause that has an applicable handler, the function containing the statement terminates, and the exception propagates “upward” along the stack of function calls to the statement that called the function. If the call to the terminated function is within a try clause that has an applicable handler, that try clause terminates, and the handler executes. Otherwise, the function containing the call terminates, and the propagation process repeats, unwinding the stack of function calls until an applicable handler is found.

If Python cannot find any applicable handler, by default the program prints an error message to the standard error stream (the file sys.stderr). The error message includes a traceback that gives details about functions terminated during propagation. ...