If you anticipate that executing a particular statement may
cause an exception and you don't want your program to
terminate and display a traceback, you can use a
try construct to specify handlers to be executed
if an exception occurs. For details, see Section 23.8, “The
try statement: Anticipate
If an exception occurs inside a function and it is not
handled at that level by a
Python will work back through the pending function calls
until it either finds a handler for that exception or
runs out of pending function calls.
If there are multiple handlers for the exception in calling functions, the innermost will be used. If there are no handlers for the exception in calling functions, you will get a stack traceback and the program will terminate.
>>> def f(): ... try: ... g() ... except ValueError, detail: ... print "Caught a ValueError:", detail.message ... >>> def g(): h() ... >>> def h(): ... raise ValueError('This is a test.') ... >>> f() Caught a ValueError: This is a test.
In the example above, function
g(), which in turn calls function
h() raises a
ValueError exception, but there is no
try: block around it. Python looks to see
if there is a
ValueError handler in
g(), but there is not. Finally a handler for
ValueError is found inside function
f(), so control resumes inside that handler.
Note that no stack traceback is displayed, because the
ValueError exception was handled