Where is CASE?????

Duncan Booth duncan at rcp.co.uk
Tue Feb 20 16:00:48 CET 2001


"Chris Richard Adams" <chrisa at ASPATECH.COM.BR> wrote in 
<mailman.982674921.12321.python-list at python.org>:

>Someone please tell me why I cannot find a CASE statement in the Python
>documentation index...
>
>There is a CASE isn't there???
>

While there are undoubtedly occasions where a CASE statement would be 
useful, they are actually much rarer in Python than you might expect. A 
large proportion of the instances where you might use CASE can be written 
better as a dictionary or an attribute lookup.

For example in most languages if you were to try to handle errors from 
opening a web URL, you might reasonably expect to find a case statement 
switching on the different error codes you could receive. If you look at the 
urllib module you will see that the Pythonic way to do this is to have class 
methods with names such as http_error_401, http_error_301, and 
http_error_default. When an error code is returned, the appropriate method 
is called (or the default method if there is no better match) by using 
getattr to look up the method by name. This technique allows the end user to 
add their own handler for any error number, even those the original class 
knows nothing about.

Another common use for CASE in some languages is when you have created a 
structure and put a type field in it. You want to process the object 
according to the type field. In any object oriented language (not just 
Python), this situation is better handled by including a method in the 
object that does the appropriate action for the object's type.

If you post a message outlining the situation where you think you need a 
CASE statement you might get suggestions of better (or just different) ways 
to solve your particular problem.



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