How to port "return False" to a class and "try/except" clause

Question

I am relatively new to Python and I am trying to learn a thing or two about classes. I have some experience with functions and I am trying to understand how classes work. My question is about error handling.

Normally with a function, I can perform some operations and if something goes wrong, I can simply return False and validate it where the function's been called before moving on with the rest of the code execution.

How can I do this with the classes?

I have a simple class with a simple method, say:

my_object = MyObj()
my_object.swim()

When I make my_object "swim", I want to be able to handle errors or problems, in a simple manner. For example:

During the execution of def swim(..): I want to be able to return False or raise different errors and handle them without interrupting the application completely.

I.e. I want to:

try:
    my_object.swim()
except:
    # it can not swim. I want it to tell me why or just tell me "False" so I can execute another codeblock and tell it to perhaps .walk_away()

I read about try/except blocks, raising errors, how they are classes. But it does not do what I like, which is to try performing a method and handling the results of that method's execution (true, false or another error, perhaps?)

How can I do this in Python?

Thank you.

If you prefer handling errors by returning false, why not just continue doing that? There's nothing particular about classes or OO that says you cannot return a success code from a method. — hmakholm left over Monica, Oct 27 '12 at 22:04
because I don't want to assign return of my method to a name-variable and than validate it. I want to try: except: especially since this method can generate errors with different codes, not just a positive or negative. — Phil, Oct 27 '12 at 22:05
But that doesn't seem to have anything in particular to do with the fact that swim() is a method rather than a plain old function. — hmakholm left over Monica, Oct 27 '12 at 22:06

Thomas Orozco · Accepted Answer · 2012-10-27T22:29:14.583

You would do the following:

class MayNotSwimException(Exception):
    def __init__(reason):
        self.reason = reason

class Person(object):
    def __init__(self, can_swim):
        self.can_swin = can_swim

    def swim(self):
        if self.can_swim:
            self.do_something()
        else:
            raise MayNotSwimException("I can't swim because...!")

    def walk_away(self):
        self.do_something_else()



p = Person(can_swim = False)
try:
    p.swim():
except MayNotSwimException as e: # In python 2.5-, use MayNotSwimException, e 
    print "That person can't swim because", e.reason
    p.walk_away()

Exceptions can be used to:

Alter the flow of control (jump back to a try/except block)
Transfer information through the exception itself. This can be achieved by raising different types of Exceptions (ie. We could define a MayNotSwimBecauseDoesntWantException and a MayNotSwimBecauseFeelsSleepyException and catch them separately) or through attributes of the Exception object itself (e.reason).

Please have a look at the documentation for a more detailed explanation!

[Shouldn't that be `except MayNotSwimException as e`](http://stackoverflow.com/q/2535760/102441)? — Eric, Oct 27 '12 at 22:24
@Eric Well, `, e` is valid in python 2.x, but you're right in saying that it's not python's greatest achievement in syntax clarity. I'll update! — Thomas Orozco, Oct 27 '12 at 22:28

score 2 · Answer 2 · answered Oct 27 '12 at 22:00

2

Put the try..except or other form of checking for failure (and the reaction code) in the swim method itself. Exceptions need not be fatal, but I suggest you limit their use where normal conditionals can be used, to keep the code clean.

answered Oct 27 '12 at 22:00

lynxlynxlynx

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How to port "return False" to a class and "try/except" clause

2 Answers2