exception handling - python isinstance vs hasattr vs try/except: What is better?

Question

I am trying to figure out the tradeoffs between different approaches of determining whether or not with object obj you can perform action do_stuff(). As I understand, there are three ways of determining if this is possible:

# Way 1
if isinstance(obj, Foo):
    obj.do_stuff()

# Way 2
if hasattr(obj, 'do_stuff'):
    obj.do_stuff()

# Way 3
try:
    obj.do_stuff()
except:
    print 'Do something else'

Which is the preferred method (and why)?

Answer 1

I believe that the last method is generally preferred by Python coders because of a motto taught in the Python community: "Easier to ask for forgiveness than permission" (EAFP).

In a nutshell, the motto means to avoid checking if you can do something before you do it. Instead, just run the operation. If it fails, handle it appropriately.

Also, the third method has the added advantage of making it clear that the operation should work.

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With that said, you really should avoid using a bare except like that. Doing so will capture any/all exceptions, even the unrelated ones. Instead, it is best to capture exceptions specifically.

Here, you will want to capture for an AttributeError:

try:
    obj.do_stuff()   # Try to invoke do_stuff
except AttributeError:
    print 'Do something else'  # If unsuccessful, do something else