Questioning pythonic type checking
- by Pace
I've seen countless times the following approach suggested for "taking in a collection of objects and doing X if X is a Y and ignoring the object otherwise"
def quackAllDucks(ducks):
for duck in ducks:
try:
duck.quack("QUACK")
except AttributeError:
#Not a duck, can't quack, don't worry about it
pass
The alternative implementation below always gets flak for the performance hit caused by type checking
def quackAllDucks(ducks):
for duck in ducks:
if hasattr(duck,"quack"):
duck.quack("QUACK")
However, it seems to me that in 99% of scenarios you would want to use the second solution because of the following:
If the user gets the parameters wrong then they will not be treated like a duck and there will be no indication. A lot of time will be wasted debugging why there is no quacking going on until the user finally realizes his silly mistake. The second solution would throw a stack trace as soon the user tried to quack.
If the user has any bugs in their quack() method which cause an AttributeError then those bugs will be silently swallowed. Once again time will be wasted digging for the bug when the second solution would simply give a stack trace showing the immediate issue.
In fact, it seems to me that the only time you would ever want to use the first method is when:
The block of code in question is in an extremely performance critical section of your application. Following the principal of "avoid premature optimization" you would only realize this of course, after you had implemented the safer approach and found it to be a bottleneck.
There are many types of quacking objects out there and you are only interested in quacking objects that quack with a very specific set of arguments (this seems to be a very rare case to me).
Given this, why is it that so many people prefer the first approach over the second approach? What is it that I am missing?
Also, I realize there are other solutions (such as using abcs) but these are the two solutions I seem to see most often for the basic case.
