Creating different instances in a loop using a dictionary

Question

I would like a user defined a series of strings

user_input = ('cat', 'cactus', 'cat')

which correspond to a series of objects to be instantiated from a dictionary of possible objects

classes = {
    'cat': Cat,
    'cactus': Cactus,
 }

Where the Cat and Cactus correspond to classes which descend from a parent class

class Pet():
    ...

class Cat(Pet):
    def __init__(self, name, colour):
        Pet.__init__(self, name, colour)
        ...

class Cactus(Pet):
    def __init__(self, name, colour):
        Pet.__init__(self, name, colour)
        ...

I try to add object type to a list

pet_types = []
for i in range(0,3):
    try:
        pet_type.append(classes[user_input[i]])
    except:
        raise Exception('type no exist')

But when I do this the exception is raised telling me "type no exist" when the string corresponds exactly to the dictionary entry! Why is this happening?

I then want to use pet_types like this

pet_500 = pet_types[500](name,colour)

Start by not using a blanket try...except. What is the actual exception being raised? — Martijn Pieters
– Martijn Pieters, Commented Oct 8, 2017 at 20:45
Side note: don't loop over range() when you could just loop ever user_input directly. — Martijn Pieters
– Martijn Pieters, Commented Oct 8, 2017 at 20:47
I think you're looking for a factory pattern. en.wikipedia.org/wiki/Factory_method_pattern — cs95
– cs95, Commented Oct 8, 2017 at 20:48

Mureinik · Accepted Answer · 2017-10-08 20:50:46Z

3

Catching any exception (except:) is a dodgy practice, as is hiding the original exception with your own, without even logging it.

The combination of these two dodgy practices in this case hid the fact that you have a typo, and you're trying to access the undefined pet_type (singular), instead of the variable you actually have - pet_types (plural). Fix it and you should be fine.

answered Oct 8, 2017 at 20:50

Mureinik

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1 Comment

Mark Corrigan Over a year ago

Yes you're correct, that is a bit embarrassing! It works fine and without the exception being thrown. In future I won't add the custom exception!!

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