Can you use a string to instantiate a class?


Question

I'm using a builder pattern to seperate a bunch of different configuration possibilities. Basically, I have a bunch of classes that are named an ID (something like ID12345). These all inherit from the base builder class. In my script, I need to instantiate an instance for each class (about 50) every time this app runs. So, I'm trying to see if instead of doing something like this:

ProcessDirector = ProcessDirector()
ID12345 = ID12345()
ID01234 = ID01234()

ProcessDirector.construct(ID12345)
ProcessDirector.construct(ID01234)

ID12345.run()
ID01234.run()

Can I do something like this (I know this doesn't work):

IDS = ["ID12345", "ID01234"]

ProcessDirector = ProcessDirector()
for id in IDS:
  builder = id() #some how instantiate class from string
  ProcessDirector.construct(builder)
  builder.run()

That way, when I need to add a new one in the future, all I have to do is add the id to the IDS list, rather than peppering the new ID throughout the code.

EDIT

Looks like there are some different opinions based on where the data is coming from. These IDs are entered in a file that no one else has access to. I'm not reading the strings from the command line, and I'd like to be able to do as little alteration when adding a new ID in the future.

1
35
12/2/2018 6:50:58 PM

Accepted Answer

Not sure this is what you want but it seems like a more Pythonic way to instantiate a bunch of classes listed in a string:

class idClasses:
    class ID12345:pass
    class ID01234:pass
# could also be: import idClasses

class ProcessDirector:
    def __init__(self):
        self.allClasses = []

    def construct(self, builderName):
        targetClass = getattr(idClasses, builderName)
        instance = targetClass()
        self.allClasses.append(instance)

IDS = ["ID12345", "ID01234"]

director = ProcessDirector()
for id in IDS:
    director.construct(id)

print director.allClasses
# [<__main__.ID12345 instance at 0x7d850>, <__main__.ID01234 instance at 0x7d918>]
17
12/20/2017 3:42:57 PM

If you wanted to avoid an eval(), you could just do:

id = "1234asdf"
constructor = globals()[id]
instance = constructor()

Provided that the class is defined in (or imported into) your current scope.


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