Polymoprhism question in python

Question

I'm trying to figure out (or find an example) of polymorphism whereby I pass a commandline argument (a string) which comports to a class (in java, you would say that it comports to a given interface but I don't know if there is such a thing in Python) then that class of that name, somehow gets instantiated from that string. This way, I can have similar classes, but have my program use various ones by simply changing the commandline argument.

Can anyone show me how this might be done in Python?

Answer 1

I'm not 100% sure I understand what you want, but my guess is you want something like this:

# A toy class.
class AClass(object):
 def __init__(self, astring):
 self.astring = astring
 def __repr__(self):
 return "%s(%r)" % (self.__class__.__name__, self.astring)

# And some variations.
class BClass(AClass):
 pass

class CClass(AClass):
 pass

# Build a dispatch table, mapping the class name to the class itself.
TABLE = {}
for cls in (AClass, BClass, CClass):
 TABLE[cls.__name__] = cls

# Get the name of the class, and an argument, from the command line.
# Or from the user. Any source of two strings will do.
# Data validation is left as an exercise.
import sys
argv = sys.argv[1:]
if not argv:
 name = raw_input("Name of the class to use? ")
 arg = raw_input("And the argument to use? ")
 argv = [name, arg]

# Instantiate.
instance = TABLE[argv[0]](argv[1])
print instance