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Polymoprhism question in python

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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?

posted May 24, 2013 by anonymous

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1 Answer

+1 vote
Best answer

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):

class CClass(AClass):

# 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
answer May 24, 2013 by anonymous
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