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Python: Random class instantiation on run-time?

post #1 of 5
Thread Starter 
Ok, I know what I want to do, but I'm not sure of the code to do it. I'm at the planning stage, so only pseudocode at the moment I'm afraid. I've got a number of class definitions, let's say 5 - Alpha, Bravo, Charlie, Delta, Echo. I'll put the names in a list. At run-time, I want a random function to select 3 of them, and create objects based on those classes. Now, how do I instantiate an object, by passing the class name from a list rather than explicitly naming it in the code?

Also, all of those classes represent the same type of thing, but with different attribute values. Is it better to use a superclass, then write in the attributes at run-time? Not sure where I'd store these values if not in a class definition - in a method of the superclass maybe? It'd be a massive class then
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post #2 of 5
Problem solved.

Code:
#! /usr/bin/python
import random

class alpha():
    def __init__(self):
        print "alpha"

class bravo():
    def __init__(self):
        print "bravo"

class charlie():
    def __init__(self):
        print "charlie"

class delta():
    def __init__(self):
        print "delta"

class echo():
    def __init__(self):
        print "echo"

def main():
    class_list = [alpha, bravo, charlie, delta, echo]
    random.seed()
    i=0;
    while(i<10):
        class_list[random.randint(0,len(class_list) - 1)]()
        i += 1

if __name__ == "__main__":
    main()
Code:
chris@dev:~$ ./sample.py
delta
charlie
charlie
delta
alpha
delta
bravo
delta
charlie
echo
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post #3 of 5
Thread Starter 
Hold on, that wouldn't create objects, in fact I can't see how that would work, as the class isn't callable without instantiating first....

But I like the thinking, I think it's something like that. I'd be removing them from the list so they don't get reselected.
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post #4 of 5
Quote:
Originally Posted by chemicalfan View Post
Hold on, that wouldn't create objects, in fact I can't see how that would work, as the class isn't callable without instantiating first....

But I like the thinking, I think it's something like that. I'd be removing them from the list so they don't get reselected.
Take a closer look at the Python docs. The __init__() method is (loosely) comparable to a constructor.

If you call the class name directly, you're constructing an object of that type, and arguments you specify get passed to the __init__() method.

In the example, an object is being created on each loop. I'm not actually doing anything with that object, but they're being created. Otherwise, you'd never see those print outs.

If you don't believe me, try running it with this slight modification. It still calls the class name, but stores the returned object into a variable called temp_obj. Then calls a second method from the class using that temp_obj.

Code:
#! /usr/bin/python
import random

class alpha():
    def __init__(self):
        self.name =  "alpha"
    def print_name(self):
        print self.name

class bravo():
    def __init__(self):
        self.name = "bravo"
    def print_name(self):
        print self.name

class charlie():
    def __init__(self):
        self.name = "charlie"
    def print_name(self):
        print self.name

class delta():
    def __init__(self):
        self.name = "delta"
    def print_name(self):
        print self.name

class echo():
    def __init__(self):
        self.name = "echo"
    def print_name(self):
        print self.name

def main():
    class_list = [alpha, bravo, charlie, delta, echo]
    random.seed()
    i=0;
    while(i<10):
        temp_obj = class_list[random.randint(0,len(class_list) - 1)]()
        temp_obj.print_name()
        i += 1

if __name__ == "__main__":
    main()
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post #5 of 5
Thread Starter 
The temp_obj way is the way I've always done object creation:

alphaobj = Alpha()

I just didn't realise you could do the "class_list[random.randint(0,len(class_list) - 1)]()" bit to select the class name
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