How to decide creating a child class without using 'if' statement

Question

class BaseCls:
    def foo(self):
        print("BaseCls")

class ChildClsA(BaseCls):
    def foo(self):
        print("ChildClsA")

class ClildClsB(BaseCls):
    def foo(self):
        print("ChildClsB")



inputStr=raw_input("press A or B\n")

if(inputStr=="A"):
    obj=ChildClsA()
if(inputStr=="B"):
    obj=ClildClsB()

obj.foo()

'if' statement can deal with this situation. However, how to decide creating a child class without using 'if' statement When I have more than one hundred children of BaseCls,

One hundred children? Sounds like a design in distress to me. — duffymo, Aug 21 '13 at 09:33

score 2 · Answer 1 · answered Aug 21 '13 at 09:38

2

You can store your classes in a dictionary:

classes = dict(A=ChildClsA, B=ChildClsB)

obj = classes[inputStr]()

answered Aug 21 '13 at 09:38

flornquake

3,156
1
21
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score 0 · Accepted Answer · answered Aug 21 '13 at 09:42

I'd double check your design, but a simplistic way would be to create a lookup of the valid subclasses from your base:

import re
lookup = {re.search('([A-Z][A-Za-z_]*$)', cls.__name__).group(1): cls for cls in BaseCls.__subclasses__()}
# {'A': <class '__main__.ChildClsA'>, 'B': <class '__main__.ClildClsB'>}

input_str = raw_input('Please enter one of {}:'.format(', '.join(lookup)))
new_class = lookup[input_str]()

Note: use whatever criteria makes sense to extract the name for lookup purposes

Ok, this is not be most "simplistic" way, but +1 because this kicks ass and pretty Pythonic. — , Aug 21 '13 at 10:11

score 0 · Answer 3 · edited May 23 '17 at 11:44

0

Another way would be to pack the user input into a string an execute that.

input_ = raw_input( "Please enter the class name:" )
try:
    exec( "%s().foo()" % input_ )
except NameError:
    print "Invalid class name entered"

I know that exec is evil :-)

So also take a look at why-should-exec-and-eval-be-avoided

edited May 23 '17 at 11:44

Community

1
1

answered Aug 21 '13 at 09:44

Robert Caspary

1,584
9
7

score 0 · Answer 4 · answered Aug 21 '13 at 10:07

This is a perfect case for a factory pattern.

Create your child classes independently en register them with the factory. Then use what key you have to call the factory and have it instantiate the desired class for you.

The simplest factory is a dictionary but you can also build a nice factory class with additional features.

How to decide creating a child class without using 'if' statement

4 Answers4