PYTHON

cls in python

For classmethods, the first parameter is the class through which the class method is invoked with instead of the usual self for instancemethods (which all methods in a class implicitly are unless specified otherwise).

Here's an example -- and for the sake of exercise, I added an exception that checks the identity of the cls parameter.

class Base(object):
    @classmethod
    def acquire(cls, param):
        if cls is Base:
            raise Exception("Must be called via subclass :(")
        return "this is the result of `acquire`ing a %r with %r" % (cls, param)

class Something(Base):
    pass

class AnotherThing(Base):
    pass

print Something.acquire("example")
print AnotherThing.acquire("another example")
print Base.acquire("this will crash")
this is the result of `acquire`ing a <class '__main__.Something'> with 'example'
this is the result of `acquire`ing a <class '__main__.AnotherThing'> with 'another example'
Traceback (most recent call last):
  File "classmethod.py", line 16, in <module>
    print Base.acquire("this will crash")
  File "classmethod.py", line 5, in acquire
    raise Exception("Must be called via subclass :(")
Exception: Must be called via subclass :(
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