What does cls() function do inside a class method?

问题:

Today I'm viewing another's code, and saw this:

class A(B): 
    # Omitted bulk of irrelevant code in the class

    def __init__(self, uid=None):
        self.uid = str(uid)

    @classmethod
    def get(cls, uid):
        o = cls(uid)
        # Also Omitted lots of code here

what does this cls() function do here?

If I got some other classes inherit this A class, call it C, when calling this get method, would this o use C class as the caller of cls()?

回答1:

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 :(

回答2:

cls is the constructor function, it will construct class A and call the __init__(self, uid=None) function.

If you enherit it (with C), the cls will hold 'C', (and not A), see AKX answer.

回答3:

It's a class factory.

Essentially it the same as calling:

o = A(uid)

cls in def get(...): is A.