Python中的构造函数和方法重写

Question

I am trying to figure out if there is a way to override the __init__ method in a python class. I have come up with this, not exactly method overriding but it has the same effect

class A():
    def __init__(self, x):
        if isinstance(x, str):
            print "calling some text parsing method"
        elif isinstance(x, int):
            print "calling some number crunching method"
        else:
            print "Oops"

Is this good practice please? Not only for constructors as in this particular question but also for other methods too

Answer 1

That's essentially what you need to do, if the actions for a string argument are very different from the actions for an integer argument. However, if one case reduces to the other, then you can define a class method as an alternate constructor. As an simple example, consider

class A():
    def __init__(self, x):
        if isinstance(x, str):
            self.x = int(x)
        elif isinstance(x, int):
            self.x = x
        else:
            raise ValueError("Cannot turn %s into an int" % (x, ))

Here, the integer case is the "fundamental" way to create an instance of A ; the string case reduces to turning the string into an integer, then proceding as in the integer case. You might rewrite this as

class A():
    # x is expected to be an integer
    def __init__(self, x):
        self.x = x

    # x is expected to be a string
    @classmethod
    def from_string(cls, x):
        try:
            return cls(int(x))
        except ValueError:
            # This doesn't really do anything except reword the exception; just an example
            raise ValueError("Cannot turn %s into an int" % (x, ))

In general, you want to avoid checking for the type of a value, because types are less important than behavior. For example, from_string above doesn't really expect a string; it just expects something that can be turned into an int . That could be a str or a float .