子类化一个复杂对象，并从其父类初始化一个实例

Question

In the following example Foo class (trying to make a datetime class that has extra methods). I also would like to be able to chain method calls (eg foo.bar().baz() ), as well as be able to pass the object to existing function which expects a datetime object.

When instantiating it I get a TypeError as shown below:

from pytz import UTC
from datetime import datetime

class Foo(datetime):
    def bar(self, timezone):
        print(self.astimezone(timezone))            
        return self

now = datetime.now(UTC)
Foo(now) # TypeError: an integer is required (got type datetime.datetime)

I am guessing the error is coming from trying to make a datetime object from a datetime object. My question is very similar to this post Creating an object from a base class object in Python . ie How does one initiate a child class from a parent object. I tried the delegation solution in the post but it resulted in recursion and some messy code. I am thinking there must be a better way!

Answer 1

You're inheriting the entire behaviour of the datetime class. The constructor behaves the same way as the datetime constructor, meaning it expects integers for year, month, day and so on. datetime(datetime.now()) would fail the same way for the same reason.

You can simply use Foo.now() to construct a Foo instance using the datetime.now() mechanics.

If you want to "augment" an existing datetime object to a Foo object, you should probably add a class method for that:

class Foo(datetime):
    @classmethod
    def from_datetime(cls, dt):
        return cls.fromtimestamp(dt.timestamp(), dt.tzinfo)

Converting the existing datetime object to a timestamp and using the fromtimestamp constructor here is probably the simplest way to copy over all the needed information. Use like:

foo = Foo.from_datetime(now)