Python - understanding from datetime import datetime module

Question

I am learning python and have acquired basic knowledge. However, I was doing my own practice using datetime module and got confused with the below line datetime.now()

from datetime import datetime
now = datetime.now()

I know that datetime is a class and now() is a method but what I am confused about is that should we not use an object to call the method or is it a static/class method which is usually called by classname.method? I tried to mimic the same by creating a module, class and tried calling it from another module so that I can only import object of the class in another module and call the methods like object.method() but I am unable to import object in another module.

I have two python files myclass.py and main.py and both are in same directory.

# myclass.py

    class MyClass:
        def __init__(self, brand):
            self.brand = brand
    
        def my_car(self):
            print(f'this is {self.brand} new car')
    
     my_obj = MyClass("BMW")

Now from main.py

# main.py

from myclass import my_obj

if __name__ == '__main__':
    my_obj.my_car()

1-Can someone please guide me how datetime.now() is being called. Is it being called using class method approach (class.method) or is it being called using datetime object (object.method). I am just curious and trying to understand the concept behind it.

2-Is it possible to import only object from another module. I read couple of links on stackoverflow which are as following but I am confused:

Instance a Python object from another Python file

Why I cannot import a function from a class in python?

I am really curious and want to know the concept. I would kindly request you to please let me know if my understanding is correct or not. Thank you.

You should look into how modules are "called". What you need is some way to ascertain that you are infact in the right directory and can do things like `from .module import my_method`. One way to do that is via `__file__`. See [this](https://stackoverflow.com/questions/9271464/what-does-the-file-variable-mean-do) for what this does. This will likely help explain what is going on with your code. — NelsonGon, May 22 '22 at 22:53

score 3 · Answer 1 · answered May 22 '22 at 22:53

3

datetime.now() is a classmethod.

Your import doesn't work because of this:

if __name__ == '__main__':
    my_obj = MyClass("BMW")
    my_obj.my_car()

The entire purpose of the if statement is to determine if the module is being imported, or is being executed directly.

If the module is being imported, __name__ is not equal to "__main__", and thus my_obj does not exist.

answered May 22 '22 at 22:53

John Gordon

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I have removed __name__=='__main__' from myclass.py. I have added __name__=='__main__': and created my_obj.my_car() in main.py. It is working fine now. Could you please check in my question section (I have modified) if it's fine now. Thank you. – brian May 22 '22 at 23:27

d-k-bo · Answer 2 · 2022-05-23T01:21:45.843

Take a look at the datetime source code:

class datetime(date):
    ...

    @classmethod
    def now(cls, tz=None):
        "Construct a datetime from time.time() and optional time zone info."
        t = _time.time()
        return cls.fromtimestamp(t, tz)

The @classmethod decorator creates a class method. It implicitly passes the class as a first argument.

This approach is often used as an alternative constructor such as

class Car:
    def __init__(self, brand):
        self.brand = brand

    @classmethod
    def from_dict(cls, d):
        return cls(d["brand"])  # or cls(**d)

from cars import Car

car_description = {"brand": "DeLorean"}

car = Car.from_dict(car_description)

Note: The class method has no self reference. In this example, the class hasn't even been instantiated yet.

Python - understanding from datetime import datetime module

2 Answers2