The structure of the project is:

project
- main.py
- session.py
- spider.py

There is a class in session.py:

import requestsclass Session:def __init__(self):self.session = requests.Session()print('Session created.')

And another class in spider.py:

from session import Sessionclass Spider:def __init__(self, sess: Session = Session()):print('Spider created.')

When I import class Spider from spider.py in main.py like this:

from spider import Spiderif __name__ == '__main__':print('Main function.')spider = Spider()

And run main.py, I get:

Session created.
Main function.
Spider created.

It confuses me. I think __init__ is the initial function used when initializing an instance, but in this case the __init__ function of Session is called when Session is imported in spider.py. I think it must be related to the default value of __init__ function in spider.py, but why?

The default values of parameters get evaluated only once in python. This is documented here and also here as stated by JETM.

Therefore a isntance of Session is created when you import spider as default value for the sess parameter of the spiders __init__ method.

If you do not wish such a behavior you can use None as default value and create the Sesssion instance inside the __init__ method if no other value was provided as pointed out by MatsLindh and FHTMitchel like this:

...
def __init__(self, sess: Session = None):if sess is None:sess = Session()print('Spider created.')