python 3, how print function changes output?

Question

The following were what I did in python shell. Can anyone explain the difference?

datetime.datetime.now() datetime.datetime(2018, 9, 29, 21, 34, 10, 847635)

print(datetime.datetime.now()) 2018-09-29 21:34:26.900063

Answer 1

The first is the result of calling repr on the datetime value, the second is the result of calling str on a datetime.
The Python shell calls repr on values other than None before printing them, while print tries str before calling repr (if str fails).

This is not dependent on the Python version.

Answer 2

When print is called, the inner __str__() of class datetime is called. This function calls isoformat() and returns the date in ISO format.

    def __str__(self):
        "Convert to string, for str()."
        return self.isoformat(sep=' ')

When an object's name is entered to the shell, __repr__() is called and returns a formal representation of the object.