Why isn't the constructor called the second time? - python-2.7

Question

Why isn't the constructor called the second time?
from datetime import datetime
class Time(datetime):
def __init__(self, *args):
print 5, args
try:
d = args[0]
print 8, d
datetime.__init__(self,
d.year, d.month, t.day, t.hour, t.minute, t.second)
except Exception:
print 12, args
datetime.__init__(self, args)
if __name__ == '__main__':
t = Time(1965, 1, 10)
print 17, t
u = Time(t)
print 19, u
Using python 2.7.2, here's the output:
bash-4.1$ python tmp.py
5 (1965, 1, 10)
8 1965
12 (1965, 1, 10)
17 1965-01-10 00:00:00
Traceback (most recent call last):
File "tmp.py", line 18, in <module>
u = Time(t)
TypeError: an integer is required
I expected to see:
5 Time(1965, 1, 10)
What function is being called if not the constructor?

Answer 1

It's the type's __new__ method.
__init__ isn't the first thing that happens when you create an object. First, the type's __new__ method is called to actually produce the object, and then __init__ is called to initialize it. For mutable types, __new__ doesn't usually do much, but for immutable types like datetime, __new__ generally creates the object pre-initialized, since otherwise __init__ would have to mutate the object to initialize it.
If you want to inherit from datetime, you have to implement __new__ as well as __init__:
def __new__(cls, *args):
print 5, args
try:
d = args[0]
print 8, d
return datetime.__new__(cls,
d.year, d.month, t.day, t.hour, t.minute, t.second)
except Exception:
print 12, args
return datetime.__new__(cls, args)
If you want to see what datetime's __new__ does, it's visible in Modules/datetimemodule.c. You'll have to know C and know or look up a bunch of Python C-api stuff to understand it, though.