Python default argument madness

I've been learning python recently. I'm just getting started so I'm running into all the stupid beginner problems, for instance the insane behaviour of default argument values. Take this example:

from datetime import datetime
import time

class Weird:

 def __init__(self, dt = datetime.now()):
  self.dt = dt

w1 = Weird()
time.sleep(1)
w2 = Weird()
w3 = Weird(datetime.now())

print w1.dt == w2.dt # True
print w1.dt == w3.dt # False

The datetime in the w1 and w2 objects will actually be the same since the default argument value in the method definition is only evaluated once! What!?!? This essentially renders default arguments useless since you always end up writing the following:

class Weird:

 def __init__(self, dt = None):
  self.dt = dt or datetime.now()

I don't understand why you would design a default argument values features in your language like this. Doing it this way certainly violates the principle of least astonishment!

What is good code?

As a project tech lead I do a lot of code review, so the "What is good code?" question comes up a lot. Of course there are many things that factor into the "good code" equation such as efficiency, correctness and elegance. For me the most important property of good code is that it should adhere to the principle of least astonishment. In other words: good code is code that does what you expect it to do in the way you expect it to be done.

Following this principle has important benefits when it comes to reading, understanding and maintaining a piece of code. It also typically improves the correctness of the code: you tend to get code that obviously has no deficiencies, in stead of no obvious deficiencies (as Tony Hoare said).

I was happy to learn that I'm in good company when it comes to attributing the principle of least astonishment to good code. Both Uncle Bob (in his excellent Clean Code book) and Peter Seibel (in his very interesting Coders at Work) ask several famous programmers about good code. A lot of the interviewees directly or indirectly mention the principle of least astonishment (for instance Ward Cunningham, Joe Armstrong and Simon Peyton Jones).