[Python-Dev] Puzzling behavior when subclassing from float

[Charles G Waldman]

Aplogies in advance if this is the wrong forum for this question.

I'm trying to understand some puzzling behavior related to subclassing
from built-in types.

I'm running Python 2.2.1

If I subclass from "dict", it seems that the base class constructor is
not being called, which is just as I would expect:

class D(dict):
    def __init__(self, spam, eggs):
        print "spam=",spam, "eggs=", eggs

>>> d = D(1,2)
spam= 1 eggs= 2
>>> print d
{}

But if I subclass from "float", some magic is happening, which I don't
quite understand - it seems that the base class constructor *is* called:

class F(float):
    def __init__(self, spam, eggs):
        print "spam=",spam, "eggs=", eggs
      
>>> f = F(1,2)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: float() takes at most 1 argument (2 given)
        

If I modify the constructor to only take a single argument, I get the
following:

class F(float): 
    def __init__(self, spam):
        print "spam=",spam
        
>>> f = F(3.14)
spam= 3.14
>>> print f
3.14
>>> print f*2
6.28

How is the value "3.14" getting associated with f?  Apparently the
base class constructor is called.  How come this is happening?