Sub-classing float type in Python, fails to catch exception in __init__()

Question

On Python 2.5 I need to use float numbers with a modified __str__() method. Also I need to know when the constructor fails.

Why I can\'t catch exception

Answer 1

float is immutable, therefore its __init__, the initializer, is basically a no-op -- nothing substantial can happen there, because the self object cannot be altered (if it's actually an instance of float rather than of a subclass -- but of course float's own __init__ must operate on that assumption;-).

Therefore, all the action happens in __new__, the constructor proper, just like for other immutable types like int, str, tuple, and so on. It's a common mistake to believe that __init__ is a constructor: it's not, it takes an already-constructed object as its first argument, self, and "initializes" it (if feasible, i.e., if that self is mutable!-) -- the construction itself happens in __new__.

So, your float subclass should start:

class My_Number(float):
  def __new__(cls, float_string):
    try: return float.__new__(cls, float_string)
    except (TypeError, ValueError): raise My_Error(float_string)

and you can remove the __init__, which is not needed. Now:

>>> n = My_Number('foo')
Traceback (most recent call last):
  File "", line 1, in 
  File "", line 4, in __new__
NameError: global name 'My_Error' is not defined

(of course, it would work even better if you did have a My_Error exception class defined;-).