class.__getattr__ behavior: feature or bug?

[Hans Nowak]

On 14 Jan 00, at 15:12, Huayin Wang wrote:

> >>> class S:
> ...    def __getattr__(self, name):
> ...       if name == 'c':
> ...          self.a = self.a + 1
> ...          return self.a
> ...       else: return None
> ... 
> >>> s=S()
> >>> s
> Traceback (innermost last):
>   File "<stdin>", line 1, in ?
> TypeError: call of non-function (type None)
> >>>

Feature. I think it works something like this: Typing the name of the 
instance (s) in interactive mode causes Python to look for the __repr__ 
method (and/or __str__, possibly), because it needs a printable 
representation... but it doesn't find it (since it's not defined, 
obviously). So it tries to find an attribute __repr__ using __getattr__... 
which returns None. Python thinks this result value should be a function 
(like __repr__ is... OK, so it's actually a method... same difference), and 
tries to call it. In this case, it ends up calling None, though, which 
leads to an error since None isn't a function.

This should work:

class S:
    def __getattr__(self, x):
        return None
    def __repr__(self):
        return str(id(self))

I must admit this error is kinda obscure. ^_^

--Hans Nowak (zephyrfalcon at hvision.nl)
Homepage: http://fly.to/zephyrfalcon
You call me a masterless man. You are wrong. I am my own master.