On Class namespaces, calling methods

[Duncan Booth]

Steven D'Aprano <steve at REMOVE-THIS-cybersource.com.au> wrote:

> On Sat, 10 Apr 2010 16:35:29 +0000, Duncan Booth wrote:
> 
>> Anyway, the moral is never, ever to use old-style classes in Python
>> 2.x. You will get weird and unexpected results.
> 
> That's a bit strong. They're only weird and unexpected if you're not 
> expecting them and don't understand them.

Which is exactly what will happen when some subclass tries to use a 
property not realising that it is actually an old style class. That kind of 
problem can be a real pain to track down.

> 
> Why are we worrying about the exact error message? New style or old 
> style, they both raise TypeError, and testing for the exact error
> message is a fragile, dangerous thing to do: error strings are not
> part of the Python API, and are subject to change without notice, for
> any reason. For all we know, Python runtimes compiled on a Tuesday
> could use different error messages from runtimes compiled on
> Wednesdays. 
> 

I'm intrigued by the exact error message because I'd like to know the 
answer to one of the questions Vicente asked: what object is being passed 
to 'm' on the system which complains "m() takes no arguments (1 given)"
I'm sure it must just be an insignificant difference, but I hate unanswered 
questions.