Re: [Pyobjc-dev] Calling methods on nil

[Martina O. <Ma...@Oe...>]

Hi Bill -

> When everything is working correctly 'message to nil silently eaten' 
> is a great convenience.  When something somewhere unexpectedly returns 
> nil, it becomes an incredibly nasty pain to try and figure out exactly 
> where the problem might be.   By the time it crops up, you may be many 
> cycles through the run loop away from where the problem actually is.

Agreed.

> In the context of PyObjC, I consider 'message to nil throws' to be a 
> feature (and not just because it is the Python way).

Unfortunately Cocoa uses the "message to nil ignored" pattern without 
documenting it. For example, the AddressBook methods which return 
arrays of people, groups etc. may return nil instead of an NSArray. As 
these details are not mentioned in Apple's documentation, they are a 
trap for PyObjc programmers.

One way to fix this might be to translate Cocoa's nil to Dinu's Null 
class. But this might break existing programs which now compare Cocoa 
return values to None. Also there might be round-trip problems (what 
happens if you stored a Python None in an NSMutableArray. When you 
retrieve it later, will you get None or a Null object?). Probably there 
will be more problems of this kind, similar to what we have seen with 
other conversions in the bridge.

Therefore I prefer the existing behaviour. However, I suggest to 
document it more detailed than now (list known examples of Cocoa 
returning a "to be ignored" nil), and to add explicit tests for nil to 
the example and tests where appropriate.

ciao
Martina