[Python-Dev] Non-string keys in type dict (original) (raw)

Guido van Rossum guido at python.org
Thu Mar 8 16:27:41 CET 2012

On Thu, Mar 8, 2012 at 1:10 AM, Lennart Regebro <regebro at gmail.com> wrote:

On Thu, Mar 8, 2012 at 08:46, Ethan Furman <ethan at stoneleaf.us> wrote:

I think it would be sad to lose that functionality.

If we are going to, though, we may as well check the string to make sure it's a valid identifier: That would break even more code. I have encountered many cases of attributes that aren't valid identifiers, in particular using dots or dashes. Admittedly this is often in cases where the object has both attribute access and key access, so you can make foo['bar-frotz'] instead. But when should we then require that it is a valid identifier and when not? --> class A: -->   pass --> setattr(A, '42', 'hrm') --> A.42  File "", line 1  A.42 ^ SyntaxError: invalid syntax Doesn't seem very useful. You have to set it with setattr, so you have to get it with getattr. I don't see the problem.

I'm with Lennart. I've spoken out on this particular question several times before; it is a feature that you can use any arbitrary string with getattr() and setattr(). However these functions should (and do!) reject non-strings.

I'm lukewarm on forbidding non-strings in namespace dicts if you can get them in there by other means. I presume that some people might use this to hide state they don't want accessible using regular attribute notation (x.foo) and maybe somebody is even using some namespace's keys as a dict to store many things with non-string keys, but that feels like abuse of the namespace to me, and there are plenty of other ways to manage such state, so if it gives a significant speed-up to use string-only dicts, so be it. (Although Python's dicts already have some string-only optimizations -- they just dynamically adapt to a more generic and slightly slower approach once the first non-key string shows up.)

As Nick says we should introduce a deprecation period if we do this.

On Wed, Mar 7, 2012 at 11:43 PM, Ethan Furman <ethan at stoneleaf.us> wrote:

Are you able to modify classes after class creation in Python 3? Without using a metaclass?

Yes, by assignment to attributes. The dict is a read-only proxy, but attribute assignment is allowed. (This is because the "new" type system introduced in Python 2.2 needs to track changes to the dict; it does this by tracking setattr/delattr calls, because dict doesn't have a way to trigger a hook on changes.)

-- --Guido van Rossum (python.org/~guido)

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