[Python-Dev] Assignment to None (original) (raw)

Michael Foord fuzzyman at voidspace.org.uk
Mon Jun 9 17:46:20 CEST 2008

Alex Martelli wrote:

The problem is more general: what if a member (of some external object we're proxying one way or another) is named print (in Python < 3), or class, or...? To allow foo.print or bar.class would require pretty big changes to Python's parser -- I have vague memories that the issue was discussed ages ago (possibly in conjunction with some early release of Jython) but never went anywhere much (including proposals to automatically append an underscore to such IDs in the proxying layer, etc etc). Maybe None in particular is enough of a special case (if it just happens to be hugely often used in dotNET libraries)?

'None' as a member does occur particularly frequently in the .NET world.

A halfway house might be to state (something like):

Python as a language disallows you from having names the same as

keywords or 'None'. An implementation restriction specific to CPython is that the same restriction also applies to member names. Alternative implementations are free to not implement this restriction, with the caveat that code using reserved member names directly will be invalid syntax for CPython.

Although this appears to raise the spectre of code written for Jython or IronPython not being compatible with CPython, in reality if they are using Java or .NET objects then they are already incompatible with CPython (IronPython doesn't work by proxying .NET objects by the way - you work directly with them but the DLR 'catches' attribute look ups etc to add Python methods to basic types and do 'other magic' like wrapping Python functions as delegates).

This at least enshrines the current IronPython behaviour with the veneer of respectability.

Michael Foord

Alex

On Mon, Jun 9, 2008 at 1:28 AM, Michael Foord <fuzzyman at voidspace.org.uk> wrote:

Steven D'Aprano wrote:

On Mon, 9 Jun 2008 12:24:55 pm Curt Hagenlocher wrote:

So, it's okay to setattr the attribute name "None" but not okay to set it directly? I suspect this is off-topic for python-dev, and would be better on comp.lang.python or similar, but for what it's worth, I consider having an attribute named 'None' bad practise, regardless of any runtime checks. But as part of Python's "we're all consenting adults here" philosophy, I wouldn't approve of expensive or extensive run-time checks specifically to prevent it. If you really have to name your attribute None, and are prepared to live with the consequences, then go ahead. FWIW Curt is asking because he is on the team responsible for implementing IronPython. In .NET you have several enumerations with members called None, for example 'Alignment.None'. It would be inconvenient for IronPython users if it enforced this particular syntax rule of Python's; that not only is assignment to None disallowed but that members named None are invalid syntax. The question is, what is the specification for Python. Is it that assignment to None is disallowed and the naming of members called None being invalid syntax is merely an artefact of the implementation of this, or does Python require this... Michael Foord In a similar fashion:

class Parrot(object): ... pass ... p = Parrot() p.1 = 'spam' File "", line 1 p.1 ^ SyntaxError: invalid syntax setattr(p, '1', 'spam')

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