[Python-Dev] The interpreter accepts f(**{'5':'foo'}); is this intentional?

[Nick Coghlan]

Michael Haggerty wrote:
> Is this behavior required somewhere by the Python language spec, or is
> it an error that just doesn't happen to be checked, or is it
> intentionally undefined whether this is allowed?

Generally speaking, Python namespace dictionaries (be it globals(),
locals(), the __dict__ attribute of an instance or a set of keyword
arguments) aren't required to enforce the use of legal identifiers (in
many cases, the CPython variants don't even enforce the use of strings).

Enforcing legal identifiers is usually the compiler's job and if you're
using dict syntax to access the contents of a namespace, the compiler
doesn't care.

That laxness is a CPython implementation detail though - other
implementations are quite free to be stricter with their namespaces
(e.g. I believe Jython namespaces use explicitly string-keyed
dictionaries, so Jython would reject the example below).

Cheers,
Nick.

P.S. An example of messing about with a class's dictionary in CPython:

Python 2.5.2 (r252:60911, Jul 31 2008, 17:28:52)
[GCC 4.2.3 (Ubuntu 4.2.3-2ubuntu7)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> class C: pass
...
>>> C.__dict__[5] = "Not an identifier!"
>>> C.5 # obviously not allowed
  File "<stdin>", line 1
    C.5 # obviously not allowed
      ^
SyntaxError: invalid syntax
>>> C.__dict__['5'] = "Still not an identifier!"
>>> C.5 # still not allowed
  File "<stdin>", line 1
    C.5 # still not allowed
      ^
SyntaxError: invalid syntax
>>> C.__dict__[5]
'Not an identifier!'
>>> getattr(C, 5)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: getattr(): attribute name must be string
>>> getattr(C, '5')
'Still not an identifier!'

Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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