Gerhard Fiedler a écrit : > On 2006-07-27 17:10:55, Bruno Desthuilliers wrote: > > >>>Isn't being on the LHS the only way to write to a non-mutable object? >> >>You *don't* "write to a non-mutable object". You rebind the name to >>another object (mutable or not, that's not the problem). > > > Ok, sloppy writing (sloppy thinking, or probably incomplete understanding > :) on my part. Let me rephrase the three original questions: > > >>>Isn't being on the LHS the only way to write to a non-mutable object? Are >>>there situations where binding happens without writing to a variable? Are >>>there write accesses to non-mutable objects where the name is not on the >>>LHS? > > > Rephrased as: > > Isn't being on the LHS (of an assignment) the only way to (re)bind a > variable?
<pedantic> s/variable/name/ </pedantic> > Are there situations where binding happens without an explicit > assignment with the variable name being on the LHS? Assignment aside, binding occurs: 1/ on evaluation of import, class and def statement see Dennis answer for an example with import. Here are examples with def and class: >>> a Traceback (most recent call last): File "<stdin>", line 1, in ? NameError: name 'a' is not defined >>> a = [] >>> a [] >>> id(a) 1078043820 >>> def a(): pass ... >>> a <function a at 0x404162cc> >>> id(a) 1078026956 >>> class a: pass ... >>> a <class __main__.a at 0x4040dadc> >>> id(a) 1077992156 >>> 2/ on function call for the function's arguments This second case won't obviously *re*bind existing names in the function's namespace, nor in the caller namespace, so we don't care here. 3/ as a result of a list comp or for loop: >>> a Traceback (most recent call last): File "<stdin>", line 1, in ? NameError: name 'a' is not defined >>> for a in range(3): pass ... >>> a 2 >>> b Traceback (most recent call last): File "<stdin>", line 1, in ? NameError: name 'b' is not defined >>> [b for b in range(3)] [0, 1, 2] >>> b 2 >>> AFAICT and IIRC, this should cover most cases - please someone correct me if I forgot something here... > Can the object > referenced by a variable name get changed without the variable name > appearing on the LHS? >>> a = [] >>> id(a) 1078043820 >>> def change(l): ... l.append(42) ... >>> change(a) >>> a [42] >>> id(a) 1078043820 >>> First assignment aside, the *name* 'a' is never on the LHS of an assignment, yet the object referenced by name 'a' got 'changed' : it was initially empty, after call to change() it contains an int. Yes, I know, this is not what you meant (and no, name 'a' has not been rebound) - but that's really what you asked : the object referenced by name 'a' got changed without name 'a' being on the LHS !-) Hence the emphasis on the difference between mutating (modifying/changing/...) an object and rebinding a name... I think your intented question was more like : "can a name be rebound without that name being on the LHS of an assignment", and it's addressed above > (I hope I got this right now... Almost !-) > asking the correct questions is not so easy > sometimes :) Indeed !-) -- http://mail.python.org/mailman/listinfo/python-list
