why cannot assign to function call
tjreedy at udel.edu
Sun Mar 15 02:45:34 CET 2009
> In article <mailman.952.1235850376.11746.python-list at python.org>,
> <rdmurray at bitdance.com> wrote:
>> I think this is the key point. I am an experienced Python programmer,
>> and I had to think long and hard about what you were saying, Mark, in
>> order to understand how it applied to Python. I think this means that
>> your model and/or your level of abstraction is not "natural" when trying
>> to understand Python programs, and I think I may have figured out why.
> Good job! I'll just add one little bit to your excellent rebuttal:
>> Assignment in Python changes a mapping (mutates a namespace object). In
>> this case the mapping being changed is the local function's namespace
>> mapping. We can't affect the caller's namespace mapping, because (in
>> this function) we don't have access to the caller's namespace object.
>> So I would diagram it like this:
>> y (in global env.) -----> 'virgin'
>> x (in function 'test') ----> 'clobbered'
> Although you later make clear that this model works well with lists, too,
> I personally find it easier to separate the concept of "name" (which is
> strictly the set of legal Python identifiers) with the more general
> concept of "target". Consider this:
>>>> L = [1, 2, 3]
>>>> L[1:1] = ['foo', 'bar']
> What do you call ``L[1:1]``?
I call that a collection slot.
> I call that a target (and in fact, so do the Python docs).
Targets encompass both names and slots.
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