En Tue, 28 Oct 2008 00:58:10 -0200, greg <[EMAIL PROTECTED]>
escribió:
Steven D'Aprano wrote:
By common usage and technical definition, C is call by value. Argument
passing in Python does not behave like C. So why insist that Python is
also call by value?
Whether it behaves like C is not the test.
Let's look at the definitions of the terms:
(1) Call by value: The actual parameter is an expression. It is
evaluated and the result is assigned to the formal parameter.
Subsequent assignments to the formal parameter do not affect
the actual parameter.
(2) Call by reference: The actual parameter is an lvalue. The
formal parameter becomes an alias for the actual parameter,
so that assigning to the formal parameter has the same
effect as assigning to the actual parameter.
Seems to me that (1) describes exactly how parameter passing
works in Python. So why insist that it's *not* call by value?
Those definitions are only applicable to unstructured, primitive types,
where the only relevant operations are "get value" and "assign value".
Structured types provide other operations too - like selection (attribute
get/set in Python). It is unspecified on both definitions above what
happens in those cases. Other posts in this same thread showed that Python
behaves similarly to call-by-reference in C++ with regard to data members
inside a structure (that is, mutable objects *can* be changed, and the
caller sees the change). Combined with your previous conclusion that
Python implements call-by-value, one should finally conclude that it can't
be neither cbv nor cbr - it's a different thing.
The term "call by object" was coined in the late '70s to describe this
behavior. More info about this ever-recurring topic can be found at
http://effbot.org/zone/call-by-object.htm
--
Gabriel Genellina
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