Elegant copy-by-value

[Carl Banks]

Martin Christensen wrote:
> Python usually uses copy-by-reference. This is all good and well, but
> sometimes we want copy-by-value. For a long time now I've been using
> copy() or deepcopy(), depending on the circumstances, from the copy
> module. Is there no more simple and elegant way of doing this? It
> seems strange that one should have to import a module to so something
> as simple as a copy-by-value. I should, of course, have researched
> this a long, long time ago, but deadlines have a way of defering such
> luxuries. :-)

For the types where it is most useful to do copy-by-value, there are
built-in ways to do shallow copies.  For example, lists (and Numeric
arrays) can do this:

    a = b[:]

Dicts have a copy method:

    a = b.copy()

Class instances don't have such a method by default, seeing that they
can have more complicated state that might not work with a simple
copy.  However, it's easy to define a class that does, if you need it.
For example, extending the Bunch class from Python Cookbook:

    class Bunch:
        def __init__(self,**kwargs): self.__dict__.update(kwargs)
	def copy(self): return Bunch(self.__dict__)


Deep copying is *not* something simple.  It is best left up to the
module, or done by hand.


-- 
CARL BANKS