Re: [Sqlalchemy-users] python copy question ?

[Shuo Yang]

On 2/7/06, Michael Bayer <[EMAIL PROTECTED]> wrote:

hey list -

I am doing a much needed reorganization with regards to the construction
and retreival of mappers to be simpler and more straightforward.   one
issue I always run into with Python is that the copy.copy () method is
*super* slow...its operation is 10 to 30 (!!!) times slower than just
constructing a new object or using __new__ with a dictionary copy:

import copy, time

# test object
class foo(object):
    def __init__(self, a, b, c):
        self.a = a
        self.b = b
        self.c = c

# create 5000 test objects
l = []
for x in range(0, 5000):
    l.append(foo('a', 'b', 'c'))

# test harness
def test(copier, method):
    x = []
    now = time.time()
    for f in l:
        x.append(copier(f))
    print method, ":",  (time.time() - now)

# run three tests ten times
for g in range(0,10):

    # test straight construction
    test(lambda x: foo(x.a, x.b, x.c), "constructor")

    # test copy function
    test(lambda x: copy.copy(x), "copy")

    # test using __new__ + dict copy
    def makenew(x):
        g = foo.__new__(foo)
        g.__dict__.update(x.__dict__)
        return g
    test(makenew, "__new__")


So what is the best way to make a shallow (new-style-class) object copy?
is this approach:

     y = klass.__new__(klass)
     y.__dict__.update(x.__dict__)

which seems to work great, safe/compatible/complete ?

http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/66507

Have you seen this?  This recipe suggests defining __copy__ to custimize  the copy.copy process if you like to use copy.  Says the __dict__.update could also work I think.

- mike



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John S. Yang