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Oct 15th, 2007

newbee: python link objects

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Why can't I have two different link objects? I create _link and _link2 as empty objects.

When I do this:
_link2 = _link,

it just creates alias _link2 to link object. Same object have 2 aliases _link2 and _link. Is there a way to copy the content of _link to _link2?

When I do this: _link2 = _link2.append("adding stuff")
why does _link2 become "None"

My code:

_link = []
site_url = "http://www.mysite.com/"
_link.append(site_url)
_link2 = []
_link2 = _link
_link2 = _link2.append("adding stuff")

print _link
print _link2

Output:

['http://www.mysite.com/', 'adding stuff']
None

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jobs

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Junior Poster in Training

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Oct 15th, 2007

Re: newbee: python link objects

The problem is that Python does assignment by reference. So the line

_link2 = _link

operates by making _link2 and _link both point to the same object. Change one, and you've changed them both.

If you want to copy the contents, do a slice:

_link2 = _link[:]

The operation on the left returns an entire copy of _link, which then gets assigned to _link2.

Jeff

jrcagle

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Oct 16th, 2007

Re: newbee: python link objects

Be careful with a slice copy ...

python Syntax (Toggle Plain Text)
old_list = [1, [2, 3,[4]], 5]
new_list = old_list[:]
 
# now change old_list
old_list[1][0] = 1111
 
print old_list  # [1, [1111, 3, [4]], 5]
print new_list  # [1, [1111, 3, [4]], 5]  oops!
old_list = [1, [2, 3,[4]], 5]
new_list = old_list[:]

# now change old_list
old_list[1][0] = 1111
 
print old_list  # [1, [1111, 3, [4]], 5]
print new_list  # [1, [1111, 3, [4]], 5]  oops!

If you want to make a copy of a list, even a deeply nested list, you best use the module copy ...

python Syntax (Toggle Plain Text)
import copy
 
old_list = [1, [2, 3,[4]], 5]
new_list = copy.deepcopy(old_list)
 
# now change old_list
old_list[1][0] = 1111
 
print old_list  # [1, [1111, 3, [4]], 5]
print new_list  # [1, [2, 3, [4]], 5]  now it works
import copy
 
old_list = [1, [2, 3,[4]], 5]
new_list = copy.deepcopy(old_list)

# now change old_list
old_list[1][0] = 1111
 
print old_list  # [1, [1111, 3, [4]], 5]
print new_list  # [1, [2, 3, [4]], 5]  now it works

vegaseat

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DaniWeb's Hypocrite

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Oct 16th, 2007

Re: newbee: python link objects

That's awkward...

jrcagle

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