On 03/05/15 00:25, Alex McFerron wrote:
step 1: x = {}
step 2: y = x
step 3: x['key'] = 'value'
# at this point if i print x or y i see {'key', 'value'}
step 4: x['key'] = 'newValue' #and at this point printing x or y i see
{'key', 'newValue'} and this is true if this was y['key']
because of the behavior in step 4, i'm thinking, ok the copy job from step
2 was a pointer only and not a by value copy job. fair enough
First mistake is to think of step 2 as a "copy job" its not.
Python variables do not work like variables in C and other
similar languages. see below.
step 5: x = {} #or y={}
step 6: print both. and what i get here is that x will be empty but y will
not (or visa verse)
question: if y=x from step 2 (the copy job) is just creating a pointer y
that points to the same thing as x then why when i set x = {} in step 5
does that also not cause y to equal {}?
what am i not understanding about python dictionaries?
This has nothing to do with dictionaries. Try this:
>>> a = 42
>>> b = a
>>> a = 66
>>> print a,b # => 66,42
The same behaviour.
Remember that unlike some other languages you may have used
variables are not bits of memory. They are labels that are
used as keys in a dictionary. Its a bit like sticking
a post-it note onto a physical object. Many notes can be
stuck to the same object. And you can take a note off an
object and stick it on a new one.
So in step 1 you take a new post-it, write the name 'x'
on it and attach it to your dictionary.
Then in step 2 you do not copy anything, rather you get
another post-it and write the name 'y' on it, then you
stick it to the same dictionary object as x
In step 5 you move the first post-it, with 'x' on it, to a
new object. It just happens to be another dictionary, but
it could have been anything. But the post-it with 'y' on is
still stuck to the original dictionary object.
Does that help?
--
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
http://www.amazon.com/author/alan_gauld
Follow my photo-blog on Flickr at:
http://www.flickr.com/photos/alangauldphotos
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