array of arrays question

[Meo]

Hi,

Here is what I'm doing in python 2.4.1 :

aOfa=[[[]]*3]*4
aOfa [[[], [], []], [[], [], []], [[], [], []], [[], [], []]]
aOfa[3][2].append(1)
aOfa

[[[1], [1], [1]], [[1], [1], [1]], [[1], [1], [1]], [[1], [1], [1]]]

Ok, so there is something I didn't understand about python's arrays.

The result I was expecting was :

[[[], [], []], [[], [], []], [[], [], []], [[], [], [1]]]

Somebody understand what's going on here?

Thank

 

[Paul Rubin]

Somebody understand what's going on here?

Yes, []*3 gives you three references to the same empty list, not three
separate empty lists. You need something like

[[] for i in xrange(3)]

to get separate lists.

Another example:

a = [1,2,3]
b = a
a[0] = 5
print b # prints [5,2,3]

Do you understand now?

 

[Meo]

yes,

thank you for your help

 

[gene tani]

I think this is addressed somewhere in a python Gotchas list. These
lists're huge, but if you scan thru these every once in a while,
they're big timesavers:

http://www.ferg.org/projects/python_gotchas.html
http://zephyrfalcon.org/labs/python_pitfalls.html
http://zephyrfalcon.org/labs/beginners_mistakes.html
http://www.python.org/doc/faq/
http://diveintopython.org/appendix/abstracts.html
http://www.onlamp.com/pub/a/python/2004/02/05/learn_python.html
http://www.norvig.com/python-iaq.html
http://www.faqts.com/knowledge_base/index.phtml/fid/245
http://amk.ca/python/writing/warts