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Explanation of list reference

Am 15.02.14 01:57, schrieb Chris Angelico:
> Can you give an example of an ambiguous case? Fundamentally, the 'is'
> operator tells you whether its two operands are exactly the same
> object, nothing more and nothing less, so I assume your "ambiguous
> cases" are ones where it's possible for two things to be either the
> same object or two indistinguishable ones.

What about the thing I posted down in this thread?

 >>> import numpy as np
 >>> a=np.array([1, 2, 3, 4])
 >>> b=a[:]
 >>> id(a)
 >>> id(b)

So, a and b are different things, right?

 >>> b[1]=37
 >>> b
array([ 1, 37,  3,  4])
 >>> a
array([ 1, 37,  3,  4])

Still they are connected. I can imagin that id() is just a debugging 
tool for extensions. What useful applications does it have outside of this?