Question

I am loading a file “data.imputation” which is 2 dimensional in variable 'x'. Variable 'y' is a copy of 'x'. I pop the first array from 'y' (y is 2D). Why is the change reflected on x? (The first array from 'x' is also popped)

ip = open('data.meanimputation','r')
x = pickle.load(ip)
y = x
y.pop(0)

At the start, len(x) == len(y). Even after y.pop(0), len(x) == len(y). Why is that? And how can I avoid it?

Answer 1

use y = x[:] instead of y = x. y = x means both y and x are now pointing to the same object.

Take a look at this example:

>>> x=[1,2,3,4]
>>> y=x
>>> y is x
True          # it means both y and x are just references to a same object [1,2,3,4], so changing either of y or x will affect [1,2,3,4]
>>> y=x[:]     # this makes a copy of x and assigns that copy to y,
>>> y is x     # y & x now point to different object, so changing one will not affect the other.
False

If x is a list is list of list then [:] is of no use:

>>> x= [[1,2],[4,5]]
>>> y=x[:]   #it makes a shallow copy,i.e if the objects inside it are mutable then it just copies their reference to the y
>>> y is x
False         # now though y and x are not same object but the object contained in them are same 
>>> y[0].append(99)
>>> x
[[1, 2, 99], [4, 5]]
>>> y
[[1, 2, 99], [4, 5]]
>>> y[0] is x[0]
True  #see both point to the same object

in such case you should use copy module’s deepcopy() function , it makes non-shallow copies of object.