Question

I’ve spent the last 2 hours on this and I’ve probably read every question on here relating to variables being passed to functions. My issue is the common one of the parameter/argument being affected by changes made inside the function, even though I have removed the reference/alias by using variable_cloned = variable[:] in the function to copy the contents across without the reference.

Here is the code:

def add_column(m):    
    #this should "clone" m without passing any reference on    
    m_cloned = m[:]
    for index, element in enumerate(m_cloned):
        # parameter m can be seen changing along with m_cloned even
        # though 'm' is not touched during this function except to 
        # pass it's contents onto 'm_cloned'        
        print "This is parameter 'm' during the for loop...", m
        m_cloned[index] += [0]
    print "This is parameter 'm' at end of for loop...", m    
    print "This is variable 'm_cloned' at end of for loop...", m_cloned
    print "m_cloned is m =", m_cloned is m, "implies there is no reference"
    return m_cloned

matrix = [[3, 2], [5, 1], [4, 7]]
print "\n"
print "Variable 'matrix' before function:", matrix
print "\n"
add_column(matrix)
print "\n"
print "Variable 'matrix' after function:", matrix

What I’m noticing is that the parameter ‘m’ in the function is changing as if is an alias of m_cloned – but as far as I can tell I have removed the alias with the first line of the function. Everywhere else I have looked online seems to suggest that this line will make sure there is no reference to parameter – but it’s not working.

I’m sure I must have made a simple mistake but after 2 hours I don’t think I’m going to find it.

Answer 1

It looks like you need a deepcopy, instead of a shallow copy, which is what [:] gives you:

from copy import deepcopy
list2 = deepcopy(list1)

Here’s a longer example comparing the two types of copy:

from copy import deepcopy

list1 = [[1], [1]]
list2 = list1[:]   # while id(list1) != id(list2), it's items have the same id()s
list3 = deepcopy(list1)

list1[0] += [3]

print list1
print list2
print list3

Outputs:

[[1, 3], [1]]  # list1
[[1, 3], [1]]  # list2
[[1], [1]]     # list3 - unaffected by reference-madness