Unexpected behavior inside function call. Returned variable is hard-linked to its initial value. Does not reflect changes

Question

I am having trouble understanding how python handles storage and retrieval of variables inside functions. In the following code, I initialize the variable 'gg' as being equal to one of the many arrays that I pass to the function. I then update the variable with some operations. Finally I have the variable returned to the base space. The problem is that the function seems to forget all the changes I have performed on the variable.

Here is an example:

XX = {hh:np.random.rand(5) for hh in np.arange(4)}

def ffun(data_in, vols = None):
    count = 0
    for voli, vol in enumerate(vols):
        if count==0:
            gg = data_in[vol]
        else:
            gg += data_in[vol]+0
        count += 1
        print([voli,vol,count,max(gg)])
        print([gg[:3]])
    # gg /= count+1
    print(count)
    print([voli,vol,count,max(gg)])
    print([gg[:3]])
    
    return gg

HH = ffun(XX,vols=np.arange(4))

HH==XX[0]

It seems as though this is some issue with internal pointers to variables stored in memory. If I initialize 'gg' with either <gg = data_in[vol] + 0> or <gg = data_in[vol].copy()>, then everything works. Everything also works if I move the operations to the base space, as in:

XX = {hh:np.random.rand(5) for hh in np.arange(4)}

data_in=XX
vols = np.arange(4)
count = 0
for voli, vol in enumerate(vols):
    if count==0:
        gg = data_in[vol]
    else:
        gg += data_in[vol]+0
    count += 1
    print([voli,vol,count,max(gg)])
    print([gg[:3]])
# gg /= count+1
print(count)
print([voli,vol,count,max(gg)])
print([gg[:3]])
    
HH==XX[0]

What am I missing? Why does gg revert to its initial value? And why does this only happen to the returned value? the variable is clearly changing within the function call.