Question

I’m writing a Python script that reads a CSV file and creates a list of deques. If I print out exactly what gets appended to the list before it gets added, it looks like what I want, but when I print out the list itself I can see that append is overwriting all of the elements in the list with the newest one.

    # Window is a list containing many instances            
def slideWindow(window, nextInstance, num_attributes):
    attribute = nextInstance.pop(0)
    window.popleft()
    for i in range(num_attributes):
        window.pop()
    window.extendleft(reversed(nextInstance))
    window.appendleft(attribute)
    return window

def convertDataFormat(filename, window_size):
    with open(filename, 'rU') as f:
        reader = csv.reader(f)

        window = deque()
        alldata = deque()

        i = 0
        for row in reader:
            if i < (window_size-1):
                window.extendleft(reversed(row[1:]))
                i+=1
            else:
                window.extendleft(reversed(row))
                break

        alldata.append(window)

        for row in reader:
            window = slideWindow(window, row, NUM_ATTRIBUTES)
            alldata.append(window)
#             print alldata

        f.close()
        return alldata

Answer 1

This is really difficult to track what you exactly want from this code. I suspect the problem lies in the following:

alldata.append(window)

for row in reader:
    window = slideWindow(window, row, NUM_ATTRIBUTES)
    alldata.append(window)

Notice that in your slideWindow function, you modify the input deque (window), and then return the modified deque. So, you’re putting a deque into the first element of your list, then you modify that object (inside slideWindow) and append another reference to the same object onto your list.

Is that what you intend to do?

The simple fix is to copy the window input in slideWindow and modify/return the copy.