Pythonic way to calculate offsets of an array

Question

I am trying to calculate the origin and offset of variable size arrays and store them in a dictionary. Here is the likely non-pythonic way that I am achieving this. I am not sure if I should be looking to use map, a lambda function, or list comprehensions to make the code more pythonic.

Essentially, I need to cut chunks of an array up based on the total size and store the xstart, ystart, x_number_of_rows_to_read, y_number_of_columns_to_read in a dictionary. The total size is variable. I can not load the entire array into memory and use numpy indexing or I definitely would. The origin and offset are used to get the array into numpy.

intervalx = xsize / xsegment #Get the size of the chunks
intervaly = ysize / ysegment #Get the size of the chunks

#Setup to segment the image storing the start values and key into a dictionary.
xstart = 0
ystart = 0
key = 0

d = defaultdict(list)

for y in xrange(0, ysize, intervaly):
    if y + (intervaly * 2) < ysize:
        numberofrows = intervaly
    else:
        numberofrows = ysize - y

    for x in xrange(0, xsize, intervalx):
        if x + (intervalx * 2) < xsize:
            numberofcolumns = intervalx

        else:
            numberofcolumns = xsize - x
        l = [x,y,numberofcolumns, numberofrows]
        d[key].append(l)
        key += 1
return d

I realize that xrange is not ideal for a port to 3.

Answer 1

This code looks fine except for your use of defaultdict. A list seems like a much better data structure because:

• Your keys are sequential
• you are storing a list whose only element is another list in your dict.

One thing you could do:

• use the ternary operator (I'm not sure if this would be an improvement, but it would be fewer lines of code)

Here's a modified version of your code with my few suggestions.

intervalx = xsize / xsegment #Get the size of the chunks
intervaly = ysize / ysegment #Get the size of the chunks

#Setup to segment the image storing the start values and key into a dictionary.
xstart = 0
ystart = 0

output = []

for y in xrange(0, ysize, intervaly):
    numberofrows = intervaly if y + (intervaly * 2) < ysize else ysize -y
    for x in xrange(0, xsize, intervalx):
        numberofcolumns = intervalx if x + (intervalx * 2) < xsize else xsize -x
        lst = [x, y, numberofcolumns, numberofrows]
        output.append(lst)

        #If it doesn't make any difference to your program, the above 2 lines could read:
        #tple = (x, y, numberofcolumns, numberofrows)
        #output.append(tple)

        #This will be slightly more efficient 
        #(tuple creation is faster than list creation)
        #and less memory hungry.  In other words, if it doesn't need to be a list due
        #to other constraints (e.g. you append to it later), you should make it a tuple.

Now to get your data, you can do offset_list=output[5] instead of offset_list=d[5][0]