Using lists in Python functions [closed]

Question

I have some code in a function that uses a list, which works, but is there a way to write a list outside of the function and use it within the function code?

For example in my code below I have a list called variables and then each type in variables is again listed in the WOTable with the code remap['type'] but I would like a code that does not relist the variable again with remap[type] in WOTable rather have a list outside the function where the list parameters can change but the function code stay the same. This script is for a weighted overlay in ArcGIS 10.2.

I have tried calling the list as weighted_overlay(variables, other parameters), and using just variables = [] in the function code, which did not work.

def weighted_overlay(raster1, raster2, raster3, out_raster):
    in_raster1 = arcpy.Raster(raster1)
    in_raster2 = arcpy.Raster(raster2)
    in_raster3 = arcpy.Raster(raster3)

    variables = ['remap1', 'remap2', 'remap3']
    remap = {}
    for type in variables:
        remap[type] = RemapValue([[1, 1], [2, 2], [3, 3], [4,4], ["NODATA", "NODATA"]])

    WOTable = WOTable([[in_raster1, 50, "VALUE", remap['remap1']],
                   [in_raster2, 35, "VALUE", remap['remap2']],
                   [in_raster3, 15, "VALUE", remap['remap3']]], [1, 9, 1])

    outWeightedOverlay = WeightedOverlay(WOTable)
    outWeightedOverlay.save(out_raster)
    return

Sorry if this has been asked before but I was unable to find a solution and I do not have a lot of experience with python lists. Eventually I would like to make this a class function as part of a larger analysis script to reuse.

Answer 1

I have tried calling the list as weighted_overlay(variables, other parameters), and using just variables = [] in the function code, which did not work.

If you are defining the list outside of the function, you do not want to then redefine it within the function.

def weighted_overlay(variables, other parameters):
    variables = []
    print variables

variables = ['remap1', 'remap2', 'remap3']
weighted_overlay(variables, other parameters)

This outputs an empty list [], since variables = [] inside the function changed its contents.

def weighted_overlay(variables, other parameters):
    print variables

variables = ['remap1', 'remap2', 'remap3']
weighted_overlay(variables, other parameters)

This outputs ['remap1', 'remap2', 'remap3'] -- the function knows what's in the list, because you passed the list to the function.

def weighted_overlay(variables, other parameters):
    print variables

weighted_overlay(['remap4', 'remap5', 'remap6'], other parameters)

You can put the variables directly inside the function call if you want to save on code space. This also outputs ['remap1', 'remap2', 'remap3'].

---

Since each of the variables entries ends up referring to the same RemapValue, I'd instead suggest still using a list, but in a different way:

value = [50, 35, 15]

def weighted_overlay(value, other parameters):
    #other code
    remap = RemapValue([[1, 1], [2, 2], [3, 3], [4,4], ["NODATA", "NODATA"]])
    WOTable = WOTable([[in_raster1, value[0], "VALUE", remap],
                       [in_raster2, value[1], "VALUE", remap],
                       [in_raster3, value[2], "VALUE", remap]], [1, 9, 1])
    #other code

Answer 2

This question is a pure python question. In addition to @artwork21 suggestion, you can make your list a global variable and then manipulate that variable in any function. Please see Using global variables in a function other than the one that created them