How do you sort multiple columns of a CSV file using python?

Question

I am having issues figuring out how to sort a large data set into more useful data.

The original file in CSV format is shown below- the data indicates x,y,z positions and finally energy. The x,y,z coordinates spread for quite a ways this is a small snippet below- basically it was an energy search over a volume.

-2.800000,-1.000000,5.470000,-0.26488315
-3.000000,1.000000,4.070000,-0.81185718
-2.800000,-1.000000,3.270000,1.29303723
-2.800000,-0.400000,4.870000,-0.51165026

Unfortunately its very difficult to plot in the requisite four dimensions so I need to trim this data. I would like to do this in such a way that I will turn the volume into a surface on the lowest energy z axis. On smaller data sets this was simple, in excel sort by X then Y and then energy, then delete all energies above the lowest. This was easy enough for small sets of data but has quickly become problematic.

I have tried various ways of doing this such as splitting the csv and using the sort command, but I am having little luck. Any advice on how to approach this would be much appreciated.

Answer 1

This does what you ask in your comment to Raymond's answer -- returns just the row with the lowest z for each x, y pair:

from operator import itemgetter
from itertools import groupby
from csv import reader


def min_z(iterable):
    # the data converted from strings to numbers
    floats = [[float(n) for n in row] for row in iterable]
    # the data sorted by x, y, z
    floats.sort(key=lambda (x, y, z, e): (x, y, z))
    # group the data by x, y
    grouped_floats = groupby(floats, key=itemgetter(slice(0, 2)))
    # return the first item from each group
    # because the data is sorted
    # the first item is the smallest z for the x, y group
    return [next(rowgroup) for xy, rowgroup in grouped_floats]


data = """-2.800000,-1.000000,5.470000,-0.26488315
-3.000000,1.000000,4.070000,-0.81185718
-2.800000,-1.000000,3.270000,1.29303723
-2.800000,-0.400000,4.870000,-0.51165026""".splitlines()


print min_z(reader(data))

Prints:

[[-3.0, 1.0, 4.07, -0.81185718], 
 [-2.8, -1.0, 3.27, 1.29303723], 
 [-2.8, -0.4, 4.87, -0.51165026]]