Sort os.listdir files Python

If have downloaded several years of data stored in files with the following naming convention, year_day.dat. For example, the file named 2014_1.dat has the data for January 1, 2014. I need to read these data files ordered by day, 2014_1.dat, 2014_2.dat, 2014_3.dat until the end of the year. In the folder they are listed in that ordered BUT when I create a list of the files in the directory they are reordered 2014_1.dat, 2014_10.dat, 2014_100.dat, 2014_101.dat...2014.199.dat, 2014_2.dat. I think I need to use a sort function but how do I force it to sort the listed files by day so I can continue processing them? Here's the code so far: ``` import sys, os, gzip, fileinput, collections # Set the input/output directories wrkDir = "C:/LJBTemp" inDir = wrkDir + "/Input" outDir = wrkDir + "/Output" # here we go inList = os.listdir(inDir) # List all the files in the 'Input' directory print inList #print to screen reveals 2014_1.dat.gz followed by 2014_10.dat.gz NOT 2014_2.dat.gz HELP d = {} for fileName in inList: # Step through each input file readFileName = inDir + "/" + fileName with gzip.open(readFileName, 'r') as f: #call built in utility to unzip file for reading for line in f: city, long, lat, elev, temp = line.split() #create dictionary d.setdefault(city, []).append(temp) #populate dictionary with city and associated temp data from each input file collections.OrderedDict(sorted(d.items(), key=lambda d: d[0])) # QUESTION? why doesn't this work #now collect and write to output file outFileName = outDir + "/" + "1981_maxT.dat" #create output file in output directory with .dat extension with open(outFileName, 'w') as f: for city, values in d.items(): f.write('{} {}\n'.format(city, ' '.join(values))) print "All done!!" raw_input("Press <enter>") # this keeps the window open until you press "enter" ```