[Tutor] raw_input() slice list

[questions anon]

I would like to use user_input() to decide how to slice a list.
This works fine until I try to leave it blank to try and select the whole
list [:]
I have posted the section of interest below and the error I get when I try
to press enter. Further below that is the entire code.
Any feedback will be greatly appreciated.
*
section of code of interest:*
        startperiod=int(raw_input("Start slice (e.g. 1 ): "))
        endperiod=int(raw_input("End slice (e.g. 2): "))
        skipperiod=int(raw_input("skip slice (e.g. 1): "))

        if startperiod=="" and endperiod=="" and skipperiod=="":
                startperiod=""
                endperiod=""
                skipperiod=""


        for (path, dirs, files) in os.walk(MainFolder):
                        for dir in dirs:
                                print dir
                        path=path+'/'

                        for ncfile in files:
                                if ncfile[-3:]=='.nc':
                                    print "dealing with ncfiles:",
path+ncfile
                                    ncfile=os.path.join(path,ncfile)
                                    ncfile=Dataset(ncfile, 'r+', 'NETCDF4')

variable=ncfile.variables[ncvariablename][:]
                                    TIME=ncfile.variables['time'][:]

fillvalue=ncfile.variables[ncvariablename]._FillValue
                                    ncfile.close()

                                    for variable, TIME in
zip((variable[startperiod:endperiod:skipperiod]),(TIME[startperiod:endperiod:skipperiod])):


*the error:*

Traceback (most recent call last):
  File "<pyshell#27>", line 1, in <module>
    plotrawdata('TSFC')
  File "D:\My Dropbox\Python_code\functions.py", line 39, in plotrawdata
    startperiod=int(raw_input("Start slice (e.g. 1 ): "))
ValueError: invalid literal for int() with base 10: ''


*THE WHOLE PROGRAM:*
from netCDF4 import Dataset
import numpy as N
import matplotlib.pyplot as plt
from numpy import ma as MA
from mpl_toolkits.basemap import Basemap
from netcdftime import utime
from datetime import datetime
import os
import matplotlib.colors as mc
import matplotlib.colorbar as c

OutputFolder=r"D:/DSE_work/temp_samples2/"
MainFolder=r"D:/DSE_work/temp_samples2/"


def plotrawdata(variable):
        if variable=='TSFC':
                ncvariablename='T_SFC'
                MainFolder=r"D:/DSE_work/temp_samples2/"
                ticks=[-5,0,5,10,15,20,25,30,35,40,45,50]
                Title='Surface Temperature'

        elif variable=='RHSFC':
                ncvariablename='RH_SFC'

MainFolder=r"E:/DSE_BushfireClimatologyProject/griddeddatasamples/temp_samples6/"
                ticks=[0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 101]
                Title='Surface RH'


fileforlatlon=Dataset("D:/DSE_work/temp_samples2/2020/01/IDZ00026_VIC_ADFD_T_SFC.nc",
'r+', 'NETCDF4')
        LAT=fileforlatlon.variables['latitude'][:]
        LON=fileforlatlon.variables['longitude'][:]

        startperiod=int(raw_input("Start slice (e.g. 1 ): "))
        endperiod=int(raw_input("End slice (e.g. 2): "))
        skipperiod=int(raw_input("skip slice (e.g. 1): "))

        if startperiod=="" and endperiod=="" and skipperiod=="":
                startperiod=str("")
                endperiod=str("")
                skipperiod=str("")


        for (path, dirs, files) in os.walk(MainFolder):
                        for dir in dirs:
                                print dir
                        path=path+'/'

                        for ncfile in files:
                                if ncfile[-3:]=='.nc':
                                    print "dealing with ncfiles:",
path+ncfile
                                    ncfile=os.path.join(path,ncfile)
                                    ncfile=Dataset(ncfile, 'r+', 'NETCDF4')

variable=ncfile.variables[ncvariablename][:]
                                    TIME=ncfile.variables['time'][:]

fillvalue=ncfile.variables[ncvariablename]._FillValue
                                    ncfile.close()

                                    for variable, TIME in
zip((variable[startperiod:endperiod:skipperiod]),(TIME[startperiod:endperiod:skipperiod])):
                                    #for variable, TIME in
zip((variable[sliceperiod]),(TIME[sliceperiod])):

                                            cdftime=utime('seconds since
1970-01-01 00:00:00')

ncfiletime=cdftime.num2date(TIME)
                                            print ncfiletime
                                            timestr=str(ncfiletime)
                                            d = datetime.strptime(timestr,
'%Y-%m-%d %H:%M:%S')
                                            date_string =
d.strftime('%Y%m%d_%H%M')
                                            #Set up basemap using mercator
projection
http://matplotlib.sourceforge.net/basemap/doc/html/users/merc.html
                                            map =
Basemap(projection='merc',llcrnrlat=-40,urcrnrlat=-33,

llcrnrlon=139.0,urcrnrlon=151.0,lat_ts=0,resolution='i')
                                            x,y=map(*N.meshgrid(LON,LAT))

map.drawcoastlines(linewidth=0.5)
                                            #map.readshapefile(shapefile1,
'DSE_REGIONS')
                                            map.drawstates()

                                            plt.title(Title+' %s
UTC'%ncfiletime)

                                            CS = map.contourf(x,y,variable,
ticks, cmap=plt.cm.jet)
                                            l,b,w,h =0.1,0.1,0.8,0.8
                                            cax = plt.axes([l+w+0.025, b,
0.025, h], )
                                            cbar=plt.colorbar(CS, cax=cax,
drawedges=True)

                                            #save map as *.png and plot
netcdf file

plt.savefig((os.path.join(OutputFolder,
ncvariablename+date_string+'UTC.png')))
                                            plt.show()
                                            plt.close() # must use
plt.close() so that colorbar works!
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