[Tutor] Funtions, modules & global variables

[questions anon]

Hi All,
I am trying to move from writing one long script each time I need to do a
new thing but I do not quite understand how functions work, even after
reading many manuals.
My first attempt is to create a python file where I have defined a number of
functions that get data in various ways.
I have also made another python file where I want to plot the data in
different ways. but I am having trouble getting that data to run in my
python plotting file.
I have made the variable I require 'global' but that doesn't seem to be
working. I keep receiving:
*
UnboundLocalError: local variable 'TSFC' referenced before assignment*

Below is a some of the script for both folder, just note that their will be
many functions in each folder
Any help will be greatly appreciated!!!

#selectdata.py
def selectalldata():
        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')
                    global TSFC
                    TSFC=ncfile.variables['T_SFC'][:,:,:]
                    LAT=ncfile.variables['latitude'][:]
                    LON=ncfile.variables['longitude'][:]
                    TIME=ncfile.variables['time'][:]
                    fillvalue=ncfile.variables['T_SFC']._FillValue
                    ncfile.close()


#plotdata.py

from selectdata import selectalldata
selectalldata()

def plotncfilewithtime():
    for TSFC, TIME in zip((TSFC[1::3]),(TIME[1::3])):
    #convert time from numbers to date and prepare it to have no symbols for
saving to filename
        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')
        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('Surface temperature at %s UTC'%ncfiletime)
        ticks=[-5,0,5,10,15,20,25,30,35,40,45,50]
        CS = map.contourf(x,y,TSFC, 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)

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

    print "end of processing"
plotncfilewithtime()
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