write python ndarray to raster

I think I understood the steps to write an array to a raster file using python. however it doesn't work in my case and I don't know why. I have an array that I want to write to a Geotiff with Swiss Oblique Mercator Projection. I have a DEM as Geotiff in the exact same location with same projections and Geotransform, so I thought I just read all parameters from that DEM and use them to write my new raster. Here's the code that I'm, using:

driver.Register()
fn = '/media/LACIE_SHARE/davos2015.tif'
inDs = gdal.Open(fn)
if inDs is None:
    print 'Could not open ' + fn 
    sys.exit(1)

rows=inDs.RasterYSize
cols=inDs.RasterXSize

driver=inDs.GetDriver()
outDs=driver.Create('/home/grassdata/test.tif',cols,rows,1,gdal.GDT_Float32)

if outDs is None:
    print "crap"
    sys.exit(1)
outBand=outDs.GetRasterBand(1)
outData=coh0 #coh0 is the array I want to convert to raster. it's an array with float32 entries. 

outBand.WriteArray(outData) # this gives 0
outBand.FlushCache()
outBand.SetNoDataValue(-99) #this gives 0
outDs.SetGeoTransform(inDs.GetGeoTransform()) #this gives 0
outDs.SetProjection(inDs.GetProjection()) #this gives 0
del outData

the lines where I commented accordingly raise 0 as result and I don't understand why... Thanks for the help!