Calculate the difference or division between bands in two different products

I need to calculate the difference and division between two bands in two different products.
In other words, I have products a and b, and I need to calculate - **b.**band as well as

In the SNAP desktop, I am able to construct proper expression, but I don’t know how should it looks like when creating BandDescriptor() in snappy/Python.

Currently I have following code (with incorrect targetBand1.expression part):

def getDiff(file1,file2,band='Soil_Moisture'):
    import snappy
    from snappy import GPF
    from snappy import ProductIO         jpy=snappy.jpy

    products = [ snappy.ProductIO.readProduct(file1), snappy.ProductIO.readProduct(file2) ]
    #verify if products contain selected band
    for prod in products:
        if not (band in prod.getBandNames()):
            print(band + " not in the " + prod.getName() + " Exiting")


    HashMap = jpy.get_type('java.util.HashMap')
    BandDescriptor = jpy.get_type('org.esa.snap.core.gpf.common.BandMathsOp$BandDescriptor')

    targetBand1 = BandDescriptor() = 'Difference'
    targetBand1.type = 'float32'
    ## Here is a problem - following expression is NOT valid:
    targetBand1.expression = band + ' - $2.' + band

    targetBands = jpy.array('org.esa.snap.core.gpf.common.BandMathsOp$BandDescriptor', 1)
    targetBands[0] = targetBand1

    parameters = HashMap()
    parameters.put('targetBands', targetBands)

    ## After executing following line, I get:
    ## org.esa.snap.core.gpf.OperatorException: Could not parse expression: 'Soil_Moisture - $2.Soil_Moisture'. Undefined symbol '$2.Soil_Moisture'.
    result = GPF.createProduct('BandMaths', parameters, products[0])

    print("Write results")
    ProductIO.writeProduct(result, 'difference_output.dim', 'BEAM-DIMAP')
    return 'difference_output.dim'

The correct way to form this expression is: band + ’ - $sourceProduct.1.’ + band
Note that the index is zero-based, so index 1 refers to the second product. I also noticed you only pass one product when calling the GPF. You should pass the whole array.

You may also put the source products in a map and assign names to them in that fashion, e.g.:
productMap = HashMap()
productMap.put(‘numberOne’, product1);
productMap.put(‘numberTwo’, product2);
The expression would then look like this: band + ’ - $numberTwo.’ + band


@TonioF thank you, it works properly now.