Feature download

def download_feature(genome='H_sapiens',version='H_sapiens_Feb_2009',chromosome='chr1',datatype='refseq',format='brs'):
    URL = 'http://netaffxdas.affymetrix.com/das2/genome'
    affy = DAS2Sources(URL)
    feature_url = ''.join((affy.prop_base,affy.capability('H_sapiens','H_sapiens_Feb_2009','features')))
    
    segment_url = ''.join((affy.prop_base,affy.capability('H_sapiens','H_sapiens_Feb_2009','segments')))
    segment_query = "segment="+'/'.join((segment_url,chromosome))

    type_url = ''.join((affy.prop_base,affy.capability('H_sapiens','H_sapiens_Feb_2009','types')))
    type_query = "type="+'/'.join((segment_url,datatype))
    
    format_query='format='+format
    
    query_param = encode_url(';'.join((segment_query,type_query,format_query)))
    
    return '?'.join((feature_url,query_param))