Download GAF files if necessary.

def dnld_gafs(species_list, prt=sys.stdout, loading_bar=True):
    """Download GAF files if necessary."""
    # Example GAF files in  http://current.geneontology.org/annotations/:
    #   http://current.geneontology.org/annotations/mgi.gaf.gz
    #   http://current.geneontology.org/annotations/fb.gaf.gz
    #   http://current.geneontology.org/annotations/goa_human.gaf.gz
    http = "http://current.geneontology.org/annotations"
    # There are two filename patterns for gene associations on geneontology.org
    fin_gafs = []
    cwd = os.getcwd()
    for species_txt in species_list: # e.g., goa_human mgi fb
        gaf_base = '{ABC}.gaf'.format(ABC=species_txt) # goa_human.gaf
        gaf_cwd = os.path.join(cwd, gaf_base) # {CWD}/goa_human.gaf
        wget_cmd = "{HTTP}/{GAF}.gz".format(HTTP=http, GAF=gaf_base)
        dnld_file(wget_cmd, gaf_cwd, prt, loading_bar)
        fin_gafs.append(gaf_cwd)
    return fin_gafs