Get enrichment arg parser.

def _init_args(self):
        """Get enrichment arg parser."""

        #pylint: disable=invalid-name
        p = argparse.ArgumentParser(__doc__,
                                    formatter_class=argparse.ArgumentDefaultsHelpFormatter)

        p.add_argument('filenames', type=str, nargs=3,
                       help='data/study data/population data/association')
        p.add_argument('--annofmt', default=None, type=str,
                       help=('Annotation file format. '
                             'Not needed if type can be determined using filename'),
                       choices=['gene2go', 'gaf', 'gpad', 'id2gos'])
        p.add_argument('--taxid', default=9606, type=int,
                       help="When using NCBI's gene2go annotation file, specify desired taxid")
        p.add_argument('--alpha', default=0.05, type=float,
                       help='Test-wise alpha for multiple testing')
        p.add_argument('--pval', default=.05, type=float,
                       help='Only print results with uncorrected p-value < PVAL.')
        p.add_argument('--pval_field', type=str,
                       help='Only print results when PVAL_FIELD < PVAL.')
        p.add_argument('--outfile', default=None, type=str,
                       help='Write enrichment results into xlsx or tsv file')
        p.add_argument('--id2sym', default=None, type=str,
                       help='ASCII file containing one geneid and its symbol per line')
        p.add_argument('--sections', default=None, type=str,
                       help=('Use sections file for printing grouped GOEA results. '
                             'Example SECTIONS values:\n'
                             'goatools.test_data.sections.gjoneska_pfenning \n'
                             'goatools/test_data/sections/gjoneska_pfenning.py \n'
                             'data/gjoneska_pfenning/sections_in.txt\n'))
        p.add_argument('--outfile_detail', type=str,
                       help=('Write enrichment results into a text file \n'
                             'containing the following information: \n'
                             '1) GOEA GO terms, grouped into sections \n\n'
                             '2) List of genes and ASCII art showing section membership \n'
                             '3) Detailed list of each gene and GO terms w/their P-values \n'))
        p.add_argument('--compare', dest='compare', default=False,
                       action='store_true',
                       help="the population file as a comparison group. if this "
                       "flag is specified, the population is used as the study "
                       "plus the `population/comparison`")
        p.add_argument('--ratio', dest='ratio', type=float, default=None,
                       help="only show values where the difference between study "
                       "and population ratios is greater than this. useful for "
                       "excluding GO categories with small differences, but "
                       "containing large numbers of genes. should be a value "
                       "between 1 and 2. ")
        p.add_argument('--indent', dest='indent', default=False,
                       action='store_true', help="indent GO terms")
        p.add_argument('--obo', default="go-basic.obo", type=str,
                       help="Specifies location and name of the obo file")
        p.add_argument('--no_propagate_counts', default=False, action='store_true',
                       help="Do not propagate counts to parent terms")
        p.add_argument('--method', default="bonferroni,sidak,holm,fdr_bh", type=str,
                       help=Methods().getmsg_valid_methods())
        p.add_argument('--pvalcalc', default="fisher", type=str,
                       help=str(FisherFactory()))
        p.add_argument('--min_overlap', default=0.7, type=float,
                       help="Check that a minimum amount of study genes are in the population")
        p.add_argument('--goslim', default='goslim_generic.obo', type=str,
                       help="The GO slim file is used when grouping GO terms.")
        p.add_argument('--ev_inc', type=str,
                       help="Include specified evidence codes and groups separated by commas")
        p.add_argument('--ev_exc', type=str,
                       help="Exclude specified evidence codes and groups separated by commas")
        p.add_argument('--ev_help', dest='ev_help', action='store_false',
                       help="Print all Evidence codes")

        if len(sys.argv) == 1:
            sys.exit(not p.print_help())
        if '--ev_help' in sys.argv:
            print('\nEVIDENCE CODE HELP: --ev_exc --ev_inc')
            print('Use any of these group names, ')
            print('like Experimental or Similarity or Experimental,Similarity,')
            print('or evidence codes, like IEA or ISS,ISO,ISA in --ev_exc or --ev_inc:\n')
            obj = EvidenceCodes()
            obj.prt_details()
            sys.exit(0)

        args = p.parse_args()  # Namespace object from argparse
        self._check_input_files(args, p)
        return args