How to use the pyfaidx.__init__.FastaVariant function in pyfaidx

def __init__(self,
                 filename,
                 vcf_file,
                 sample=None,
                 het=True,
                 hom=True,
                 call_filter=None,
                 **kwargs):
        super(FastaVariant, self).__init__(filename, **kwargs)
        try:
            import pysam
        except ImportError:
            raise ImportError("pysam must be installed for FastaVariant.")
        try:
            import vcf
        except ImportError:
            raise ImportError("PyVCF must be installed for FastaVariant.")
        if call_filter is not None:
            try:
                key, expr, value = call_filter.split()  # 'GQ > 30'
            except IndexError:
                raise ValueError(
                    "call_filter must be a string in the format 'XX <>!= NN'")
            assert all([x in self.expr for x in list(expr)])
            assert all([x in string.ascii_uppercase for x in list(key)])

def variant_sites(self):
        if isinstance(self._fa, FastaVariant):
            pos = []
            var = self._fa.vcf.fetch(self.name, 0, len(self))
            for site in var:
                if site.is_snp:
                    sample = site.genotype(self._fa.sample)
                    if sample.gt_type in self._fa.gt_type and eval(
                            self._fa.filter):
                        pos.append(site.POS)
            return tuple(pos)
        else:
            raise NotImplementedError(
                "variant_sites() only valid for FastaVariant.")