Iterate over variants and call a function on each.

def visit_variants(self, func, variants=None, **kwargs):
        """Iterate over variants and call a function on each."""
        if variants:
            present_variants = range(self.package.num_variants)
            invalid_variants = set(variants) - set(present_variants)
            if invalid_variants:
                raise BuildError(
                    "The package does not contain the variants: %s"
                    % ", ".join(str(x) for x in sorted(invalid_variants)))

        # iterate over variants
        results = []
        num_visited = 0

        for variant in self.package.iter_variants():
            if variants and variant.index not in variants:
                self._print_header(
                    "Skipping variant %s (%s)..."
                    % (variant.index, self._n_of_m(variant)))
                continue

            # visit the variant
            result = func(variant, **kwargs)
            results.append(result)
            num_visited += 1

        return num_visited, results