def install_pip(self, package, save=True, save_dev=False):
        """Installs a pip package.

        :param save: if `True`, pins the package to the Hydrogen requirements
            YAML file.
        :param save_dev: if `True`, pins the package as a development
            dependency to the Hydrogen requirements YAML file.
        :param return: a **single** :class:`Requirement` object, representing
            the installed version of the given package.
        """
        requirement = Requirement.coerce(package)
        click.echo("pip install " + requirement.package)
        cmd = envoy.run("pip install {}".format(str(requirement)))
        if cmd.status_code == 0:
            installed_packages = get_installed_pypackages()
            package = installed_packages[requirement.package]
            requirement.version = "=={}".format(package.version)
            if save:
                self.requirements["all"].add(requirement)
            if save_dev:
                self.requirements["dev"].add(requirement)
            if save or save_dev:
                self.requirements.save()
            return requirement
        else:
            fatal(cmd.std_err)
def is_immutable_rev_checkout(self, url, dest):
        # type: (str, str) -> bool
        _, rev_options = self.get_url_rev_options(hide_url(url))
        if not rev_options.rev:
            return False
        if not self.is_commit_id_equal(dest, rev_options.rev):
            # the current commit is different from rev,
            # which means rev was something else than a commit hash
            return False
        # return False in the rare case rev is both a commit hash
        # and a tag or a branch; we don't want to cache in that case
        # because that branch/tag could point to something else in the future
        is_tag_or_branch = bool(
            self.get_revision_sha(dest, rev_options.rev)[0]
        )
        return not is_tag_or_branch