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 toggle_comment(self):
        "Toggle the selected comment tree between visible and hidden"

        current_index = self.nav.absolute_index
        self.content.toggle(current_index)

        # This logic handles a display edge case after a comment toggle. We
        # want to make sure that when we re-draw the page, the cursor stays at
        # its current absolute position on the screen. In order to do this,
        # apply a fixed offset if, while inverted, we either try to hide the
        # bottom comment or toggle any of the middle comments.
        if self.nav.inverted:
            data = self.content.get(current_index)
            if data['hidden'] or self.nav.cursor_index != 0:
                window = self._subwindows[-1][0]
                n_rows, _ = window.getmaxyx()
                self.nav.flip(len(self._subwindows) - 1)
                self.nav.top_item_height = n_rows