def add(self, elem, replace=False):
        """Add a requirement.

        :param elem: a string or :class:`Requirement` instance.
        :param replace: if `True`, packages in the set with the same name will
            be removed first.
        """
        if isinstance(elem, text_type):
            elem = Requirement.coerce(elem)
        if replace and elem in self:
            self.remove(elem)
        super(Requirements, self).add(elem)
def install_secret_key(app, filename='secret_key'):
    """Configure the SECRET_KEY from a file
    in the instance directory.

    If the file does not exist, print instructions
    to create it from a shell with a random key,
    then exit.
    """
    filename = os.path.join(app.instance_path, filename)

    try:
        app.config['SECRET_KEY'] = open(filename, 'rb').read()
    except IOError:
        print('Error: No secret key. Create it with:')
        full_path = os.path.dirname(filename)
        if not os.path.isdir(full_path):
            print('mkdir -p {filename}'.format(filename=full_path))
        print('head -c 24 /dev/urandom > {filename}'.format(filename=filename))
        sys.exit(1)