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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def push(self, cart, env=None, callback=None):
 """ `cart` - Release cart to push items from `callback` - Optional callback to call if juicer.utils.upload_rpm succeeds Pushes the items in a release cart to the pre-release environment. """

        juicer.utils.Log.log_debug("Initializing push of cart '%s'" % cart.cart_name)

        if not env:
            env = self._defaults['start_in']

        cart.current_env = env
        self.sign_cart_for_env_maybe(cart, env)
        self.upload(env, cart, callback)
        return True