diff --git a/adversarial/advbox/attacks/gradientsign.py b/adversarial/advbox/attacks/gradientsign.py
index 33fd92e71b7dc11c2c6ab3417a193faded3785f9..2d7a445020214ad0a36a1688eb6fb19ee3871080 100644
--- a/adversarial/advbox/attacks/gradientsign.py
+++ b/adversarial/advbox/attacks/gradientsign.py
@@ -43,6 +43,7 @@ class IteratorGradientSignAttack(Attack):
     This attack was originally implemented by Alexey Kurakin(Google Brain).
     Paper link: https://arxiv.org/pdf/1607.02533.pdf
     """
+
     def _apply(self, image_label, epsilons=100, steps=10):
         """
         Apply the iterative gradient sign attack.