Avoid mutating class attributes
Avoid mutating class attributes

keisuke-umezawa/chainer,okuta/chainer,chainer/chainer,hvy/chainer,wkentaro/chainer,okuta/chainer,chainer/chainer,tkerola/chainer,keisuke-umezawa/chainer,chainer/chainer,wkentaro/chainer,hvy/chainer,keisuke-umezawa/chainer,okuta/chainer,wkentaro/chainer,pfnet/chainer,niboshi/chainer,keisuke-umezawa/chainer,hvy/chainer,niboshi/chainer,niboshi/chainer,wkentaro/chainer,hvy/chainer,chainer/chainer,niboshi/chainer,okuta/chainer