How to solve the AttributeError:'list' object has no attribute 'astype'?

Question

I am just wondering how to solve the attribute error in python3.6. The error is

'list' object has no attribute 'astype'.

My related code is as blow.

def _init_mean_std(self, data):
    data = data.astype('float32')
    self.mean, self.std = np.mean(data), np.std(data)
    self.save_meanstd()
    return data

Is there anyone who can advice to me?

Thank you!