code: def apply_train(self, df:DataFrame):
          "Compute the means and stds of `self.cont_names` columns to normalize them."
          self.means,self.stds = {},{}
          for n in self.cont_names:
              assert is_numeric_dtype(df[n]), (f)
              self.means[n],self.stds[n] = df[n].mean(),df[n].std()
              df[n] = (df[n]-self.means[n]) / (1e-7 + self.stds[n])
docs: Compute the means and stds of `self.cont_names` columns to normalize them.