Permuted MNIST class. AttributeError: can't set attribute

Running this code for getting permuted MNIST raises an error on the bolded if statement:
AttributeError: can’t set attribute
Can anyone help me with understanding the reason behind error and how to fix it?

import random
import torch
from torchvision import datasets

class PermutedMNIST(datasets.MNIST):

    def __init__(self, root="~/.torch/data/mnist", train=True, permute_idx=None):
        super(PermutedMNIST, self).__init__(root, train, download=True)
        assert len(permute_idx) == 28 * 28
        **if self.train:**
**            self.train_data = torch.stack([img.float().view(-1)[permute_idx] / 255 for img in self.train_data()])**
            self.test_data = torch.stack([img.float().view(-1)[permute_idx] / 255 for img in self.test_data()])

    def __getitem__(self, index):

        if self.train:
            img, target = self.train_data[index], self.train_labels[index]
            img, target = self.test_data[index], self.test_labels[index]

        return img, target

    def get_sample(self, sample_size):
        sample_idx = random.sample(range(len(self)), sample_size)
        return [img for img in self.train_data[sample_idx]]
    def train_data(self):
        warnings.warn("train_data has been renamed data")

self.train_data is a read-only property. Try another name such as self.my_data.