TypeError：无法解压不可迭代的 NoneType 对象

我知道这个问题之前已经被问过，但我似乎无法让我的问题工作。

import numpy as np


def load_dataset():
    def download(filename, source="http://yaan.lecun.com/exdb/mnist/"):
        print ("Downloading ",filename)
        import urllib
        urllib.urlretrieve(source+filename,filename)

    import gzip
    
    def load_mnist_images(filename):
        if not os.path.exists(filename):
            download(filename)
        with gzip.open(filename,"rb") as f:
            data=np.frombuffer(f.read(), np.uint8, offset=16)
            
            data = data.reshape(-1,1,28,28)
            
            return data/np.float32(256)

        def load_mnist_labels(filename):
            if not os.path.exists(filename):
                download(filename)
            with gzip.open(filename,"rb") as f:
                data = np.frombuffer(f.read(), np.uint8, offset=8)
            return data

        X_train = load_mnist_images("train-images-idx3-ubyte.gz")
        y_train = load_mnist_labels("train-labels-idx1-ubyte.gz")
        X_test = load_mnist_images("t10k-images-idx3-ubyte.gz")
        y_test = load_mnist_labels("t10k-labels-idx1-ubyte.gz")

        return X_train, y_train, X_test, y_test


X_train, y_train, X_test, y_test = load_dataset()


import matplotlib
matplotlib.use("TkAgg")

import matplotlib.pyplot as plt
plt.show(plt.imshow(X_train[3][0]))

这是我收到的错误：

Traceback (most recent call last):
  File "C:\Users\nehad\Desktop\Neha\Non-School\Python\Handwritten Digits 
Recognition.py", line 38, in <module>
    X_train, y_train, X_test, y_test = load_dataset()
TypeError: cannot unpack non-iterable NoneType object

我是机器学习的新手。我只是错过了一些简单的事情吗？我正在为我的学校科学展览尝试一个手写数字识别项目。

当您执行多重分配时，您会收到此错误None（这是NoneType）。例如：

X_train, y_train, X_test, y_test = None

TypeError：无法解压不可迭代的 NoneType 对象

因此，如果您收到此错误，则错误很可能是作业的右侧部分不是您所期望的（什么都没有）。