[ Read set of images into 4D Numpy array with dimension (num_img,channel, dim1, dim2) ]

I have a set of 1000 gray scale images (28x28), I want to read them into 4D numpy array (number of images, 1, img_dim1,img_dim2). Following is my code but it doesn't work properly. Any idea how I can fix the issue in the code?

from PIL import Image
import numpy as np
import os

mypath=os.path.dirname('path/to/directory/')
def load_dataset( ) :
    data =np.zeros((1000,1,28,28), dtype=np.float64)
    for fname in os.listdir(mypath):
        pathname = os.path.join(mypath, fname)
        img = Image.open(pathname)
        data = np.dstack((data, img))
    return data
data=load_dataset()
print(data.shape)

Answer 1

The problem has been solved by using append and adding a new axis np.newaxis

from PIL import Image
import numpy as np
import os

mypath=os.path.dirname('path/to/directory/')
def load_dataset( ) :
    data =[]
    for fname in os.listdir(mypath):
        pathname = os.path.join(mypath, fname)
        img = Image.open(pathname)
        img1 = img[np.newaxis,:,:]
        data.append(img1)
    return data

data= load_dataset()
data_x=np.array(data)
print data_x.shape