Implement checking for loaded image

imgrate.py

@@ -2,15 +2,28 @@
 import cv2
 import numpy as np
 from argparse import ArgumentParser
+from functools import wraps
+
+
+def image_required(fun):
+    @wraps(fun)
+    def wrapper(self, *args, **kwargs):
+        if self.image is None:
+            raise RuntimeError('Method {}.{} requires an image!'
+                               .format(self.__class__.__name__, fun.__name__))
+        return fun(self, *args, **kwargs)
+    return wrapper
 
 
 class imgrate:
     def __init__(self, image=None):
         self.image = image
 
+    @image_required
     def laplacian_variance(self):
         return cv2.Laplacian(self.image, cv2.CV_64F).var()
 
+    @image_required
     def fdibm(self):
         F = np.fft.fft2(self.image)
         Fcentered = np.fft.fftshift(F)
@@ -28,4 +41,4 @@ if __name__ == '__main__':
     image = cv2.imread(args.images[0])
     grayscale = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)
 
-    print(imgrate(grayscale).fdibm())
+    print(imgrate(grayscale).laplacian_variance())