samples: fix python samples compatibility with demo.py

samples/python/dft.py

@@ -11,8 +11,9 @@ USAGE:
 # Python 2/3 compatibility
 from __future__ import print_function
 
-import cv2 as cv
 import numpy as np
+import cv2 as cv
+
 import sys
 
 
@@ -62,8 +63,8 @@ def shift_dft(src, dst=None):
 
     return dst
 
-if __name__ == "__main__":
 
+def main():
     if len(sys.argv) > 1:
         fname = sys.argv[1]
     else:
@@ -110,4 +111,10 @@ if __name__ == "__main__":
     cv.imshow("magnitude", log_spectrum)
 
     cv.waitKey(0)
+    print('Done')
+
+
+if __name__ == '__main__':
+    print(__doc__)
+    main()
     cv.destroyAllWindows()