# Nested list comprehensions act like real for loops: >>> a = [[None for i in range(3)] for j in range(3)] >>> a [[None, None, None], [None, None, None], [None, None, None]] >>> a[0][0] = 5 >>> a [[5, None, None], [None, None, None], [None, None, None]] # Side-effect: i and j will be changed.
# Another way (much less clear to my eyes): >>> a = map(lambda x: map(lambda x: None, range(3)), range(3)) >>> a[0][0]=5 >>> a [[5, None, None], [None, None, None], [None, None, None]] -- http://mail.python.org/mailman/listinfo/python-list
