AttributeError: 'module' object has no attribute 'Graph'

[SeekPoint]

mldl@ub1604:/ub16_prj/FacebookCommunityDetection$ python marketSegment.py
Traceback (most recent call last):
File "marketSegment.py", line 12, in
g= ig.Graph.Read_Ncol('./data/fb_caltech_small_edgelist.txt',directed=False)
AttributeError: 'module' object has no attribute 'Graph'
mldl@ub1604:/ub16_prj/FacebookCommunityDetection$