UserWarning: module dap was already imported from None

I installed python-matploglib and python-mpltoolkits.basemap using apt, but when I tried to import Basemap using from mpltoolkits.basemap import Basemap, the following warning shows up:

usr/lib/pymodules/python2.7/mpl_toolkits/__init__.py:2: UserWarning: Module dap was already imported from None, but /usr/lib/python2.7/dist-packages is being added to sys.path
  __import__('pkg_resources').declare_namespace(__name__)
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