title: No module named _gdal
body: <p>I am trying to run a Python script test.py to test the gdal library.  The script contains the line:</p>
  
  <pre><code>from osgeo import gdal
  </code></pre>
  
  <p>At that line I get the traceback:</p>
  
  <pre><code>File "~/test.py", line 9, in &lt;module&gt;
      from osgeo import gdal
  File "/usr/local/lib/python2.7/site-packages/GDAL-2.2.0/osgeo/__init__.py", line 21, in &lt;module&gt;
      gdal = swing_import_helper()
  File "/usr/local/lib/python2.7/site-packages/GDAL-2.2.0/osgeo/__init__.py", line 13, in swig_import_helper
      import gdal
  ImportError: No module named _gdal
  </code></pre>
  
  <p>I don't see any _gdal Python files or any scripts that define _gdal.  Do I need to install another (prerequisite) library that defines _gdal?  The GDAL package should include everything needed to install the library and run scripts that import gdal.</p>