Check for presence of gzip and bz2 modules

wrappers/python/simtk/openmm/app/statedatareporter.py

@@ -31,8 +31,16 @@ USE OR OTHER DEALINGS IN THE SOFTWARE.
 __author__ = "Peter Eastman"
 __version__ = "1.0"
 
-import bz2
-import gzip
+try:
+    import bz2
+    have_bz2 = True
+except: have_bz2 = False
+
+try:
+    import gzip
+    have_gzip = True
+except: have_gzip = False
+
 import simtk.openmm as mm
 import simtk.unit as unit
 import math
@@ -71,8 +79,12 @@ class StateDataReporter(object):
             # Detect the desired compression scheme from the filename extension
             # and open all files unbuffered
             if file.endswith('.gz'):
+                if not have_gzip:
+                    raise RuntimeError("Cannpt write .gz file because Python could not import gzip library")
                 self._out = gzip.GzipFile(fileobj=open(file, 'wb', 0))
             elif file.endswith('.bz2'):
+                if not have_bz2:
+                    raise RuntimeError("Cannpt write .bz2 file because Python could not import bz2 library")
                 self._out = bz2.BZ2File(file, 'w', 0)
             else:
                 self._out = open(file, 'w', 0)