docstring

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

@@ -41,19 +41,44 @@ class CheckpointReporter(object):
     will be saved in the file.
 
     To use it, create a CheckpointReporter, then add it to the Simulation's
-    list of reporters.
+    list of reporters. To load a checkpoint file a continue a simulation,
+    use the following recipe:
+
+    >>> with open('checkput.chk', 'rb') as f:
+    >>>     simulation.context.loadCheckpoint(f.read())
+
+    Notes:
+    A checkpoint contains not only publicly visible data such as the particle
+    positions and velocities, but also internal data such as the states of
+    random number generators.  Ideally, loading a checkpoint should restore the
+    Context to an identical state to when it was written, such that continuing
+    the simulation will produce an identical trajectory.  This is not strictly
+    guaranteed to be true, however, and should not be relied on.  For most
+    purposes, however, the internal state should be close enough to be
+    reasonably considered equivalent.
+
+    A checkpoint contains data that is highly specific to the Context from
+    which it was created. It depends on the details of the System, the
+    Platform being used, and the hardware and software of the computer it was
+    created on.  If you try to load it on a computer with different hardware,
+    or for a System that is different in any way, loading is likely to fail.
+    Checkpoints created with different versions of OpenMM are also often
+    incompatible.  If a checkpoint cannot be loaded, that is signaled by
+    throwing an exception.
+
     """
     def __init__(self, file, reportInterval):
         """Create a CheckpointReporter.
 
         Parameters:
-         - file (string or open file object) The file to write to
+         - file (string or open file object) The file to write to. Any current
+           contents will be overwritten.
          - reportInterval (int) The interval (in time steps) at which to write checkpoints
         """
 
         self._reportInterval = reportInterval
         if isinstance(file, basestring):
-            self._out = open(file, 'wb')
+            self._out = open(file, 'w+b')
         else:
             self._out = file
 
@@ -78,4 +103,5 @@ class CheckpointReporter(object):
          - state (State) The current state of the simulation
         """
         self._out.seek(0)
-        self._out.write(simulation.context.createCheckpoint())
+        chk = simulation.context.createCheckpoint()
+        self._out.write(chk)