Correct outputs for describeNextReport and add testing.

mdareporter/mdareporter.py

@@ -36,9 +36,8 @@
 USE OR OTHER DEALINGS IN THE SOFTWARE.
 """
 
-
-
 import MDAnalysis as mda
+from MDAnalysis.lib.util import get_ext
 from MDAnalysis.lib.mdamath import triclinic_box
 from openmm import unit
 import numpy as np
@@ -102,7 +101,28 @@ def describeNextReport(self, simulation):
             positions should be wrapped to lie in a single periodic box.
         """
         steps = self._reportInterval - simulation.currentStep%self._reportInterval
-        return (steps, True, False, False, False, self._enforcePeriodicBox)
+
+        try:
+            # this will be called again inside mda.Writer but we need the ext here
+            root, ext = get_ext(self._filename)
+        except (TypeError, AttributeError):
+            errmsg = f'File format could not be guessed from "{self._filename}"'
+            raise ValueError(errmsg) from None
+
+        if ext in ["trr"]:
+            positions, velocities, forces = True, True, True
+        elif ext in ["ncdf", "nc"]:
+            positions = True
+            velocities = self._writer_kwargs.get("positions", False)
+            forces = self._writer_kwargs.get("forces", False)
+        elif ext in ["h5md"]:
+            positions = self._writer_kwargs.get("positions", True)
+            velocities = self._writer_kwargs.get("velocities", True)
+            forces = self._writer_kwargs.get("forces", True)
+        else:
+            positions, velocities, forces = True, False, False
+
+        return steps, positions, velocities, forces, False, self._enforcePeriodicBox
 
     def report(self, simulation, state):
         """Generate a report.

mdareporter/tests/test_mdareporter.py

@@ -165,4 +165,22 @@ def test_mdareporter_boxvectors(file_ext, simulation):
 
         print("max error in positions is ",np.max(max_errors)," Angstrom")
 
-
+
+@pytest.mark.parametrize("file_ext, expected_returns", [
+    ("dcd", (1, True, False, False)),
+    ("ncdf", (1, True, False, False)),
+    ("trr", (1, True, True, True)),
+    ("nc", (1, True, False, False)),
+    ("h5md", (1, True, True, True)),
+])
+def test_describenextreport(file_ext, expected_returns, simulation):
+    report_interval = 1
+    reporter = MDAReporter(f"output.{file_ext}", report_interval, enforcePeriodicBox=True)
+
+    simulation.currentStep = 0
+    steps, positions, velocities, forces, _, _ = reporter.describeNextReport(simulation)
+
+    assert steps == report_interval
+    assert positions == expected_returns[1]
+    assert velocities == expected_returns[2]
+    assert forces == expected_returns[3]