better interval description and warning if unset

Author: tylerflex

CHANGELOG.md

@@ -19,6 +19,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Warnings for too many frequencies in monitors; too many modes requested in a ``ModeSpec``; too many number of grid points in a mode monitor or mode source.
 
 ### Changed
+- Time domain monitors warn about data usage if all defaults used in time sampler specs.
 
 ### Fixed
 - Faster sorting of modes in `ModeSolverData.overlap_sort` by avoiding excessive data copying.

tests/test_components/test_monitor.py

@@ -12,6 +12,28 @@ def test_stop_start():
         td.FluxTimeMonitor(size=(1, 1, 0), name="f", start=2, stop=1)
 
 
+# interval, start, stop, log_out
+time_sampling_tests = [
+    (None, 0.0, None, "WARNING"),  # all defaults
+    (1, 0.0, None, None),  # interval set (=1)
+    (2, 0.0, None, None),  # interval set (=2)
+    (None, 1e-12, None, None),  # start specified
+    (None, 0.0, 5e-12, None),  # stop specified
+]
+
+
+@pytest.mark.parametrize("interval, start, stop, log_desired", time_sampling_tests)
+def test_monitor_interval_warn(log_capture, interval, start, stop, log_desired):
+    """Assert time monitor interval warning handled as expected."""
+
+    mnt = td.FluxTimeMonitor(size=(1, 1, 0), name="f", interval=interval, stop=stop, start=start)
+    assert_log_level(log_capture, log_desired)
+
+    # make sure it got set to either 1 (undefined) or the specified value
+    mnt_interval = interval if interval else 1
+    assert mnt.interval == mnt_interval
+
+
 def test_time_inds():
     M = td.FluxTimeMonitor(size=(1, 1, 0), name="f", start=0, stop=1)
     assert M.time_inds(tmesh=[]) == (0, 0)

tidy3d/components/monitor.py

@@ -192,11 +192,40 @@ class TimeMonitor(Monitor, ABC):
     )
 
     interval: pydantic.PositiveInt = pydantic.Field(
-        1,
+        None,
         title="Time interval",
-        description="Number of time step intervals between monitor recordings.",
+        description="Sampling rate of the monitor: number of time steps between each measurement. "
+        "Set ``inverval`` to 1 for the highest possible resolution in time. "
+        "Higher integer values downsample the data by measuring every ``interval`` time steps. "
+        "This can be useful for reducing data storage as needed by the application.",
     )
 
+    @pydantic.validator("interval", always=True)
+    def _warn_interval_default(cls, val, values):
+        """If all defaults used for time sampler, warn and set ``interval=1`` internally."""
+
+        if val is None:
+            start = values.get("start")
+            stop = values.get("stop")
+            if start == 0.0 and stop is None:
+                log.warning(
+                    "The monitor 'interval' field was left as its default value, "
+                    "which will set it to 1 internally. "
+                    "A value of 1 means that the data will be sampled at every time step, "
+                    "which may potentially produce more data than desired, "
+                    "depending on the use case. "
+                    "To reduce data storage, one may downsample the data by setting 'interval > 1'"
+                    " or by choosing alternative 'start' and 'stop' values for the time sampling. "
+                    "If you intended to use the highest resolution time sampling, "
+                    "you may suppress this warning "
+                    "by explicitly setting 'interval=1' in the monitor."
+                )
+
+            # set 'interval = 1' for backwards compatibility
+            val = 1
+
+        return val
+
     @pydantic.validator("stop", always=True, allow_reuse=True)
     def stop_greater_than_start(cls, val, values):
         """Ensure sure stop is greater than or equal to start."""