Stop deriving Activity fields for a file holding more than one activity

src/activity_parser/activity.py

@@ -94,6 +94,9 @@ def fill_activity(activity: Activity, records: pd.DataFrame, laps: pd.DataFrame)
     A field the file itself reports is never overwritten; this only fills fields the
     file left ``None``. ``total_timer_time``, ``total_ascent`` and ``total_descent`` are
     not derived and are left as the file reported them, ``None`` included.
+
+    Should not be called for a file holding more than one activity, since the filled
+    fields would aggregate over all activities.
     """
     # Prefer sum of lap data, fall back to calculating from records
     total_elapsed_time = _coalesce(

src/activity_parser/parse_fit.py

@@ -314,7 +314,8 @@ def parse_fit(
     )
 
     activity = build_activity(convert_units_mapping(session or {}, SESSION_UNITS), file_id)
-    activity = fill_activity(activity, records, laps)
+    if len(messages.get("session", [])) <= 1:
+        activity = fill_activity(activity, records, laps)
 
     return records, laps, activity
 

src/activity_parser/parse_tcx_gpx.py

@@ -221,7 +221,9 @@ def parse_tcx(
     etree.strip_elements(root, "{*}Track")
     laps = build_dataframe(root.iter("{*}Lap"), TCX_LAP_FIELDS)
 
-    activity = fill_activity(tcx_activity(root), records, laps)
+    activity = tcx_activity(root)
+    if len(root.findall(".//{*}Activity")) <= 1:
+        activity = fill_activity(activity, records, laps)
 
     return records, laps, activity
 
@@ -261,6 +263,8 @@ def parse_gpx(
     records = build_dataframe(root.iter("{*}trkpt"), GPX_TRACKPOINT_FIELDS)
     records = index_by_time(records, "time")
 
-    activity = fill_activity(gpx_activity(root), records, pd.DataFrame())
+    activity = gpx_activity(root)
+    if len(root.findall("{*}trk")) <= 1:
+        activity = fill_activity(activity, records, pd.DataFrame())
 
     return records, pd.DataFrame(), activity

tests/test_parse_fit.py

@@ -402,6 +402,9 @@ def test_parse_fit_multi_session_uses_first_session_and_file_id():
     assert activity.sport == "running"
     assert activity.total_elapsed_time == 60.0
     assert activity.creator == "garmin 1111"
+    # Nothing from the second session leaks in.
+    assert activity.total_distance is None
+    assert activity.avg_heart_rate is None
 
 
 def test_parse_fit_chained_files(tmp_path):

tests/test_parse_tcx_gpx.py

@@ -227,6 +227,12 @@ def test_tcx_multi_activity_merges_records_and_laps_first_activity_wins_summary(
     assert laps["total_distance"].tolist() == pytest.approx([0.5, 1.0])
     assert activity.sport == "cycling"
     assert activity.start_time == pd.Timestamp("2026-01-05T08:00:00Z")
+    # Computed fields must not mix the two Activities' merged records/laps together.
+    assert activity.total_elapsed_time is None
+    assert activity.total_distance is None
+    assert activity.avg_heart_rate is None
+    assert activity.max_heart_rate is None
+    assert activity.avg_speed is None
 
 
 # ---------------------------------------------------------------------------
@@ -401,8 +407,10 @@ def test_gpx_1_0_base_fields():
 
 def test_gpx_multiple_tracks_merged():
     # Two separate <trk> elements, unlike multi_segment.gpx's two <trkseg> in one.
-    records, _, _ = ActivityParser().parse(MULTI_TRACK_GPX)
+    records, _, activity = ActivityParser().parse(MULTI_TRACK_GPX)
     assert records["latitude"].tolist() == pytest.approx([37.0, 37.5])
+    # Not derived from the merged records' time span.
+    assert activity.total_elapsed_time is None
 
 
 # ---------------------------------------------------------------------------

tests/test_readme.py

@@ -41,5 +41,5 @@ def test_readme_laps_table_matches_default_columns():
 
 def test_readme_activity_table_matches_dataclass_fields():
     text = README.read_text()
-    section = _section(text, "### Activity", "## Parser notes")
+    section = _section(text, "### Activity", "## Examples")
     assert _table_column_names(section) == [f.name for f in fields(Activity)]