Raise ValueError in dehumanize() for negative numbers (#1278)

arrow/arrow.py

@@ -1380,6 +1380,16 @@ def dehumanize(self, input_string: str, locale: str = "en_us") -> "Arrow":
 
         current_time = self.fromdatetime(self._datetime)
 
+        # Humanized strings encode direction with the locale's past/future
+        # wording ("ago"/"in"), not a sign on the number. A negative value such
+        # as "in -1 hours" would otherwise have its sign dropped by the number
+        # regex below and be parsed as positive, so reject it explicitly.
+        if re.search(r"-\d", input_string):
+            raise ValueError(
+                "Invalid input string: negative numbers are not supported. "
+                "Use the past or future wording instead, e.g. '1 hour ago' or 'in 1 hour'."
+            )
+
         # Create an object containing the relative time info
         time_object_info = dict.fromkeys(
             ["seconds", "minutes", "hours", "days", "weeks", "months", "years"], 0

tests/test_arrow.py

@@ -2930,6 +2930,17 @@ def test_no_units_modified(self, locale_list_no_weeks: List[str]):
             with pytest.raises(ValueError):
                 arw.dehumanize(empty_future_string, locale=lang)
 
+    # Negative numbers are not valid: direction comes from "ago"/"in" wording,
+    # not a sign, and the sign would otherwise be silently dropped.
+    def test_negative_number(self):
+        arw = arrow.Arrow(2000, 6, 18, 5, 55, 0)
+
+        with pytest.raises(ValueError):
+            arw.dehumanize("in -1 hours")
+
+        with pytest.raises(ValueError):
+            arw.dehumanize("-2 days ago")
+
     def test_slavic_locales(self, slavic_locales: List[str]):
         # Relevant units for Slavic locale plural logic
         units = [