Reject negative numbers in dehumanize()

arrow/arrow.py

@@ -1380,6 +1380,17 @@ def dehumanize(self, input_string: str, locale: str = "en_us") -> "Arrow":
 
         current_time = self.fromdatetime(self._datetime)
 
+        # Humanized strings express direction with words ("ago"/"in"), never
+        # with an arithmetic sign, so ``humanize`` itself never emits one. A
+        # minus sign attached to a number (e.g. "in -1 hours") would otherwise
+        # be silently dropped by the unsigned ``\d+`` extraction below and yield
+        # a result in the opposite direction from what was written, so reject it.
+        if re.search(r"-\d", input_string):
+            raise ValueError(
+                f"Invalid input string: {input_string!r}. Negative numbers are "
+                "not supported; use 'ago' or 'in' to indicate direction."
+            )
+
         # 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

@@ -2868,6 +2868,15 @@ def test_normalized_locale(self):
         assert arw.dehumanize(second_ago_string, locale="zh_hk") == second_ago
         assert arw.dehumanize(second_future_string, locale="zh_hk") == second_future
 
+    # Ensures a negative number is rejected rather than silently dropping the
+    # sign and producing a result in the opposite direction (see gh-1278).
+    def test_negative_number(self):
+        arw = arrow.Arrow(2000, 6, 18, 5, 55, 0)
+
+        for input_string in ("in -1 hours", "in -2 days", "-3 minutes ago"):
+            with pytest.raises(ValueError):
+                arw.dehumanize(input_string)
+
     # Ensures relative units are required in string
     def test_require_relative_unit(self, locale_list_no_weeks: List[str]):
         for lang in locale_list_no_weeks: