fix: handle negative values in dehumanize()

arrow/arrow.py

@@ -1430,6 +1430,26 @@ def dehumanize(self, input_string: str, locale: str = "en_us") -> "Arrow":
                 else:
                     change_value = int(num_match.group())
 
+                    # Reject explicitly-negative values such as "in -1 hours".
+                    # The digit-only num_pattern above never captures a sign,
+                    # so the minus was previously dropped silently and
+                    # "in -1 hours" produced the same (wrong) result as
+                    # "in 1 hours". ``humanize`` always expresses direction
+                    # through the past/future templates ("... ago" / "in ...")
+                    # with a positive magnitude, so a negative number here is
+                    # malformed input rather than a second, conflicting
+                    # direction signal. Detect the sign by inspecting the
+                    # character that immediately precedes the matched digits in
+                    # the original input string.
+                    num_start = match.start() + num_match.start()
+                    if num_start > 0 and input_string[num_start - 1] == "-":
+                        raise ValueError(
+                            f"Invalid input string. Negative time value "
+                            f"'-{num_match.group()}' is not supported; express "
+                            "direction using the past ('... ago') or future "
+                            "('in ...') form with a positive value instead."
+                        )
+
                 # No time to update if now is the unit
                 if unit == "now":
                     unit_visited[unit] = True

tests/test_arrow.py

@@ -2888,6 +2888,29 @@ def test_require_relative_unit(self, locale_list_no_weeks: List[str]):
             with pytest.raises(ValueError):
                 arw.dehumanize(second_future_string, locale=lang)
 
+    def test_negative_values(self):
+        # Negative numeric values are malformed input: humanize only ever
+        # emits positive magnitudes and expresses direction through the
+        # past/future templates. They were previously silently dropped, so
+        # "in -1 hours" returned the same (wrong) result as "in 1 hours".
+        arw = arrow.Arrow(2000, 6, 18, 5, 55, 0)
+
+        # Sanity check: the corresponding positive strings still work and the
+        # buggy result must not equal the positive result.
+        assert arw.dehumanize("in 1 hours") == arw.shift(hours=1)
+        assert arw.dehumanize("1 hours ago") == arw.shift(hours=-1)
+
+        negative_strings = [
+            "in -1 hours",
+            "in -5 days",
+            "-3 hours ago",
+            "in -2 years",
+            "-10 seconds ago",
+        ]
+        for negative_string in negative_strings:
+            with pytest.raises(ValueError):
+                arw.dehumanize(negative_string)
+
     # Test for scrambled input
     def test_scrambled_input(self, locale_list_no_weeks: List[str]):
         for lang in locale_list_no_weeks: