Reject negative numbers in dehumanize instead of dropping the sign

arrow/arrow.py

@@ -1378,6 +1378,24 @@ def dehumanize(self, input_string: str, locale: str = "en_us") -> "Arrow":
                 f"Dehumanize does not currently support the {locale} locale, please consider making a contribution to add support for this locale."
             )
 
+        # A humanized string carries its direction in the "ago"/"in" wording, so
+        # a sign on the number is never meaningful. The number pattern below
+        # matches unsigned digits only, which would silently drop the sign and
+        # return a time shifted the opposite way. No locale's timeframe strings
+        # contain a hyphen, so one in front of a digit can only have come from
+        # the caller.
+        #
+        # A hyphen *between* digits is a separator, not a sign: "2020-01-01" is
+        # invalid input, but it is not a negative number, and saying so would send
+        # the caller looking for a sign that is not there. The lookbehind leaves
+        # those to the ordinary "not a valid humanized string" error below.
+        if re.search(r"(?<!\d)-\d", input_string):
+            raise ValueError(
+                "Invalid input String. String contains a negative number. "
+                "Humanized strings express direction with words, not signs. "
+                "Ex: '1 hour ago' rather than 'in -1 hours'."
+            )
+
         current_time = self.fromdatetime(self._datetime)
 
         # Create an object containing the relative time info

tests/test_arrow.py

@@ -2930,6 +2930,43 @@ def test_no_units_modified(self, locale_list_no_weeks: List[str]):
             with pytest.raises(ValueError):
                 arw.dehumanize(empty_future_string, locale=lang)
 
+    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",
+            "in -1 years",
+        ]:
+            with pytest.raises(ValueError, match="negative number"):
+                arw.dehumanize(input_string)
+
+    def test_negative_number_not_confused_with_positive(self):
+        arw = arrow.Arrow(2000, 6, 18, 5, 55, 0)
+
+        assert arw.dehumanize("in 2 hours") == arw.shift(hours=2)
+        assert arw.dehumanize("2 hours ago") == arw.shift(hours=-2)
+
+    def test_hyphen_between_digits_is_not_a_negative_number(self):
+        arw = arrow.Arrow(2000, 6, 18, 5, 55, 0)
+
+        # Still invalid input, but a separator is not a sign. Reporting these as
+        # negative numbers would send the caller looking for a sign that is not
+        # there, instead of at the string not being a humanized one.
+        for input_string in ["2020-01-01", "2020-01-01 00:00:00"]:
+            with pytest.raises(ValueError, match="Input string not valid"):
+                arw.dehumanize(input_string)
+
+    def test_leading_negative_number_is_still_rejected(self):
+        arw = arrow.Arrow(2000, 6, 18, 5, 55, 0)
+
+        # The lookbehind must not lose the cases the guard exists for: a sign at
+        # the start of the string, or after a space or an opening bracket.
+        for input_string in ["-1 hours ago", "in -1 hours", "(-1 hours ago)"]:
+            with pytest.raises(ValueError, match="negative number"):
+                arw.dehumanize(input_string)
+
     def test_slavic_locales(self, slavic_locales: List[str]):
         # Relevant units for Slavic locale plural logic
         units = [