Fix handling of negative time deltas (#173)

Fixes #18. 
Fixes #171.

The [Python docs
say](https://docs.python.org/3/library/datetime.html#datetime.timedelta):

> String representations of
[timedelta](https://docs.python.org/3/library/datetime.html#datetime.timedelta)
objects are normalized similarly to their internal representation. This
leads to somewhat unusual results for negative timedeltas. For example:

```pycon
>>> timedelta(hours=-5)
datetime.timedelta(days=-1, seconds=68400)
>>> print(_)
-1 day, 19:00:00
```

However, we assumed all components have the same sign, and used absolute
values of some of those, leading to inconsistent behaviour (on Linux and
macOS; it still worked on Windows).

Instead, we should convert the whole `timedelta` into its absolute
value, and then use its components directly.

Also test all the `timedelta`s with positive and negative values.

.pre-commit-config.yaml

@@ -28,7 +28,8 @@ repos:
     hooks:
       - id: mypy
         additional_dependencies: [pytest, types-freezegun, types-setuptools]
-        args: [--strict, --pretty, --show-error-codes]
+        args: [--strict, --pretty, --show-error-codes, .]
+        pass_filenames: false
 
   - repo: https://github.com/tox-dev/pyproject-fmt
     rev: 1.7.0

src/humanize/time.py

@@ -141,14 +141,13 @@ def naturaldelta(
 
     use_months = months
 
-    seconds = abs(delta.seconds)
-    days = abs(delta.days)
-    years = days // 365
-    days = days % 365
+    delta = abs(delta)
+    years = delta.days // 365
+    days = delta.days % 365
     num_months = int(days // 30.5)
 
     if not years and days < 1:
-        if seconds == 0:
+        if delta.seconds == 0:
             if min_unit == Unit.MICROSECONDS and delta.microseconds < 1000:
                 return (
                     _ngettext("%d microsecond", "%d microseconds", delta.microseconds)
@@ -165,24 +164,24 @@ def naturaldelta(
                 )
             return _("a moment")
 
-        if seconds == 1:
+        if delta.seconds == 1:
             return _("a second")
 
-        if seconds < 60:
-            return _ngettext("%d second", "%d seconds", seconds) % seconds
+        if delta.seconds < 60:
+            return _ngettext("%d second", "%d seconds", delta.seconds) % delta.seconds
 
-        if 60 <= seconds < 120:
+        if 60 <= delta.seconds < 120:
             return _("a minute")
 
-        if 120 <= seconds < 3600:
-            minutes = seconds // 60
+        if 120 <= delta.seconds < 3600:
+            minutes = delta.seconds // 60
             return _ngettext("%d minute", "%d minutes", minutes) % minutes
 
-        if 3600 <= seconds < 3600 * 2:
+        if 3600 <= delta.seconds < 3600 * 2:
             return _("an hour")
 
-        if 3600 < seconds:
-            hours = seconds // 3600
+        if 3600 < delta.seconds:
+            hours = delta.seconds // 3600
             return _ngettext("%d hour", "%d hours", hours) % hours
 
     elif years == 0:

tests/test_time.py

@@ -95,6 +95,7 @@ def test_naturaldelta_nomonths(test_input: dt.timedelta, expected: str) -> None:
         (1, "a second"),
         (23.5, "23 seconds"),
         (30, "30 seconds"),
+        (dt.timedelta(microseconds=13), "a moment"),
         (dt.timedelta(minutes=1, seconds=30), "a minute"),
         (dt.timedelta(minutes=2), "2 minutes"),
         (dt.timedelta(hours=1, minutes=30, seconds=30), "an hour"),
@@ -129,6 +130,8 @@ def test_naturaldelta_nomonths(test_input: dt.timedelta, expected: str) -> None:
 )
 def test_naturaldelta(test_input: float | dt.timedelta, expected: str) -> None:
     assert humanize.naturaldelta(test_input) == expected
+    if not isinstance(test_input, str):
+        assert humanize.naturaldelta(-test_input) == expected
 
 
 @freeze_time("2020-02-02")