Fix precise_diff counting partial months as full months

rust/src/python/helpers.rs

@@ -1,5 +1,4 @@
 #![allow(clippy::useless_conversion)]
-use std::cmp::Ordering;
 
 use pyo3::{
     prelude::*,
@@ -308,6 +307,9 @@ pub fn precise_diff<'py>(
         hour_diff -= 1;
     }
 
+    // The time of day of dt2 is earlier than that of dt1
+    let time_borrow = hour_diff < 0;
+
     if hour_diff < 0 {
         hour_diff += 24;
         day_diff -= 1;
@@ -332,26 +334,17 @@ pub fn precise_diff<'py>(
         let days_in_month =
             DAYS_PER_MONTHS[usize::from(helpers::is_leap(dtinfo2.year))][dtinfo2.month as usize];
 
-        match day_diff.cmp(&(days_in_month - days_in_last_month)) {
-            Ordering::Less => {
-                // We don't have a full month, we calculate days
-                if days_in_last_month < dtinfo1.day {
-                    day_diff += dtinfo1.day;
-                } else {
-                    day_diff += days_in_last_month;
-                }
-            }
-            Ordering::Equal => {
-                // We have exactly a full month
-                // We remove the days difference
-                // and add one to the months difference
-                day_diff = 0;
-                month_diff += 1;
-            }
-            Ordering::Greater => {
-                // We have a full month
-                day_diff += days_in_last_month;
-            }
+        if dtinfo2.day == days_in_month && !time_borrow {
+            // dt2 is the last day of its month and dt1's day does not
+            // exist in it, so adding the months to dt1 lands exactly
+            // on dt2 (e.g. Jan 31 + 1 month = Feb 28): a full month
+            day_diff = 0;
+            month_diff += 1;
+        } else if days_in_last_month < dtinfo1.day {
+            // We don't have a full month, we calculate days
+            day_diff += dtinfo1.day;
+        } else {
+            day_diff += days_in_last_month;
         }
 
         month_diff -= 1;

src/pendulum/_helpers.py

@@ -253,6 +253,8 @@ def precise_diff(
             hour_diff += 24
             d_diff -= 1
 
+    # A negative d_diff here means the time of day of d2 is earlier than d1's
+    time_borrow = d_diff < 0
     y_diff = d2.year - d1.year
     m_diff = d2.month - d1.month
     d_diff += d2.day - d1.day
@@ -272,20 +274,16 @@ def precise_diff(
         days_in_last_month = DAYS_PER_MONTHS[leap][month]
         days_in_month = DAYS_PER_MONTHS[int(is_leap(d2.year))][d2.month]
 
-        if d_diff < days_in_month - days_in_last_month:
-            # We don't have a full month, we calculate days
-            if days_in_last_month < d1.day:
-                d_diff += d1.day
-            else:
-                d_diff += days_in_last_month
-        elif d_diff == days_in_month - days_in_last_month:
-            # We have exactly a full month
-            # We remove the days difference
-            # and add one to the months difference
+        if d2.day == days_in_month and not time_borrow:
+            # d2 is the last day of its month and d1's day does not
+            # exist in it, so adding the months to d1 lands exactly
+            # on d2 (e.g. Jan 31 + 1 month = Feb 28): a full month
             d_diff = 0
             m_diff += 1
+        elif days_in_last_month < d1.day:
+            # We don't have a full month, we calculate days
+            d_diff += d1.day
         else:
-            # We have a full month
             d_diff += days_in_last_month
 
         m_diff -= 1

tests/date/test_diff.py

@@ -261,7 +261,7 @@ def test_diff_for_humans_other_and_month():
 
         assert today.diff_for_humans(today.add(months=1)) == "1 month before"
 
-    with pendulum.travel_to(pendulum.datetime(2017, 1, 31)):
+    with pendulum.travel_to(pendulum.local(2017, 1, 31)):
         today = pendulum.today().date()
 
         assert today.diff_for_humans(today.add(weeks=4)) == "1 month before"
@@ -314,7 +314,7 @@ def test_diff_for_humans_other_and_future_month():
         assert today.diff_for_humans(today.subtract(weeks=4)) == "4 weeks after"
         assert today.diff_for_humans(today.subtract(months=1)) == "1 month after"
 
-    with pendulum.travel_to(pendulum.datetime(2017, 2, 28)):
+    with pendulum.travel_to(pendulum.local(2017, 2, 28)):
         today = pendulum.today().date()
 
         assert today.diff_for_humans(today.subtract(weeks=4)) == "1 month after"

tests/test_helpers.py

@@ -86,6 +86,33 @@ def test_precise_diff() -> None:
     assert_diff(diff, days=6, hours=0)
 
 
+def test_precise_diff_partial_month() -> None:
+    # May 12 to June 11 is 30 days, not a full month
+    diff = precise_diff(datetime(2020, 5, 12, 4), datetime(2020, 6, 11, 10))
+    assert_diff(diff, days=30, hours=6)
+
+    diff = precise_diff(datetime(2020, 6, 11, 10), datetime(2020, 5, 12, 4))
+    assert_diff(diff, days=-30, hours=-6)
+
+    # https://github.com/python-pendulum/pendulum/issues/790
+    diff = precise_diff(datetime(2024, 1, 3), datetime(2024, 2, 1))
+    assert_diff(diff, days=29)
+
+
+def test_precise_diff_end_of_month() -> None:
+    # Jan 31 + 1 month is clamped to Feb 28, so this is a full month
+    diff = precise_diff(datetime(2023, 1, 31), datetime(2023, 2, 28))
+    assert_diff(diff, months=1)
+
+    # https://github.com/python-pendulum/pendulum/issues/402
+    diff = precise_diff(datetime(2019, 8, 30), datetime(2020, 2, 29))
+    assert_diff(diff, months=6)
+
+    # Not a full month if the time of day has not been reached yet
+    diff = precise_diff(datetime(2023, 1, 31, 10), datetime(2023, 2, 28, 9))
+    assert_diff(diff, days=27, hours=23)
+
+
 def test_precise_diff_timezone() -> None:
     paris = pendulum.timezone("Europe/Paris")
     toronto = pendulum.timezone("America/Toronto")