Fixed Period and PeriodIndex formatting (pandas-dev#46361)

Author: smarie

doc/source/whatsnew/v1.5.0.rst

@@ -440,6 +440,7 @@ I/O
 Period
 ^^^^^^
 - Bug in subtraction of :class:`Period` from :class:`PeriodArray` returning wrong results (:issue:`45999`)
+- Bug in :meth:`Period.strftime` and :meth:`PeriodIndex.strftime`, directives ``%l`` and ``%u`` were giving wrong results (:issue:`46252`)
 -
 
 Plotting

pandas/_libs/tslibs/period.pyx

@@ -9,6 +9,7 @@ from cpython.object cimport (
     PyObject_RichCompareBool,
 )
 from numpy cimport (
+    int32_t,
     int64_t,
     ndarray,
 )
@@ -1217,39 +1218,56 @@ cdef str _period_strftime(int64_t value, int freq, bytes fmt):
         char *formatted
         bytes pat, brepl
         list found_pat = [False] * len(extra_fmts)
-        int year, quarter
+        int quarter
+        int32_t us, ps
         str result, repl
 
     get_date_info(value, freq, &dts)
+
+    # Find our additional directives in the pattern and replace them with
+    # placeholders that are not processed by c_strftime
     for i in range(len(extra_fmts)):
         pat = extra_fmts[i][0]
         brepl = extra_fmts[i][1]
         if pat in fmt:
             fmt = fmt.replace(pat, brepl)
             found_pat[i] = True
 
+    # Execute c_strftime to process the usual datetime directives
     formatted = c_strftime(&dts, <char*>fmt)
 
     result = util.char_to_string(formatted)
     free(formatted)
 
+    # Now we will fill the placeholders corresponding to our additional directives
+
+    # First prepare the contents
+    # Save these to local vars as dts can be modified by get_yq below
+    us = dts.us
+    ps = dts.ps
+    if any(found_pat[0:3]):
+        # Note: this modifies `dts` in-place so that year becomes fiscal year
+        # However it looses the us and ps
+        quarter = get_yq(value, freq, &dts)
+    else:
+        quarter = 0
+
+    # Now do the filling per se
     for i in range(len(extra_fmts)):
         if found_pat[i]:
 
-            quarter = get_yq(value, freq, &dts)
-
-            if i == 0:
-                repl = str(quarter)
-            elif i == 1:  # %f, 2-digit year
+            if i == 0:  # %q, 1-digit quarter.
+                repl = f"{quarter}"
+            elif i == 1:  # %f, 2-digit 'Fiscal' year
                 repl = f"{(dts.year % 100):02d}"
-            elif i == 2:
+            elif i == 2:  # %F, 'Fiscal' year with a century
                 repl = str(dts.year)
-            elif i == 3:
-                repl = f"{(value % 1_000):03d}"
-            elif i == 4:
-                repl = f"{(value % 1_000_000):06d}"
-            elif i == 5:
-                repl = f"{(value % 1_000_000_000):09d}"
+            elif i == 3:  # %l, milliseconds
+                repl = f"{(us // 1_000):03d}"
+            elif i == 4:  # %u, microseconds
+                repl = f"{(us):06d}"
+            elif i == 5:  # %n, nanoseconds
+                repl = f"{((us * 1000) + (ps // 1000)):09d}"
 
             result = result.replace(str_extra_fmts[i], repl)
 
@@ -2302,7 +2320,8 @@ cdef class _Period(PeriodMixin):
         containing one or several directives.  The method recognizes the same
         directives as the :func:`time.strftime` function of the standard Python
         distribution, as well as the specific additional directives ``%f``,
-        ``%F``, ``%q``. (formatting & docs originally from scikits.timeries).
+        ``%F``, ``%q``, ``%l``, ``%u``, ``%n``.
+        (formatting & docs originally from scikits.timeries).
 
         +-----------+--------------------------------+-------+
         | Directive | Meaning                        | Notes |
@@ -2349,11 +2368,20 @@ cdef class _Period(PeriodMixin):
         |           | AM or PM.                      |       |
         +-----------+--------------------------------+-------+
         | ``%q``    | Quarter as a decimal number    |       |
-        |           | [01,04]                        |       |
+        |           | [1,4]                          |       |
         +-----------+--------------------------------+-------+
         | ``%S``    | Second as a decimal number     | \(4)  |
         |           | [00,61].                       |       |
         +-----------+--------------------------------+-------+
+        | ``%l``    | Millisecond as a decimal number|       |
+        |           | [000,999].                     |       |
+        +-----------+--------------------------------+-------+
+        | ``%u``    | Microsecond as a decimal number|       |
+        |           | [000000,999999].               |       |
+        +-----------+--------------------------------+-------+
+        | ``%n``    | Nanosecond as a decimal number |       |
+        |           | [000000000,999999999].         |       |
+        +-----------+--------------------------------+-------+
         | ``%U``    | Week number of the year        | \(5)  |
         |           | (Sunday as the first day of    |       |
         |           | the week) as a decimal number  |       |

pandas/core/arrays/datetimelike.py

@@ -1593,6 +1593,14 @@ def strftime(self, date_format: str) -> npt.NDArray[np.object_]:
         of the string format can be found in `python string format
         doc <%(URL)s>`__.
 
+        Formats supported by the C `strftime` API but not by the python string format
+        doc (such as `"%%R"`, `"%%r"`) are not officially supported and should be
+        preferably replaced with their supported equivalents (such as `"%%H:%%M"`,
+        `"%%I:%%M:%%S %%p"`).
+
+        Note that `PeriodIndex` support additional directives, detailed in
+        `Period.strftime`.
+
         Parameters
         ----------
         date_format : str
@@ -1609,6 +1617,8 @@ def strftime(self, date_format: str) -> npt.NDArray[np.object_]:
         DatetimeIndex.normalize : Return DatetimeIndex with times to midnight.
         DatetimeIndex.round : Round the DatetimeIndex to the specified freq.
         DatetimeIndex.floor : Floor the DatetimeIndex to the specified freq.
+        Timestamp.strftime : Format a single Timestamp.
+        Period.strftime : Format a single Period.
 
         Examples
         --------

pandas/core/arrays/period.py

@@ -642,17 +642,17 @@ def _format_native_types(
         values = self.astype(object)
 
         if date_format:
-            formatter = lambda dt: dt.strftime(date_format)
+            formatter = lambda per: per.strftime(date_format)
         else:
-            formatter = lambda dt: str(dt)
+            formatter = lambda per: str(per)
 
         if self._hasna:
             mask = self._isnan
             values[mask] = na_rep
             imask = ~mask
-            values[imask] = np.array([formatter(dt) for dt in values[imask]])
+            values[imask] = np.array([formatter(per) for per in values[imask]])
         else:
-            values = np.array([formatter(dt) for dt in values])
+            values = np.array([formatter(per) for per in values])
         return values
 
     # ------------------------------------------------------------------

pandas/tests/io/formats/test_format.py

@@ -1,7 +1,6 @@
 """
 Test output formatting for Series/DataFrame, including to_string & reprs
 """
-
 from datetime import datetime
 from io import StringIO
 import itertools
@@ -3167,6 +3166,65 @@ def test_str(self):
         assert str(NaT) == "NaT"
 
 
+class TestPeriodIndexFormat:
+    def test_period_format_and_strftime_default(self):
+        per = pd.PeriodIndex([datetime(2003, 1, 1, 12), None], freq="H")
+
+        # Default formatting
+        formatted = per.format()
+        assert formatted[0] == "2003-01-01 12:00"  # default: minutes not shown
+        assert formatted[1] == "NaT"
+        # format is equivalent to strftime(None)...
+        assert formatted[0] == per.strftime(None)[0]
+        assert per.strftime(None)[1] is np.nan  # ...except for NaTs
+
+        # Same test with nanoseconds freq
+        per = pd.period_range("2003-01-01 12:01:01.123456789", periods=2, freq="n")
+        formatted = per.format()
+        assert (formatted == per.strftime(None)).all()
+        assert formatted[0] == "2003-01-01 12:01:01.123456789"
+        assert formatted[1] == "2003-01-01 12:01:01.123456790"
+
+    def test_period_custom(self):
+        # GH#46252 custom formatting directives %l (ms) and %u (us)
+
+        # 3 digits
+        per = pd.period_range("2003-01-01 12:01:01.123", periods=2, freq="l")
+        formatted = per.format(date_format="%y %I:%M:%S (ms=%l us=%u ns=%n)")
+        assert formatted[0] == "03 12:01:01 (ms=123 us=123000 ns=123000000)"
+        assert formatted[1] == "03 12:01:01 (ms=124 us=124000 ns=124000000)"
+
+        # 6 digits
+        per = pd.period_range("2003-01-01 12:01:01.123456", periods=2, freq="u")
+        formatted = per.format(date_format="%y %I:%M:%S (ms=%l us=%u ns=%n)")
+        assert formatted[0] == "03 12:01:01 (ms=123 us=123456 ns=123456000)"
+        assert formatted[1] == "03 12:01:01 (ms=123 us=123457 ns=123457000)"
+
+        # 9 digits
+        per = pd.period_range("2003-01-01 12:01:01.123456789", periods=2, freq="n")
+        formatted = per.format(date_format="%y %I:%M:%S (ms=%l us=%u ns=%n)")
+        assert formatted[0] == "03 12:01:01 (ms=123 us=123456 ns=123456789)"
+        assert formatted[1] == "03 12:01:01 (ms=123 us=123456 ns=123456790)"
+
+    def test_period_tz(self):
+        # Formatting periods created from a datetime with timezone.
+
+        # This timestamp is in 2013 in Europe/Paris but is 2012 in UTC
+        dt = pd.to_datetime(["2013-01-01 00:00:00+01:00"], utc=True)
+
+        # Converting to a period looses the timezone information
+        # Since tz is currently set as utc, we'll see 2012
+        with tm.assert_produces_warning(UserWarning, match="will drop timezone"):
+            per = dt.to_period(freq="H")
+        assert per.format()[0] == "2012-12-31 23:00"
+
+        # If tz is currently set as paris before conversion, we'll see 2013
+        dt = dt.tz_convert("Europe/Paris")
+        with tm.assert_produces_warning(UserWarning, match="will drop timezone"):
+            per = dt.to_period(freq="H")
+        assert per.format()[0] == "2013-01-01 00:00"
+
+
 class TestDatetimeIndexFormat:
     def test_datetime(self):
         formatted = pd.to_datetime([datetime(2003, 1, 1, 12), NaT]).format()