REF: implement get_unit_from_dtype

pandas/_libs/tslibs/conversion.pyx

@@ -36,6 +36,7 @@ from pandas._libs.tslibs.np_datetime cimport (
     dtstruct_to_dt64,
     get_datetime64_unit,
     get_datetime64_value,
+    get_unit_from_dtype,
     npy_datetime,
     npy_datetimestruct,
     pandas_datetime_to_datetimestruct,
@@ -234,7 +235,9 @@ def ensure_datetime64ns(arr: ndarray, copy: bool = True):
             result = result.copy()
         return result
 
-    unit = get_datetime64_unit(arr.flat[0])
+    if arr.dtype.kind != "M":
+        raise TypeError("ensure_datetime64ns arr must have datetime64 dtype")
+    unit = get_unit_from_dtype(arr.dtype)
     if unit == NPY_DATETIMEUNIT.NPY_FR_GENERIC:
         # without raising explicitly here, we end up with a SystemError
         # built-in function ensure_datetime64ns returned a result with an error

pandas/_libs/tslibs/np_datetime.pxd

@@ -1,3 +1,4 @@
+cimport numpy as cnp
 from cpython.datetime cimport (
     date,
     datetime,
@@ -79,3 +80,5 @@ cdef NPY_DATETIMEUNIT get_datetime64_unit(object obj) nogil
 cdef int _string_to_dts(str val, npy_datetimestruct* dts,
                         int* out_local, int* out_tzoffset,
                         bint want_exc) except? -1
+
+cdef NPY_DATETIMEUNIT get_unit_from_dtype(cnp.dtype dtype)

pandas/_libs/tslibs/np_datetime.pyi

@@ -1 +1,6 @@
+import numpy as np
+
 class OutOfBoundsDatetime(ValueError): ...
+
+# only exposed for testing
+def py_get_unit_from_dtype(dtype: np.dtype): ...

pandas/_libs/tslibs/np_datetime.pyx

@@ -19,6 +19,9 @@ from cpython.object cimport (
 
 PyDateTime_IMPORT
 
+cimport numpy as cnp
+
+cnp.import_array()
 from numpy cimport int64_t
 
 from pandas._libs.tslibs.util cimport get_c_string_buf_and_size
@@ -42,6 +45,8 @@ cdef extern from "src/datetime/np_datetime.h":
 
     npy_datetimestruct _NS_MIN_DTS, _NS_MAX_DTS
 
+    PyArray_DatetimeMetaData get_datetime_metadata_from_dtype(cnp.PyArray_Descr *dtype);
+
 cdef extern from "src/datetime/np_datetime_strings.h":
     int parse_iso_8601_datetime(const char *str, int len, int want_exc,
                                 npy_datetimestruct *out,
@@ -74,6 +79,22 @@ cdef inline NPY_DATETIMEUNIT get_datetime64_unit(object obj) nogil:
     """
     return <NPY_DATETIMEUNIT>(<PyDatetimeScalarObject*>obj).obmeta.base
 
+
+cdef NPY_DATETIMEUNIT get_unit_from_dtype(cnp.dtype dtype):
+    # NB: caller is responsible for ensuring this is *some* datetime64 or
+    #  timedelta64 dtype, otherwise we can segfault
+    cdef:
+        cnp.PyArray_Descr* descr = <cnp.PyArray_Descr*>dtype
+        PyArray_DatetimeMetaData meta
+    meta = get_datetime_metadata_from_dtype(descr)
+    return meta.base
+
+
+def py_get_unit_from_dtype(dtype):
+    # for testing get_unit_from_dtype; adds 896 bytes to the .so file.
+    return get_unit_from_dtype(dtype)
+
+
 # ----------------------------------------------------------------------
 # Comparison
 

pandas/_libs/tslibs/src/datetime/np_datetime.c

@@ -768,3 +768,15 @@ void pandas_timedelta_to_timedeltastruct(npy_timedelta td,
                             "invalid base unit");
     }
 }
+
+
+/*
+ * This function returns a pointer to the DateTimeMetaData
+ * contained within the provided datetime dtype.
+ *
+ * Copied near-verbatim from numpy/core/src/multiarray/datetime.c
+ */
+PyArray_DatetimeMetaData
+get_datetime_metadata_from_dtype(PyArray_Descr *dtype) {
+    return (((PyArray_DatetimeDTypeMetaData *)dtype->c_metadata)->meta);
+}

pandas/_libs/tslibs/src/datetime/np_datetime.h

@@ -75,5 +75,12 @@ int cmp_npy_datetimestruct(const npy_datetimestruct *a,
 void
 add_minutes_to_datetimestruct(npy_datetimestruct *dts, int minutes);
 
+/*
+ * This function returns the DateTimeMetaData
+ * contained within the provided datetime dtype.
+ */
+PyArray_DatetimeMetaData get_datetime_metadata_from_dtype(
+        PyArray_Descr *dtype);
+
 
 #endif  // PANDAS__LIBS_TSLIBS_SRC_DATETIME_NP_DATETIME_H_

pandas/tests/tslibs/test_np_datetime.py

@@ -0,0 +1,37 @@
+import numpy as np
+
+from pandas._libs.tslibs.np_datetime import py_get_unit_from_dtype
+
+
+def test_get_unit_from_dtype():
+    # datetime64
+    assert py_get_unit_from_dtype(np.dtype("M8[Y]")) == 0
+    assert py_get_unit_from_dtype(np.dtype("M8[M]")) == 1
+    assert py_get_unit_from_dtype(np.dtype("M8[W]")) == 2
+    # B has been deprecated and removed -> no 3
+    assert py_get_unit_from_dtype(np.dtype("M8[D]")) == 4
+    assert py_get_unit_from_dtype(np.dtype("M8[h]")) == 5
+    assert py_get_unit_from_dtype(np.dtype("M8[m]")) == 6
+    assert py_get_unit_from_dtype(np.dtype("M8[s]")) == 7
+    assert py_get_unit_from_dtype(np.dtype("M8[ms]")) == 8
+    assert py_get_unit_from_dtype(np.dtype("M8[us]")) == 9
+    assert py_get_unit_from_dtype(np.dtype("M8[ns]")) == 10
+    assert py_get_unit_from_dtype(np.dtype("M8[ps]")) == 11
+    assert py_get_unit_from_dtype(np.dtype("M8[fs]")) == 12
+    assert py_get_unit_from_dtype(np.dtype("M8[as]")) == 13
+
+    # timedelta64
+    assert py_get_unit_from_dtype(np.dtype("m8[Y]")) == 0
+    assert py_get_unit_from_dtype(np.dtype("m8[M]")) == 1
+    assert py_get_unit_from_dtype(np.dtype("m8[W]")) == 2
+    # B has been deprecated and removed -> no 3
+    assert py_get_unit_from_dtype(np.dtype("m8[D]")) == 4
+    assert py_get_unit_from_dtype(np.dtype("m8[h]")) == 5
+    assert py_get_unit_from_dtype(np.dtype("m8[m]")) == 6
+    assert py_get_unit_from_dtype(np.dtype("m8[s]")) == 7
+    assert py_get_unit_from_dtype(np.dtype("m8[ms]")) == 8
+    assert py_get_unit_from_dtype(np.dtype("m8[us]")) == 9
+    assert py_get_unit_from_dtype(np.dtype("m8[ns]")) == 10
+    assert py_get_unit_from_dtype(np.dtype("m8[ps]")) == 11
+    assert py_get_unit_from_dtype(np.dtype("m8[fs]")) == 12
+    assert py_get_unit_from_dtype(np.dtype("m8[as]")) == 13