implement abbrev_to_npy_unit (#47395)

Author: jbrockmendel

pandas/_libs/tslibs/conversion.pyx

@@ -31,7 +31,10 @@ from cpython.datetime cimport (
 import_datetime()
 
 from pandas._libs.tslibs.base cimport ABCTimestamp
-from pandas._libs.tslibs.dtypes cimport periods_per_second
+from pandas._libs.tslibs.dtypes cimport (
+    abbrev_to_npy_unit,
+    periods_per_second,
+)
 from pandas._libs.tslibs.np_datetime cimport (
     NPY_DATETIMEUNIT,
     NPY_FR_ns,
@@ -139,35 +142,36 @@ cpdef inline (int64_t, int) precision_from_unit(str unit):
     cdef:
         int64_t m
         int p
+        NPY_DATETIMEUNIT reso = abbrev_to_npy_unit(unit)
 
-    if unit == "Y":
+    if reso == NPY_DATETIMEUNIT.NPY_FR_Y:
         m = 1_000_000_000 * 31556952
         p = 9
-    elif unit == "M":
+    elif reso == NPY_DATETIMEUNIT.NPY_FR_M:
         m = 1_000_000_000 * 2629746
         p = 9
-    elif unit == "W":
+    elif reso == NPY_DATETIMEUNIT.NPY_FR_W:
         m = 1_000_000_000 * 3600 * 24 * 7
         p = 9
-    elif unit == "D" or unit == "d":
+    elif reso == NPY_DATETIMEUNIT.NPY_FR_D:
         m = 1_000_000_000 * 3600 * 24
         p = 9
-    elif unit == "h":
+    elif reso == NPY_DATETIMEUNIT.NPY_FR_h:
         m = 1_000_000_000 * 3600
         p = 9
-    elif unit == "m":
+    elif reso == NPY_DATETIMEUNIT.NPY_FR_m:
         m = 1_000_000_000 * 60
         p = 9
-    elif unit == "s":
+    elif reso == NPY_DATETIMEUNIT.NPY_FR_s:
         m = 1_000_000_000
         p = 9
-    elif unit == "ms":
+    elif reso == NPY_DATETIMEUNIT.NPY_FR_ms:
         m = 1_000_000
         p = 6
-    elif unit == "us":
+    elif reso == NPY_DATETIMEUNIT.NPY_FR_us:
         m = 1000
         p = 3
-    elif unit == "ns" or unit is None:
+    elif reso == NPY_DATETIMEUNIT.NPY_FR_ns or reso == NPY_DATETIMEUNIT.NPY_FR_GENERIC:
         m = 1
         p = 0
     else:

pandas/_libs/tslibs/dtypes.pxd

@@ -4,6 +4,7 @@ from pandas._libs.tslibs.np_datetime cimport NPY_DATETIMEUNIT
 
 
 cdef str npy_unit_to_abbrev(NPY_DATETIMEUNIT unit)
+cdef NPY_DATETIMEUNIT abbrev_to_npy_unit(str abbrev)
 cdef NPY_DATETIMEUNIT freq_group_code_to_npy_unit(int freq) nogil
 cpdef int64_t periods_per_day(NPY_DATETIMEUNIT reso=*) except? -1
 cdef int64_t periods_per_second(NPY_DATETIMEUNIT reso) except? -1

pandas/_libs/tslibs/dtypes.pyx

@@ -313,6 +313,39 @@ cdef str npy_unit_to_abbrev(NPY_DATETIMEUNIT unit):
         raise NotImplementedError(unit)
 
 
+cdef NPY_DATETIMEUNIT abbrev_to_npy_unit(str abbrev):
+    if abbrev == "Y":
+        return NPY_DATETIMEUNIT.NPY_FR_Y
+    elif abbrev == "M":
+        return NPY_DATETIMEUNIT.NPY_FR_M
+    elif abbrev == "W":
+        return NPY_DATETIMEUNIT.NPY_FR_W
+    elif abbrev == "D" or abbrev == "d":
+        return NPY_DATETIMEUNIT.NPY_FR_D
+    elif abbrev == "h":
+        return NPY_DATETIMEUNIT.NPY_FR_h
+    elif abbrev == "m":
+        return NPY_DATETIMEUNIT.NPY_FR_m
+    elif abbrev == "s":
+        return NPY_DATETIMEUNIT.NPY_FR_s
+    elif abbrev == "ms":
+        return NPY_DATETIMEUNIT.NPY_FR_ms
+    elif abbrev == "us":
+        return NPY_DATETIMEUNIT.NPY_FR_us
+    elif abbrev == "ns":
+        return NPY_DATETIMEUNIT.NPY_FR_ns
+    elif abbrev == "ps":
+        return NPY_DATETIMEUNIT.NPY_FR_ps
+    elif abbrev == "fs":
+        return NPY_DATETIMEUNIT.NPY_FR_fs
+    elif abbrev == "as":
+        return NPY_DATETIMEUNIT.NPY_FR_as
+    elif abbrev is None:
+        return NPY_DATETIMEUNIT.NPY_FR_GENERIC
+    else:
+        raise ValueError(f"Unrecognized unit {abbrev}")
+
+
 cdef NPY_DATETIMEUNIT freq_group_code_to_npy_unit(int freq) nogil:
     """
     Convert the freq to the corresponding NPY_DATETIMEUNIT to pass