REF: de-duplicate precision_from_unit attempt 2 (#51594)

* Remove also-unraisable raising * Restore Y/M cases

Author: jbrockmendel

pandas/_libs/tslib.pyx

@@ -56,7 +56,6 @@ from pandas._libs.tslibs.conversion cimport (
  convert_timezone,
  get_datetime64_nanos,
  parse_pydatetime,
- precision_from_unit,
 )
 from pandas._libs.tslibs.nattype cimport (
  NPY_NAT,
@@ -258,7 +257,6 @@ def array_with_unit_to_datetime(
  """
  cdef:
  Py_ssize_t i, n=len(values)
- int64_t mult
  bint is_ignore = errors == "ignore"
  bint is_coerce = errors == "coerce"
  bint is_raise = errors == "raise"
@@ -275,8 +273,6 @@ def array_with_unit_to_datetime(
  )
  return result, tz
 
- mult, _ = precision_from_unit(unit)
-
  result = np.empty(n, dtype="M8[ns]")
  iresult = result.view("i8")
 

pandas/_libs/tslibs/conversion.pyx

@@ -37,6 +37,7 @@ from pandas._libs.tslibs.np_datetime cimport (
  NPY_FR_us,
  check_dts_bounds,
  convert_reso,
+ get_conversion_factor,
  get_datetime64_unit,
  get_datetime64_value,
  get_implementation_bounds,
@@ -83,9 +84,9 @@ TD64NS_DTYPE = np.dtype("m8[ns]")
 # Unit Conversion Helpers
 
 cdef int64_t cast_from_unit(
-  object ts,
-  str unit,
-  NPY_DATETIMEUNIT out_reso=NPY_FR_ns
+ object ts,
+ str unit,
+ NPY_DATETIMEUNIT out_reso=NPY_FR_ns
 ) except? -1:
  """
  Return a casting of the unit represented to nanoseconds
@@ -104,12 +105,6 @@ cdef int64_t cast_from_unit(
  int64_t m
  int p
 
- m, p = precision_from_unit(unit, out_reso)
-
- # just give me the unit back
- if ts is None:
- return m
-
  if unit in ["Y", "M"]:
  if is_float_object(ts) and not ts.is_integer():
  # GH#47267 it is clear that 2 "M" corresponds to 1970-02-01,
@@ -126,6 +121,8 @@ cdef int64_t cast_from_unit(
  dt64obj = np.datetime64(ts, unit)
  return get_datetime64_nanos(dt64obj, out_reso)
 
+ m, p = precision_from_unit(unit, out_reso)
+
  # cast the unit, multiply base/frac separately
  # to avoid precision issues from float -> int
  try:
@@ -148,8 +145,8 @@ cdef int64_t cast_from_unit(
 
 
 cpdef inline (int64_t, int) precision_from_unit(
-  str unit,
-  NPY_DATETIMEUNIT out_reso=NPY_DATETIMEUNIT.NPY_FR_ns,
+ str unit,
+ NPY_DATETIMEUNIT out_reso=NPY_DATETIMEUNIT.NPY_FR_ns,
 ):
  """
  Return a casting of the unit represented to nanoseconds + the precision
@@ -166,34 +163,21 @@ cpdef inline (int64_t, int) precision_from_unit(
  int p
  NPY_DATETIMEUNIT reso = abbrev_to_npy_unit(unit)
 
- multiplier = periods_per_second(out_reso)
-
+ if reso == NPY_DATETIMEUNIT.NPY_FR_GENERIC:
+ reso = NPY_DATETIMEUNIT.NPY_FR_ns
  if reso == NPY_DATETIMEUNIT.NPY_FR_Y:
  # each 400 years we have 97 leap years, for an average of 97/400=.2425
  # extra days each year. We get 31556952 by writing
  # 3600*24*365.2425=31556952
+ multiplier = periods_per_second(out_reso)
  m = multiplier * 31556952
  elif reso == NPY_DATETIMEUNIT.NPY_FR_M:
  # 2629746 comes from dividing the "Y" case by 12.
+ multiplier = periods_per_second(out_reso)
  m = multiplier * 2629746
- elif reso == NPY_DATETIMEUNIT.NPY_FR_W:
- m = multiplier * 3600 * 24 * 7
- elif reso == NPY_DATETIMEUNIT.NPY_FR_D:
- m = multiplier * 3600 * 24
- elif reso == NPY_DATETIMEUNIT.NPY_FR_h:
- m = multiplier * 3600
- elif reso == NPY_DATETIMEUNIT.NPY_FR_m:
- m = multiplier * 60
- elif reso == NPY_DATETIMEUNIT.NPY_FR_s:
- m = multiplier
- elif reso == NPY_DATETIMEUNIT.NPY_FR_ms:
- m = multiplier // 1_000
- elif reso == NPY_DATETIMEUNIT.NPY_FR_us:
- m = multiplier // 1_000_000
- elif reso == NPY_DATETIMEUNIT.NPY_FR_ns or reso == NPY_DATETIMEUNIT.NPY_FR_GENERIC:
- m = multiplier // 1_000_000_000
  else:
- raise ValueError(f"cannot cast unit {unit}")
+ m = get_conversion_factor(reso, out_reso)
+
  p = <int>log10(m) # number of digits in 'm' minus 1
  return m, p
 

pandas/_libs/tslibs/np_datetime.pyx

@@ -571,6 +571,8 @@ cdef int64_t get_conversion_factor(
  return 1000 * get_conversion_factor(NPY_DATETIMEUNIT.NPY_FR_fs, to_unit)
  elif from_unit == NPY_DATETIMEUNIT.NPY_FR_fs:
  return 1000 * get_conversion_factor(NPY_DATETIMEUNIT.NPY_FR_as, to_unit)
+ else:
+ raise ValueError("Converting from M or Y units is not supported.")
 
 
 cdef int64_t convert_reso(