REF: Implement NullFrequencyError in pd.errors (pandas-dev#34144)

Author: jbrockmendel

doc/source/reference/general_utility_functions.rst

@@ -40,6 +40,7 @@ Exceptions and warnings
    errors.EmptyDataError
    errors.OutOfBoundsDatetime
    errors.MergeError
+   errors.NullFrequencyError
    errors.NumbaUtilError
    errors.ParserError
    errors.ParserWarning

pandas/_libs/tslibs/__init__.py

@@ -13,7 +13,6 @@
     "ints_to_pytimedelta",
     "Timestamp",
     "tz_convert_single",
-    "NullFrequencyError",
 ]
 
 
@@ -22,5 +21,5 @@
 from .np_datetime import OutOfBoundsDatetime
 from .period import IncompatibleFrequency, Period
 from .timedeltas import Timedelta, delta_to_nanoseconds, ints_to_pytimedelta
-from .timestamps import NullFrequencyError, Timestamp
+from .timestamps import Timestamp
 from .tzconversion import tz_convert_single

pandas/_libs/tslibs/timestamps.pyx

@@ -183,15 +183,6 @@ def round_nsint64(values, mode, freq):
 
 # ----------------------------------------------------------------------
 
-class NullFrequencyError(ValueError):
-    """
-    Error raised when a null `freq` attribute is used in an operation
-    that needs a non-null frequency, particularly `DatetimeIndex.shift`,
-    `TimedeltaIndex.shift`, `PeriodIndex.shift`.
-    """
-    pass
-
-
 def integer_op_not_supported(obj):
     # GH#22535 add/sub of integers and int-arrays is no longer allowed
     # Note we return rather than raise the exception so we can raise in

pandas/errors/__init__.py

@@ -6,7 +6,17 @@
 
 from pandas._config.config import OptionError
 
-from pandas._libs.tslibs import NullFrequencyError, OutOfBoundsDatetime
+from pandas._libs.tslibs import OutOfBoundsDatetime
+
+
+class NullFrequencyError(ValueError):
+    """
+    Error raised when a null `freq` attribute is used in an operation
+    that needs a non-null frequency, particularly `DatetimeIndex.shift`,
+    `TimedeltaIndex.shift`, `PeriodIndex.shift`.
+    """
+
+    pass
 
 
 class PerformanceWarning(Warning):

pandas/tests/tslibs/test_api.py

@@ -30,7 +30,6 @@ def test_namespace():
         "iNaT",
         "is_null_datetimelike",
         "nat_strings",
-        "NullFrequencyError",
         "OutOfBoundsDatetime",
         "Period",
         "IncompatibleFrequency",