BUG: ensure_timedelta64ns overflows

doc/source/reference/general_utility_functions.rst

@@ -43,6 +43,7 @@ Exceptions and warnings
    errors.NullFrequencyError
    errors.NumbaUtilError
    errors.OutOfBoundsDatetime
+   errors.OutOfBoundsTimedelta
    errors.ParserError
    errors.ParserWarning
    errors.PerformanceWarning

pandas/_libs/tslibs/__init__.py

@@ -7,6 +7,7 @@
     "nat_strings",
     "is_null_datetimelike",
     "OutOfBoundsDatetime",
+    "OutOfBoundsTimedelta",
     "IncompatibleFrequency",
     "Period",
     "Resolution",
@@ -26,7 +27,7 @@
 ]
 
 from . import dtypes
-from .conversion import localize_pydatetime
+from .conversion import OutOfBoundsTimedelta, localize_pydatetime
 from .dtypes import Resolution
 from .nattype import NaT, NaTType, iNaT, is_null_datetimelike, nat_strings
 from .np_datetime import OutOfBoundsDatetime

pandas/_libs/tslibs/conversion.pyx

@@ -51,6 +51,15 @@ DT64NS_DTYPE = np.dtype('M8[ns]')
 TD64NS_DTYPE = np.dtype('m8[ns]')
 
 
+class OutOfBoundsTimedelta(ValueError):
+    """
+    Raised when encountering a timedelta value that cannot be represented
+    as a timedelta64[ns].
+    """
+    # Timedelta analogue to OutOfBoundsDatetime
+    pass
+
+
 # ----------------------------------------------------------------------
 # Unit Conversion Helpers
 
@@ -228,11 +237,34 @@ def ensure_timedelta64ns(arr: ndarray, copy: bool=True):
 
     Returns
     -------
-    result : ndarray with dtype timedelta64[ns]
-
+    ndarray[timedelta64[ns]]
     """
-    return arr.astype(TD64NS_DTYPE, copy=copy)
-    # TODO: check for overflows when going from a lower-resolution to nanos
+    assert arr.dtype.kind == "m", arr.dtype
+
+    if arr.dtype == TD64NS_DTYPE:
+        return arr.copy() if copy else arr
+
+    # Re-use the datetime64 machinery to do an overflow-safe `astype`
+    dtype = arr.dtype.str.replace("m8", "M8")
+    dummy = arr.view(dtype)
+    try:
+        dt64_result = ensure_datetime64ns(dummy, copy)
+    except OutOfBoundsDatetime as err:
+        # Re-write the exception in terms of timedelta64 instead of dt64
+
+        # Find the value that we are going to report as causing an overflow
+        tdmin = arr.min()
+        tdmax = arr.max()
+        if np.abs(tdmin) >= np.abs(tdmax):
+            bad_val = tdmin
+        else:
+            bad_val = tdmax
+
+        raise OutOfBoundsTimedelta(
+            f"Out of bounds for nanosecond {arr.dtype.name} {bad_val}"
+        )
+
+    return dt64_result.view(TD64NS_DTYPE)
 
 
 # ----------------------------------------------------------------------

pandas/core/internals/blocks.py

@@ -2302,7 +2302,8 @@ class TimeDeltaBlock(DatetimeLikeBlockMixin, IntBlock):
 
     def __init__(self, values, placement, ndim=None):
         if values.dtype != TD64NS_DTYPE:
-            values = conversion.ensure_timedelta64ns(values)
+            # e.g. non-nano or int64
+            values = TimedeltaArray._from_sequence(values)._data
         if isinstance(values, TimedeltaArray):
             values = values._data
         assert isinstance(values, np.ndarray), type(values)

pandas/errors/__init__.py

@@ -6,7 +6,7 @@
 
 from pandas._config.config import OptionError
 
-from pandas._libs.tslibs import OutOfBoundsDatetime
+from pandas._libs.tslibs import OutOfBoundsDatetime, OutOfBoundsTimedelta
 
 
 class NullFrequencyError(ValueError):

pandas/tests/tslibs/test_api.py

@@ -32,6 +32,7 @@ def test_namespace():
         "is_null_datetimelike",
         "nat_strings",
         "OutOfBoundsDatetime",
+        "OutOfBoundsTimedelta",
         "Period",
         "IncompatibleFrequency",
         "Resolution",

pandas/tests/tslibs/test_conversion.py

@@ -4,7 +4,13 @@
 import pytest
 from pytz import UTC
 
-from pandas._libs.tslibs import conversion, iNaT, timezones, tzconversion
+from pandas._libs.tslibs import (
+    OutOfBoundsTimedelta,
+    conversion,
+    iNaT,
+    timezones,
+    tzconversion,
+)
 
 from pandas import Timestamp, date_range
 import pandas._testing as tm
@@ -89,6 +95,13 @@ def test_ensure_datetime64ns_bigendian():
     tm.assert_numpy_array_equal(result, expected)
 
 
+def test_ensure_timedelta64ns_overflows():
+    arr = np.arange(10).astype("m8[Y]") * 100
+    msg = r"Out of bounds for nanosecond timedelta64\[Y\] 900"
+    with pytest.raises(OutOfBoundsTimedelta, match=msg):
+        conversion.ensure_timedelta64ns(arr)
+
+
 class SubDatetime(datetime):
     pass