Added check that timedelta can be represented by an integer number of nanoseconds

Author: rosstitmarsh

pandas/_libs/tslibs/timedeltas.pyx

@@ -687,19 +687,25 @@ cdef timedelta_from_spec(object number, object frac, object unit):
     unit : a list of unit characters
     """
     cdef:
-        str n
+        object ts
+        int64_t result
 
     unit = "".join(unit)
     if unit in ["M", "Y", "y"]:
         raise ValueError(
-            "Units 'M', 'Y' and 'y' do not represent unambiguous timedelta "
-            "values and are not supported."
+            "Units 'M', 'Y' and 'y' do not represent unambiguous timedelta values and "
+            "are not supported."
         )
-
     unit = parse_timedelta_unit(unit)
 
-    n = "".join(number) + "." + "".join(frac)
-    return cast_from_unit(float(n), unit)
+    ts = float("".join(number) + "." + "".join(frac))
+    result = cast_from_unit(ts, unit)
+
+    # Check that ts can be represented by an integer number of nanoseconds
+    if result == 0 and ts > 0:
+        raise ValueError("Resolution too high, maximum resolution is 1ns")
+
+    return result
 
 
 cpdef inline str parse_timedelta_unit(str unit):

pandas/tests/scalar/timedelta/test_constructors.py

@@ -407,13 +407,26 @@ def test_iso_constructor(fmt, exp):
 )
 def test_iso_constructor_raises_ambiguous_units(fmt):
     msg = (
-        "Units 'M', 'Y' and 'y' do not represent unambiguous timedelta values "
-        "and are not supported."
+        "Units 'M', 'Y' and 'y' do not represent unambiguous timedelta values and are "
+        "not supported."
     )
     with pytest.raises(ValueError, match=msg):
         Timedelta(fmt)
 
 
+@pytest.mark.parametrize(
+    "fmt",
+    [
+        "P1DT0H0M0.0000000001S",
+        "PT0.00000000001M",
+    ],
+)
+def test_iso_constructor_raises_resolution(fmt):
+    msg = "Resolution too high, maximum resolution is 1ns"
+    with pytest.raises(ValueError, match=msg):
+        Timedelta(fmt)
+
+
 @pytest.mark.parametrize(
     "fmt",
     [
@@ -432,8 +445,6 @@ def test_iso_constructor_raises_ambiguous_units(fmt):
         "P1S",
         # Duplicate time separator
         "P1DTT1H",
-        # Precision too high
-        "P1DT0H0M0.0000000001S",
         # Only lowest order component may have be frational
         "P1DT2.5H3M",
         # Number start with in dot