RangeIndex` with datetime64 bounds cannot be printed (`Unknown format code 'g'`)

[d-chambers]

What happened?

A RangeIndex built from datetime64 start/stop and a timedelta64 step works: labels are computed correctly, and isel and sel(method="nearest") keep the index lazy. But its repr formats start, stop and step with :.3g, which only real numbers accept, so printing the index, or any DataArray/Dataset which holds it, raises.

A lazy time axis is the main reason we want RangeIndex: a long, evenly sampled acquisition has 1e8+ time labels and a handful of channels, so the time coordinate is the one worth not materializing.

What did you expect to happen?

The repr to print, as it does for a float range, e.g.

Indexes:
    time     RangeIndex (start=2023-01-01T00:00:00.000000000, stop=2023-01-01T00:00:10.000000000, step=1 seconds)

Minimal Complete Verifiable Example

# /// script
# requires-python = ">=3.11"
# dependencies = [
#   "xarray[complete]@git+https://github.com/pydata/xarray.git@main",
# ]
# ///
#
# This script automatically imports the development branch of xarray to check for issues.
# Please delete this header if you have _not_ tested this script with `uv run`!

import xarray as xr
import numpy as np
from xarray.indexes import RangeIndex
xr.show_versions()

t0 = np.datetime64("2023-01-01", "ns")
step = np.timedelta64(1, "s")
index = RangeIndex.arange(t0, t0 + 10 * step, step, dim="time")
da = xr.DataArray(np.zeros(10), dims="time", coords=xr.Coordinates.from_xindex(index))

print(da.time.values[:2])  # ['2023-01-01T00:00:00.000000000' '2023-01-01T00:00:01.000000000']
print(da)

Steps to reproduce

Just run it

MVCE confirmation

• Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
• Complete example — the example is self-contained, including all data and the text of any traceback.
• Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
• New issue — a search of GitHub Issues suggests this is not a duplicate.
• Recent environment — the issue occurs with the latest version of xarray and its dependencies.

Relevant log output

Anything else we need to know?

No response

Environment

Details

Reproduced on both, in clean uv environments:
xarray 2026.7.0 (latest release)
xarray 2026.7.1.dev56+g8de862c29 (main at commit 8de862c, 2026-09-21)

INSTALLED VERSIONS

commit: None
python: 3.13.1 (main, Dec 6 2024, 18:40:43) [Clang 18.1.8 ]
python-bits: 64
OS: Linux
OS-release: 7.0.0-31-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: ('en_US', 'UTF-8')
libhdf5: None
libnetcdf: None

xarray: 2026.7.0
pandas: 3.0.6
numpy: 2.5.3
scipy: None
netCDF4: None
pydap: None
h5netcdf: None
h5py: None
zarr: None
cftime: None
nc_time_axis: None
iris: None
bottleneck: None
dask: None
distributed: None
matplotlib: None
cartopy: None
seaborn: None
numbagg: None
fsspec: None
cupy: None
pint: None
sparse: None
flox: None
numpy_groupies: None
setuptools: None
pip: 24.3.1
conda: None
pytest: None
mypy: None
IPython: None
sphinx: None