look up time values from a pandas.DataFrame

[raybellwaves]

I've got some data stored in a dataframe. I would to use the dataframe to select points from a Dataset. It seems to struggle with time values.

import xarray as xr
import pandas as pd

ds = xr.tutorial.open_dataset("air_temperature")
df = pd.DataFrame({"lat": ds["lat"].values[0:2], "lon": ds["lon"].values[0:2], "time": ds["time"].values[0:2]})

ds.sel(time=df["time"])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/ray/miniconda3/envs/main/lib/python3.9/site-packages/xarray/core/dataset.py", line 2474, in sel
    pos_indexers, new_indexes = remap_label_indexers(
  File "/Users/ray/miniconda3/envs/main/lib/python3.9/site-packages/xarray/core/coordinates.py", line 421, in remap_label_indexers
    pos_indexers, new_indexes = indexing.remap_label_indexers(
  File "/Users/ray/miniconda3/envs/main/lib/python3.9/site-packages/xarray/core/indexing.py", line 117, in remap_label_indexers
    idxr, new_idx = index.query(labels, method=method, tolerance=tolerance)
  File "/Users/ray/miniconda3/envs/main/lib/python3.9/site-packages/xarray/core/indexes.py", line 200, in query
    raise ValueError(
ValueError: cannot use a dict-like object for selection on a dimension that does not have a MultiIndex

ds.sel(lat=df["lat"]) # works

[max-sixty]

Hi @raybellwaves ! Hope all's well.

Is this because the df['lat'] is returning a series? Adding .values seems to return something — is that what you're hoping for?

In [6]: ds.sel(time=df["time"].values)
Out[6]:
<xarray.Dataset>
Dimensions:  (lat: 25, time: 2, lon: 53)
Coordinates:
  * lat      (lat) float32 75.0 72.5 70.0 67.5 65.0 ... 25.0 22.5 20.0 17.5 15.0
  * lon      (lon) float32 200.0 202.5 205.0 207.5 ... 322.5 325.0 327.5 330.0
  * time     (time) datetime64[ns] 2013-01-01 2013-01-01T06:00:00
Data variables:
    air      (time, lat, lon) float32 ...
Attributes:
    Conventions:  COARDS
    title:        4x daily NMC reanalysis (1948)
    description:  Data is from NMC initialized reanalysis\n(4x/day).  These a...
    platform:     Model
    references:   http://www.esrl.noaa.gov/psd/data/gridded/data.ncep.reanaly...

[raybellwaves]

Thanks yes .values solves this issue. I'm more interested in what it'll take for it to work for a pandas datetime. I believe time in xarray in determined by time (types) in numpy hence you have to do when wanting to pandas datetime operations you have to do .to_index() first. e.g. changing time to a local timezone:

ds = ds.assign_coords(
    {
        "time": ds["time"]
        .to_index()
        .tz_localize("UTC")
        .tz_convert("America/Los_Angeles")
        .tz_localize(None)
        .values
    }
)

If there are known issues on this welcome to link them here. This probably falls under https://github.com/pydata/xarray/issues?q=is%3Aopen+is%3Aissue+label%3Atopic-indexing and possibly #5692