ds = xr.tutorial.load_dataset("air_temperature") with 0.18 needs engine argument

[keewis]

From xarray-contrib/xarray-tutorial#43 by @scottyhq:

Many notebooks out there start with the line ds = xr.tutorial.load_dataset("air_temperature"). That now gives an error traceback with xarray>=0.18:

Traceback (most recent call last):
  File "/Users/scott/GitHub/zarrdata/./create_zarr.py", line 6, in <module>
    ds = xr.tutorial.load_dataset("air_temperature")
  File "/Users/scott/miniconda3/envs/zarrdata/lib/python3.9/site-packages/xarray/tutorial.py", line 179, in load_dataset
    with open_dataset(*args, **kwargs) as ds:
  File "/Users/scott/miniconda3/envs/zarrdata/lib/python3.9/site-packages/xarray/tutorial.py", line 100, in open_dataset
    ds = _open_dataset(filepath, **kws)
  File "/Users/scott/miniconda3/envs/zarrdata/lib/python3.9/site-packages/xarray/backends/api.py", line 485, in open_dataset
    engine = plugins.guess_engine(filename_or_obj)
  File "/Users/scott/miniconda3/envs/zarrdata/lib/python3.9/site-packages/xarray/backends/plugins.py", line 112, in guess_engine
    raise ValueError("cannot guess the engine, try passing one explicitly")
ValueError: cannot guess the engine, try passing one explicitly

It's an easy fix though, just add ds = xr.tutorial.load_dataset("air_temperature", engine="netcdf4"), new users might be thrown by that though. Also a note that unless the netcdf4 library is explicitly put into the software environment, even adding the engine=netcdf4 can result in an error: "ValueError: unrecognized engine netcdf4 must be one of: ['store', 'zarr']", so I think a minimal environment definition to run would be:

name: xarray-tutorial
channels:
  - conda-forge
dependencies:
  - xarray=0.18
  - pooch=1.3
  - netcdf4=1.5
  - zarr=2.8

[keewis]

I think there's something wrong with your environment: I can't reproduce in a environment created by

mamba create -n test python=3.9 mamba xarray=0.18 pooch=1.3 netcdf4=1.5 zarr=2.8

Edit: you could also try clearing ~/.xarray_tutorial_data / ~/.cache/xarray_tutorial_data

[keewis]

we could definitely improve the error message, though. Something like "unknown engine {engine}, please choose one of the installed engines: {engines}", maybe?

[scottyhq]

Thanks @keewis I should have been more clear about the environment. I was recently going over the tutorial with someone and started with:

1. conda create -n xarray-tutorial xarray
   running ds = xr.tutorial.load_dataset("air_temperature") --> ImportError: using the tutorial data requires pooch
2. we install pooch and then hit: ValueError: cannot guess the engine, try passing one explicitly
3. after consulting the docstring we then try ds = xr.tutorial.load_dataset("air_temperature", engine="netcdf4") and hit ValueError: unrecognized engine netcdf4 must be one of: ['store']
4. being familiar with xarray we then installed netcdf4 into our environment and all is well.

I do think these error messages are not obvious to fix for new xarray users trying out the tutorial (especially # 3 above)

we could definitely improve the error message, though. Something like "unknown engine {engine}, please choose one of the installed engines: {engines}", maybe?

Yes. Perhaps with a link to docs with a list of engines? for the tutorial case specifically could also update the ImportError message to read ImportError: please install 'pooch' and 'netcdf4' to use xarray tutorial data?

[shoyer]

This is indeed unfortunate, thanks for the report!

I believe xarray.tutorial.load_dataset("air_temperature") works -- at least in my testing -- if either scipy or netCDF4 is installed (along with Pooch).

But if you don't have either scipy or netCDF4 installed, you get the very unhelpful error message ValueError: cannot guess the engine, try passing one explicitly

At the very least, we should report which engines are installed -- and ideally make a specific recommendation like installing scipy or netCDF4.

[max-sixty]

But if you don't have either scipy or netCDF4 installed, you get the very unhelpful error message ValueError: cannot guess the engine, try passing one explicitly

At the very least, we should report which engines are installed -- and ideally make a specific recommendation like installing scipy or netCDF4.

Yes and is this a problem with passing an engine? Could doing what it says ("passing one explicitly") make it work?

[shoyer]

Good point -- in this case specifying an engine explicitly cannot help, because the required engine is not installed.

[shoyer]

CC @alexamici @aurghs

I would suggest three levels of fixes:

1. Better fall-back message when engine guessing fails: Better error message when no backend engine is found. #5300
2. Suggesting specific dependencies to install (e.g., netCDF4) based on the provided file, from inside open_dataset.
3. Suggesting specific dependencies to install when using tutorial datasets.

If we do a nice job of (2) than maybe (3) will not be necessary. I'll open a new issue to discuss my proposed solution for (2).

[shoyer]

I've added a temporary work-around to provide better errors for the tutorial datasets into #5300, i.e., solution (3) from my previouso comment.

This is challenging to unit-test, so I'll just share my IPython session(s) from a new environment to show how it works:

(xarray-min)   ~/dev/xarray better-no-plugins-errors $ ipython
Python 3.9.4 (default, Apr  9 2021, 09:32:38)
Type 'copyright', 'credits' or 'license' for more information
IPython 7.22.0 -- An enhanced Interactive Python. Type '?' for help.

In [1]: import xarray

In [2]: xarray.tutorial.open_dataset('air_temperature')
---------------------------------------------------------------------------
ModuleNotFoundError                       Traceback (most recent call last)
~/dev/xarray/xarray/tutorial.py in open_dataset(name, cache, cache_dir, engine, **kws)
    113     try:
--> 114         import pooch
    115     except ImportError:

ModuleNotFoundError: No module named 'pooch'

During handling of the above exception, another exception occurred:

ImportError                               Traceback (most recent call last)
<ipython-input-2-e970a0b41221> in <module>
----> 1 xarray.tutorial.open_dataset('air_temperature')

~/dev/xarray/xarray/tutorial.py in open_dataset(name, cache, cache_dir, engine, **kws)
    114         import pooch
    115     except ImportError:
--> 116         raise ImportError("using the tutorial data requires pooch")
    117
    118     logger = pooch.get_logger()

ImportError: using the tutorial data requires pooch

In [3]: ! pip install pooch
Collecting pooch
  Using cached pooch-1.3.0-py3-none-any.whl (51 kB)
Requirement already satisfied: requests in /Users/shoyer/miniconda3/envs/xarray-min/lib/python3.9/site-packages (from pooch) (2.25.1)
Requirement already satisfied: appdirs in /Users/shoyer/miniconda3/envs/xarray-min/lib/python3.9/site-packages (from pooch) (1.4.4)
Requirement already satisfied: packaging in /Users/shoyer/miniconda3/envs/xarray-min/lib/python3.9/site-packages (from pooch) (20.9)
Requirement already satisfied: pyparsing>=2.0.2 in /Users/shoyer/miniconda3/envs/xarray-min/lib/python3.9/site-packages (from packaging->pooch) (2.4.7)
Requirement already satisfied: certifi>=2017.4.17 in /Users/shoyer/miniconda3/envs/xarray-min/lib/python3.9/site-packages (from requests->pooch) (2020.12.5)
Requirement already satisfied: urllib3<1.27,>=1.21.1 in /Users/shoyer/miniconda3/envs/xarray-min/lib/python3.9/site-packages (from requests->pooch) (1.26.4)
Requirement already satisfied: idna<3,>=2.5 in /Users/shoyer/miniconda3/envs/xarray-min/lib/python3.9/site-packages (from requests->pooch) (2.10)
Requirement already satisfied: chardet<5,>=3.0.2 in /Users/shoyer/miniconda3/envs/xarray-min/lib/python3.9/site-packages (from requests->pooch) (4.0.0)
Installing collected packages: pooch
Successfully installed pooch-1.3.0

In [4]: xarray.tutorial.open_dataset('air_temperature')
---------------------------------------------------------------------------
ModuleNotFoundError                       Traceback (most recent call last)
~/dev/xarray/xarray/tutorial.py in _check_netcdf_engine_installed(name)
     51         try:
---> 52             import scipy
     53         except ImportError:

ModuleNotFoundError: No module named 'scipy'

During handling of the above exception, another exception occurred:

ModuleNotFoundError                       Traceback (most recent call last)
~/dev/xarray/xarray/tutorial.py in _check_netcdf_engine_installed(name)
     54             try:
---> 55                 import netCDF4
     56             except ImportError:

ModuleNotFoundError: No module named 'netCDF4'

During handling of the above exception, another exception occurred:

ImportError                               Traceback (most recent call last)
<ipython-input-4-e970a0b41221> in <module>
----> 1 xarray.tutorial.open_dataset('air_temperature')

~/dev/xarray/xarray/tutorial.py in open_dataset(name, cache, cache_dir, engine, **kws)
    128             default_extension = ".nc"
    129             if engine is None:
--> 130                 _check_netcdf_engine_installed(name)
    131             path = path.with_suffix(default_extension)
    132         elif path.suffix == ".grib":

~/dev/xarray/xarray/tutorial.py in _check_netcdf_engine_installed(name)
     55                 import netCDF4
     56             except ImportError:
---> 57                 raise ImportError(
     58                     f"opening tutorial dataset {name} requires either scipy or "
     59                     "netCDF4 to be installed."

ImportError: opening tutorial dataset air_temperature requires either scipy or netCDF4 to be installed.

In [5]: ! pip install netcdf4
Collecting netcdf4
  Using cached netCDF4-1.5.6-cp39-cp39-macosx_10_9_x86_64.whl (4.0 MB)
Requirement already satisfied: numpy>=1.9 in /Users/shoyer/miniconda3/envs/xarray-min/lib/python3.9/site-packages (from netcdf4) (1.20.1)
Requirement already satisfied: cftime in /Users/shoyer/miniconda3/envs/xarray-min/lib/python3.9/site-packages (from netcdf4) (1.4.1)
Installing collected packages: netcdf4
Successfully installed netcdf4-1.5.6

In [6]: xarray.tutorial.open_dataset('air_temperature')
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-6-e970a0b41221> in <module>
----> 1 xarray.tutorial.open_dataset('air_temperature')

~/dev/xarray/xarray/tutorial.py in open_dataset(name, cache, cache_dir, engine, **kws)
    138     # retrieve the file
    139     filepath = pooch.retrieve(url=url, known_hash=None, path=cache_dir)
--> 140     ds = _open_dataset(filepath, engine=engine, **kws)
    141     if not cache:
    142         ds = ds.load()

~/dev/xarray/xarray/backends/api.py in open_dataset(filename_or_obj, engine, chunks, cache, decode_cf, mask_and_scale, decode_times, decode_timedelta, use_cftime, concat_characters, decode_coords, drop_variables, backend_kwargs, *args, **kwargs)
    478
    479     if engine is None:
--> 480         engine = plugins.guess_engine(filename_or_obj)
    481
    482     backend = plugins.get_backend(engine)

~/dev/xarray/xarray/backends/plugins.py in guess_engine(store_spec)
    118         )
    119     else:
--> 120         raise ValueError(
    121             "xarray is unable to open this file because it has no currently "
    122             "installed IO backends. Xarray's read/write support requires "

ValueError: xarray is unable to open this file because it has no currently installed IO backends. Xarray's read/write support requires installing optional dependencies:
http://xarray.pydata.org/en/stable/getting-started-guide/installing.html
http://xarray.pydata.org/en/stable/user-guide/io.html

In [7]:
Do you really want to exit ([y]/n)? y
(xarray-min)   ~/dev/xarray better-no-plugins-errors $ ipython
Python 3.9.4 (default, Apr  9 2021, 09:32:38)
Type 'copyright', 'credits' or 'license' for more information
IPython 7.22.0 -- An enhanced Interactive Python. Type '?' for help.

In [1]: import xarray

In [2]: xarray.tutorial.open_dataset('air_temperature')
Out[2]:
<xarray.Dataset>
Dimensions:  (lat: 25, lon: 53, time: 2920)
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 ... 2014-12-31T18:00:00
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...

This isn't quite perfect -- the user needs to restart their session to actually load the data -- but hopefully this is much more obvious and roughly within user expectations.

[max-sixty]

Thanks a lot @shoyer .

Is there a way of having pooch as a default but not required dependency? I'm not sure how python works with this — can we specify a minimal / required dependency set, but have pooch installed by default?

It's a pretty light dependency, and it's unfortunate to add any sand into the gears of getting someone to the tutorial. (though an order of magnitude better since the change above)