Dataset.__repr__ causing dask to be computed.

[andrewdhicks]

Printing a Dataset object will show array values (#206), but it will also cause a dask-backed array to be computed so that the data variable values can be computed:

<xarray.Dataset>
Dimensions:  (time: 182, x: 420, y: 489)
Coordinates:
  * time     (time) datetime64[ns] 1990-03-02T23:11:16 1990-03-02T23:11:39 ...
  * y        (y) float64 -3.919e+06 -3.919e+06 -3.919e+06 -3.919e+06 ...
  * x        (x) float64 1.538e+06 1.538e+06 1.538e+06 1.538e+06 1.538e+06 ...
Data variables:
    band_20  (time, y, x) float64 729.0 612.0 579.0 629.0 862.0 1.027e+03 ...
    band_10  (time, y, x) float64 476.0 375.0 357.0 416.0 586.0 708.0 730.0 ...
    band_50  (time, y, x) float64 2.175e+03 634.0 245.0 546.0 1.788e+03 ...
...

When a DataArray is printed, it doesn't compute values when using dask:

<xarray.DataArray u'ls5_nbar_albers' (time: 182, y: 489, x: 420)>
dask.array<concate..., shape=(182, 489, 420), dtype=float64, chunksize=(1, 489, 420)>
Coordinates:
  * time      (time) datetime64[ns] 1990-03-02T23:11:16 1990-03-02T23:11:39 ...
  * y         (y) float64 -3.919e+06 -3.919e+06 -3.919e+06 -3.919e+06 ...
  * x         (x) float64 1.538e+06 1.538e+06 1.538e+06 1.538e+06 1.538e+06 ...

There is a check to make sure the data is not remote, but not based on dask status, see: https://github.com/pydata/xarray/blob/master/xarray/core/formatting.py#L173
Is there a way to indicate that computing a particular dask is an expensive operation and it should not be calculated?

[shoyer]

Showing a preview of values in a dask dataset can be very convenient for interactive use, so I'm loathe to turn it off entirely. I agree that it's not always useful, though.

The current check for remote data is a complete hack that should probably be removed :).

What do you think about adding an user configurable option to disable printing lazily computed values in datasets? You could then write something like xarray.set_options(preview_lazy_data=False):
http://xarray.pydata.org/en/stable/generated/xarray.set_options.html

[andrewdhicks]

That sounds like a great solution. I'll get a PR up with the preview_lazy_data option preventing formatting when var._in_memory is false.

[stale]

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