ENH: Add DataFrame tabular repr

[sinhrks]

closes #1604. Fixed to be distinguishable from pandas output.

[mrocklin]

@jcrist any comments on this?

[mrocklin]

Taking another look at this (sorry for the long silence). Some feedback based on the following example:

Some subjective thoughts:

• I think we should show fewer empty lines for divisions. Maybe four or five
• I think that we should remove the text-based representation above the tabular repr.
• I think that we should include the number of partitions somewhere, perhaps where it currently says "divisions"
• It would be nice to also state how many tasks are in the computation. I'm not sure exactly where this would go, but we could consider using some of the empty space below the dtypes.
• If the computation of df.head is cheap we could consider adding the first few lines as a sample

head = df.head(compute=False)
head.dask = head._optimize(head.dask, head._keys())
if len(head.dask) < 10:
    ...

I'm not sure about this though. This sort of guess work can get us in trouble. Just throwing it out there as a thought.

[sinhrks]

Sorry not to follow this up. updated based on your suggestions.

• show fewer empty lines for divisions. Maybe four or five
• remove the text-based representation above the tabular repr.
• include the number of partitions somewhere

I think it's nice to display dask key name and number of tasks. Do we already have a function to count current tasks?

[mrocklin]

Do we already have a function to count current tasks?

I would just use len(self.dask)

[mrocklin]

Trying this out locally now. It feels very nice to me.

[mrocklin]

Oh, and I see that it works nicely as a text repr as well.

[sinhrks]

I would just use len(self.dask)

Ah you're right. I've tried to count something other than dask tasks.

Fixed to include it to html / string repr. Note that to_string only contains data repr, but I do not have specific preference.

[mrocklin]

If we wanted to use more than 7 or 10 characters then we might want to use the dask.utils.key_split function introduced in #1919 .

[mrocklin]

I've added the use of key_split to determine the name. Any objections @sinhrks ?

I would like to merge this soon.

[sinhrks]

thx, no objections of course:)

[mrocklin]

Merged. Thanks @sinhrks ! I think this change will make several people happy.