DataArray.diff dim argument should be optional as is in docstring

[smartass101]

The dosctring of DataArray.diff lists the dim arg as optional, but it isn't. IMHO it should indeed be optional as it is quite convenient to apply diff to 1D signals without specifying the dimension.

[shoyer]

Seems reasonable to me. Want to put together a PR?

[dcherian]

@smartass101 I was about to open an issue for this. Looks like you have already solved it. Wanna open a PR?

[stale]

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[keewis]

this is still relevant. Should we update the docstring or the implementation? If we choose the latter we might be able to pick the commit by @smartass101 (or he might open a PR for it if he's still around?)

[max-sixty]

I'm low confidence keener on a consistent API that doesn't depend on the number of dimensions, rather than saving a kwarg, so would vote +0.1 on retaining the implementation and changing the docstring, such that it's required to pass a dimension name even with one dimension.

[dcherian]

I agree.

[smartass101]

These days I mostly use .differentiate('coord') anyways, so I'm also for updating the docstring.