Allow string formatting of scalar DataArrays

[fmaussion]

• Closes Should str.format() work on xarray scalars? #5976
• Tests added
• Passes pre-commit run --all-files
• User visible changes (including notable bug fixes) are documented in whats-new.rst

This is a first try at formatting dataarray scalars. Here is the current behavior:

In [1]: importxarrayasxr
...: importnumpyasnpIn [2]: a=np.array(1)
...: da=xr.DataArray(a)
In [3]: print(a)
1In [4]: print(da)<xarray.DataArray ()>array(1)
In [5]: print('{}'.format(a))
1In [6]: print('{}'.format(da))
<xarray.DataArray ()>array(1)
In [7]: print('{:.3f}'.format(a))
1.000In [8]: print('{:.3f}'.format(da))
1.000In [9]: a=np.array([1, 2])
...: da=xr.DataArray(a)
In [10]: print('{}'.format(a))
[12]
In [11]: print('{}'.format(da))
<xarray.DataArray (dim_0: 2)>array([1, 2])
Dimensionswithoutcoordinates: dim_0In [12]: print('{:.3f}'.format(a))
---------------------------------------------------------------------------TypeErrorTraceback (mostrecentcalllast)
<ipython-input-12-c5afc7863e89>in<module>---->1print('{:.3f}'.format(a))
TypeError: unsupportedformatstringpassedtonumpy.ndarray.__format__In [13]: print('{:.3f}'.format(da))
---------------------------------------------------------------------------TypeErrorTraceback (mostrecentcalllast)
<ipython-input-13-bddebd8462bd>in<module>---->1print('{:.3f}'.format(da))
~/disk/Dropbox/HomeDocs/git/xarray/xarray/core/common.pyin__format__(self, format_spec)
162returnformatting.array_repr(self)
163# Else why fall back to numpy-->164returnself.values.__format__(format_spec)
165166def_iter(self: Any) ->Iterator[Any]:
TypeError: unsupportedformatstringpassedtonumpy.ndarray.__format__

I don't think there is any backwards compatibility issue but lets see if the tests pass

[fmaussion]

Note that there would be a way to change the behavior to:

print(da) ->lengthyreprprint('{}'.format(da) ->fallbacktonumpy.__format__

This would break backwards compatibility, but I think it would be my preference - there are some chances that it brakes some code (in documentation pages maybe?), but I don't think that many people rely on '{}'.format(da) to return __repr__ ...

[Illviljan]

Suggested change

	assertvar.__format__("") =="<xarray.DataArray ()>\narray(0)"
	assertvar.__format__("d") =="0"
	assertvar.__format__(".2f") =="0.00"
	var=xr.DataArray([0])
	assertvar.__format__("") == (
	"<xarray.DataArray (dim_0: 1)>\narray([0])"
	"\nDimensions without coordinates: dim_0"
	)
	assertvar.__format__("") ==var.__repr__()
	assertvar.__format__("d") =="0"
	assertvar.__format__(".2f") =="0.00"
	var=xr.DataArray([0])
	assertvar.__format__("") ==var.__repr__()

repr style can change over time. I think this should work.

[fmaussion]

Thanks that makes sense!

However I think I would like to simplify this even more and always call ndarray.__format__ when DataArray.__format__ is called. This won't be backwards compatible but I think this would be better and more predictable.

[fmaussion]

Side note: with this design (no call to repr() in format()) we would mimick numpy's behavior:

a=np.array([0.1, 0.2])
a.__format__("")
Out[16]: '[0.1 0.2]'a.__repr__()
Out[17]: 'array([0.1, 0.2])'

However I'm quite surprised as to why .__format__("") works on non-scalar arrays but not with other specifiers - this is discussed somehow in numpy/numpy#5543 . I think it's okay for xarray to defer to whatever numpy is doing here, but of course we can also keep the status-quo, it's quite a small change after all.

[fmaussion]

@Illviljan and other devs - I still think this small change would be a nice addition for accessibility and teaching. Let me know if we can purse or if I should close this.

[Illviljan]

Thanks @fmaussion!