Consider adding _len_ method to `Table`

[SimonGreenhill]

It would be useful to have a __len__ method on Table instances so this works:

if len(cldf['LanguageTable']):
   print("we have languages!")

...but I guess this requires iterating over all rows to know, so it's expensive (unless you want to try load that from the metadata)

[xrotwang]

if next(cldf['LanguageTable']):
    ...

should be cheap.

[SimonGreenhill]

btw -- next(cldf['table']) raises TypeError: 'Table' object is not an iterator, there's no Table.__next__()

[xrotwang]

Ah, so we should add that!

[xflr6]

I don't think we want to make the table an exhaustable iterator (it is already iterable, AFAICT).

@xrotwang : maybe you meant if next(iter(clds['LanguageTable'])): above?

[xflr6]

For the use case at hand it might be enough to add .__bool__() so we can do if cldf['LanguageTable']

[xrotwang]

Ah, yes. That sounds better.

Sebastian Bank notifications@github.com schrieb am Fr., 31. Jan. 2020, 18:35:

For the use case at hand it might be enough to add bool so we can do if cldf['LanguageTable']

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[xrotwang]

Doing a bit more thinking, I'm now inclined to not add any method at all. Adding __bool__ would break backwards compatibility (think of code like pycldf.Dataset.add_table) which returns a table but might also return None - and checking the result with if table. And the missing __len__ method can be correctly interpreted as "that's hard (or expensive) to know".

Please re-open, if you disagree.