Add n argument to TakeFirst processor

[kmike]

What do you think about adding "n" argument to TakeFirst processor?

• By default it would be None, meaning the existing behaviour.
• TakeFirst(3) would mean "return a list with at most 3 non-empty values".

[Digenis]

It lets the return type depend on an argument, TakeFirst(None)(List) would return just an element of the list while TakeFirst(1)(List) an element of the list wrapped in a list.
Personally I don't like making the types less predictable but python does this already:

• operator.itemgetter() from the builtins behaves similarly, the return type implicitly depends on the number of its arguments.
• "str %" formatting expects a single right term or a tuple of them depending on the number of % placeholders in the string.

On the one hand TakeFirst(3) looks like natural language but on the other it implies a different return type.

I would rather see a function, like an argument to filter, passed to TakeFirst() allowing for customization of the default "value is not None and value != '' " (I am just saying this here/now because it conflicts with the above proposal)

[Digenis]

From my perspective, the loader.processor lacks two loaders.

I 'd go a bit further, since your TakeFirst(-1) made me think of:
lambda n: lambda l: filter(None, l)[:n]
In the snippet notice that the first argument to filter() could also be customized, (without making the return type depend on the argument to the factory):
lambda f=None: lambda l: filter(f, l)
(Of course I oversimplify it here with None just to keep the snippet simple)
Let us call it "Filter" since "TakeFirst" return a single element.

Now "Slice":
Slice = lambda a, b: lambda l: l[a, b]
the operator module lacks a slicegetter anyway
Now the example with TakeFirst(3) would be written: Compose(Filter(), Slice(None, 3))
and why not name the composition as processor.TakeSlice or TakeFirstN, a shortcut.

Most processors offer practical shortcuts for long compositions of lambdas, "functools" and "operator". I see TakeFirst as a shortcut of shorcuts.

To say this in plain code Digenis/scrapy@1f088d4
and doctests to demonstrate their behaviour https://github.com/Digenis/scrapy/blob/0e9c95dfcccb1628d51bf500eca60b683ab95fb5/scrapy/contrib/loader/processor.py#L52-L100

[redapple]

(just digging out old issues)
If we really want that (disclaimer: I don't use loaders myself), I'd favor a seperate processor TakeFirstN(n), always returning a list of at most n elements

[cathalgarvey]

@kmike - Is this something you'd still like to see? We might resolve some of @Digenis' concerns with a new processor, TakeFirstN, which would simplify typing and clarity issues a lot?

[kmike]

TakeFirstN sounds fine to me, though I haven't used item loaders for a long time.