Fix #3771: Add fallback css and xpath selectors #3772
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Gallaecio
reviewed
May 14, 2019
| @@ -385,12 +385,26 @@ ItemLoader objects | |||
| :param xpath: the XPath to extract data from | |||
| :type xpath: str | |||
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| :param stop_on_first_match: when ``xpath`` is a list of selectors, should it | |||
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I wonder whether or not this parameter should be added, since I believe the same effect can be achieved already with TakeFirst.
Or are you adding it for performance reasons?
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Truth be told, I forgot that we could get the same behavior with TakeFirst.
Not sure if performance is a problem at all.
My concern was related to semantics.
loader.add_css('field', ['sel1', 'sel2']) gives the idea of a composition, field is a composition of both 'sel1' and 'sel2'.
loader.add_css('field', ['sel1', 'sel2'], stop_on_first_match=True) would mean they are fallbacks because of different page layouts. If 'sel1' fails, then it try 'sel2'.
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Fixes #3771
So, scrapy already handled
loader.add_css('my_field', ['selector1', 'selector2']).Though, I added this behavior to the docs.
I added a new kwarg called
stop_on_first_matchto bothadd_cssandadd_xpath.This way we can control if the field is a composite of various selectors or if it is a set of fallbacks.
I'm not sure about the name
stop_on_first_match.processor.Composehas astop_on_noneargument (https://docs.scrapy.org/en/latest/topics/loaders.html#scrapy.loader.processors.Compose).Maybe we can use another that is similar, or even
stop_on_noneit is checking explicitly forNoneright now.