Why this inconsistent behaviour using scrapy shell printing results?

问题

Load the scrapy shell

scrapy shell "http://www.worldfootball.net/all_matches/eng-premier-league-2015-2016/"

Try a selector:

response.xpath('(//table[@class="standard_tabelle"])[1]/tr[not(th)]')

Note: it prints results.

But now use that selector as a for statement:

 for row in response.xpath('(//table[@class="standard_tabelle"])[1]/tr[not(th)]'):
     row.xpath(".//a[contains(@href, 'report')]/@href").extract_first()

Hit return twice, nothing is printed. To print results inside the for loop, you have to wrap the selector in a print function. Like so:

print(row.xpath(".//a[contains(@href, 'report')]/@href").extract_first())

Why?

Edit

If I do the exact same thing as Liam's post below, my output is this:

rmp:www rmp$ scrapy shell "http://www.worldfootball.net/all_matches/eng-premier-league-2015-2016/"
2016-03-05 06:13:28 [scrapy] INFO: Scrapy 1.0.5 started (bot: scrapybot)
2016-03-05 06:13:28 [scrapy] INFO: Optional features available: ssl, http11
2016-03-05 06:13:28 [scrapy] INFO: Overridden settings: {'LOGSTATS_INTERVAL': 0, 'DUPEFILTER_CLASS': 'scrapy.dupefilters.BaseDupeFilter'}
2016-03-05 06:13:28 [scrapy] INFO: Enabled extensions: CloseSpider, TelnetConsole, CoreStats, SpiderState
2016-03-05 06:13:28 [scrapy] INFO: Enabled downloader middlewares: HttpAuthMiddleware, DownloadTimeoutMiddleware, UserAgentMiddleware, RetryMiddleware, DefaultHeadersMiddleware, MetaRefreshMiddleware, HttpCompressionMiddleware, RedirectMiddleware, CookiesMiddleware, ChunkedTransferMiddleware, DownloaderStats
2016-03-05 06:13:28 [scrapy] INFO: Enabled spider middlewares: HttpErrorMiddleware, OffsiteMiddleware, RefererMiddleware, UrlLengthMiddleware, DepthMiddleware
2016-03-05 06:13:28 [scrapy] INFO: Enabled item pipelines: 
2016-03-05 06:13:28 [scrapy] DEBUG: Telnet console listening on 127.0.0.1:6023
2016-03-05 06:13:28 [scrapy] INFO: Spider opened
2016-03-05 06:13:29 [scrapy] DEBUG: Crawled (200) <GET http://www.worldfootball.net/all_matches/eng-premier-league-2015-2016/> (referer: None)
[s] Available Scrapy objects:
[s]   crawler    <scrapy.crawler.Crawler object at 0x108c89c10>
[s]   item       {}
[s]   request    <GET http://www.worldfootball.net/all_matches/eng-premier-league-2015-2016/>
[s]   response   <200 http://www.worldfootball.net/all_matches/eng-premier-league-2015-2016/>
[s]   settings   <scrapy.settings.Settings object at 0x10a25bb10>
[s]   spider     <DefaultSpider 'default' at 0x10c1201d0>
[s] Useful shortcuts:
[s]   shelp()           Shell help (print this help)
[s]   fetch(req_or_url) Fetch request (or URL) and update local objects
[s]   view(response)    View response in a browser
2016-03-05 06:13:29 [root] DEBUG: Using default logger
2016-03-05 06:13:29 [root] DEBUG: Using default logger


In [1]: for row in response.xpath('(//table[@class="standard_tabelle"])[1]/tr[not(th)]'):
...:        row.xpath(".//a[contains(@href, 'report')]/@href").extract_first()
...:     

But with print added?

In [2]: for row in response.xpath('(//table[@class="standard_tabelle"])[1]/tr[not(th)]'):
...:        print row.xpath(".//a[contains(@href, 'report')]/@href").extract_first()
...:     
/report/premier-league-2015-2016-manchester-united-tottenham-hotspur/
/report/premier-league-2015-2016-afc-bournemouth-aston-villa/
/report/premier-league-2015-2016-everton-fc-watford-fc/
/report/premier-league-2015-2016-leicester-city-sunderland-afc/
/report/premier-league-2015-2016-norwich-city-crystal-palace/

回答1:

This just worked for me.

>>>scrapy shell "http://www.worldfootball.net/all_matches/eng-premier-league-2015-2016/"



>>> for row in response.xpath('(//table[@class="standard_tabelle"])[1]/tr[not(th)]'):
...     row.xpath(".//a[contains(@href, 'report')]/@href").extract_first()
...

u'/report/premier-league-2015-2016-manchester-united-tottenham-hotspur/'
u'/report/premier-league-2015-2016-afc-bournemouth-aston-villa/'
u'/report/premier-league-2015-2016-everton-fc-watford-fc/'
u'/report/premier-league-2015-2016-leicester-city-sunderland-afc/'
u'/report/premier-league-2015-2016-norwich-city-crystal-palace/'
u'/report/premier-league-2015-2016-chelsea-fc-swansea-city/'
u'/report/premier-league-2015-2016-arsenal-fc-west-ham-united/'
u'/report/premier-league-2015-2016-newcastle-united-southampton-fc/'
u'/report/premier-league-2015-2016-stoke-city-liverpool-fc/'
u'/report/premier-league-2015-2016-west-bromwich-albion-manchester-city/'

does this not show the same results for you?

来源：https://stackoverflow.com/questions/35790211/why-this-inconsistent-behaviour-using-scrapy-shell-printing-results