[MRG] Resolved issue #546. Output format parsing from filename extension.

[denysbutenko]

Instead of explicitly specify the output format type, we can learn it from the file extension.

[kmike]

I think we shouldn't drop support for explicit -t option.

[kmike]

Also, some tests for this feature would be great!

[kmike]

I think this will raise an unhelpful exception when user runs scrapy crawl myspider -o ./data.

[denysbutenko]

Yep, I'm not noticed what os not imported.
After import:
Runspider

~/dev/spider/pws/spiders ❯ ~/dev/scrapy/bin/scrapy runspider pws_spider.py -o ./data.                                                 
Usage
=====
  scrapy runspider [options] <spider_file>

runspider: error: Invalid/unrecognized output format: , Expected ['xml', 'jsonlines', 'json', 'csv', 'pickle', 'marshal']

Crawl:

~/dev/spider/pws/spiders ❯ ~/dev/scrapy/bin/scrapy crawl pws_spider.py -o ./data.                                                    
Usage
=====
  scrapy crawl [options] <spider>

crawl: error: Invalid/unrecognized output format: , Expected ['xml', 'jsonlines', 'json', 'csv', 'pickle', 'marshal']

[kmike]

What about

scrapy crawl myspider -o ./data

(without trailing dot)?

[denysbutenko]

Function os.path.splitext() will return tuple of file name and file extension. Then we select file extension and replace dot. Of course may be needed replace only first dot, but I not see file types with double extension.

[kmike]

Yes, you're right.

[denysbutenko]

Without trailing dot:

~/dev/spider/pws/spiders ❯ ~/dev/scrapy/bin/scrapy crawl pws_spider.py -o ./data                                                      
Usage
=====
  scrapy crawl [options] <spider>

crawl: error: Invalid/unrecognized output format: , Expected ['xml', 'jsonlines', 'json', 'csv', 'pickle', 'marshal']

Result of splitext of ./data will be empty file ext.

>>> import os
>>> fname = "./data"
>>> print os.path.splitext(fname)
('./data', '')

[nyov]

Could we have a more verbose error message, for less confusion with the newbies?

raise UsageError("Unrecognized output format '%s', set one using the '-t' switch or as a fileextension from the supported list %s" % (opts.output_format, tuple(valid_output_formats)))

[kmike]

This PR looks good to me. There is one possible improvement: check docs and tutorials and remove -t option from them when possible. But I'm fine with merging it without this improvement.