add the default parameter to get_output_value

[noviluni]

Hey guys! Good job decoupling this from Scrapy!

I’ve been thinking about the possibility of adding a default value for the get_output_value().

So as an example, you could do:

item_loader.get_output_value('name', 'Foobar')

and you would receive the value for name if it's available or 'Foobar' if it's not available.

The implementation seems quite simple, but there are a lot of scenarios that I tried to reproduce in the tests.

Could you?

1. Let me know if you like the idea.
I decided to implement it because I frequently find something similar to:

item_loader.get_output_value('name') or ''

2. Check the tests I added and see if it's what you expect or not.
If you see any missed case you can also let me know it.

Thanks in advance!

[codecov]

Codecov Report

Merging #26 into master will increase coverage by 1.58%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #26      +/-   ##
==========================================
+ Coverage   98.02%   99.60%   +1.58%     
==========================================
  Files           4        4              
  Lines         253      256       +3     
==========================================
+ Hits          248      255       +7     
+ Misses          5        1       -4     

Impacted Files	Coverage Δ
itemloaders/__init__.py	99.36% <100.00%> (+1.94%)	⬆️
itemloaders/utils.py	100.00% <0.00%> (+3.12%)	⬆️

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[noviluni]

Updated @Gallaecio

[ejulio]

I just have one consideration about the API.
In a dict, get will only return the default value if the key was not set.
Here, the default value is returned even if the field was set, but processing the value returned an "empty value" ('', 0, ...).

It looks good to me, though I think we should be clear as much as possible about this behavior.

Also, not sure if we need to add something to docs about it

[Gallaecio]

You are right, we definitely need to update the documentation.