Item default value is appended to the processor_in output in ItemLoader

[sergiogup]

Description

If I create an Item using a dataclass and define a default value, that default value is appended at the start of the resulting array of the output-processor input. Should not the default value be overriden or at least be at the end of the resulting array when the input processor result is not None? Is this intended behavior?

If so, it means that the user has to create a new Loader function, i.e. TakeSecond(), which is identical in functioning as TakeFirst() but it takes the second value in the array if they want to provide a non-None default value and takes the first value in case the input processor received a None value

Steps to Reproduce

1. 
   import scrapy
   from dataclasses import dataclass
   from typing import Optional

from itemloaders.processors import TakeFirst, MapCompose, Join, Compose, Identity from scrapy.loader import ItemLoader

Item definition

@dataclass class ArticleItem: user_rating: Optional[float] = -999

Item Loader definition

class RappiLoader(ItemLoader):

default_output_processor = TakeFirst()

user_rating_in = MapCompose(lambda x: x.get('score') if isinstance(x, dict) else None)

**Expected behavior:**
resulting item should be `ArticleItem(user_rating=4.5)`

**Actual behavior:**
Result is `ArticleItem(user_rating=-999)`

If I inspect the value that gets fed to the output processor it is `[-999, 4.5]`. That's why when using TakeFirst(), it seems as if the input processor did not receive a valid value from the spider, which is not the case. 

**Reproduces how often:**
Every time

### Versions

Scrapy       : 2.6.2
lxml         : 4.8.0.0
libxml2      : 2.9.10
cssselect    : 1.1.0
parsel       : 1.6.0
w3lib        : 1.22.0
Twisted      : 22.4.0
Python       : 3.9.13 | packaged by conda-forge | (main, May 27 2022, 17:00:52) - [Clang 13.0.1 ]
pyOpenSSL    : 22.0.0 (OpenSSL 1.1.1s  1 Nov 2022)
cryptography : 37.0.4
Platform     : macOS-13.1-x86_64-i386-64bit

[wRAR]

Moving this to itemloaders, though on a second thought (and without checking any code) it may be, though not likely, itemdapter-specific.

[VMRuiz]

Hello @wRAR , this is related to how itemloaders build the items.

What's happening here is that ItemLoader will create an instance of ArticleItem without values. Because of that, the user_rating is set to -999 during the instance creation, and ItemLoader will assume as the first value discovered to that field. This is the same behaviour as to passing an already pre-filled dictionary to an item loader:

>>> loader = RappiLoader(item={"user_rating": 2.5})
>>> loader.add_value("user_rating", 0)  # Add fallback value in case user_rating was not defined 
>>> loader.load_item()
{'user_rating': 2.5}

In this case however, the expectation would be to keep the initial value already provided.

I think this shouldn't be fixed. The list of values are always added in the same order. Then, is up to the user to decide what to do with them - e.g. select the first one, last one, all of them, apply some filters, or manipulate them-.

You can implementing a custom selector that fits your needs.