[bug] FlaskDynaconf breaks default getter of flask config

[shsmad]

Describe the bug
In pure Flask config if you query config for nonexisting key you get KeyError exception.
If use FlaskDynaconf if you query config for nonexisting key you get value None.

Such differences break thirdparty libraries, which suppose that flask config will raise KeyError when queriing nonexisting key (for example, python-social-auth: https://github.com/python-social-auth/social-core/blob/master/social_core/strategy.py#L45 which goes to https://github.com/python-social-auth/social-app-flask/blob/master/social_flask/strategy.py#L19)

To Reproduce
Correct behavior:

from flask import Flask

app = Flask(__name__)
print("NON_EXISTING" in app.config)
print(app.config["NON_EXISTING"])

will give you

False
Traceback (most recent call last):
  File "dc_poc/1_no_dynaconf.py", line 6, in <module>
    print(app.config["NON_EXISTING"])
KeyError: 'NON_EXISTING'

Incorrect behavior:

from flask import Flask
from dynaconf import FlaskDynaconf

app = Flask(__name__)
FlaskDynaconf(app=app)
print("NON_EXISTING" in app.config)
print(app.config["NON_EXISTING"])

will give you

False
None

1. Having the following folder structure

Project structure

.
├── 1_no_dynaconf.py
└── 2_dynaconf.py

0 directories, 2 files

2. Having the following config files:

Config files

No config files used at all

3. Having the following app code:

Code

1_no_dynaconf.py

from flask import Flask

app = Flask(__name__)
print("NON_EXISTING" in app.config)
print(app.config["NON_EXISTING"])

2_dynaconf.py

from flask import Flask
from dynaconf import FlaskDynaconf

app = Flask(__name__)
FlaskDynaconf(app=app)
print("NON_EXISTING" in app.config)
print(app.config["NON_EXISTING"])

4. Executing under the following environment

Execution

used Python 3.8.5, Flask 1.1.2, dynaconf 3.1.2

# other commands and details?
# virtualenv activation?

$ python 1_no_dynaconf.py
$ python 2_dynaconf.py

Expected behavior
Queriing non existing key via config[key] should raise KeyError, not returning None.

Environment (please complete the following information):

• OS: Linux/Kubuntu20.04
• Dynaconf Version 3.1.2
• Flask 1.1.2

Additional context
Add any other context about the problem here.

[rochacbruno]

Duplicate of #501 already merged in #502 and coming in the next release

[shsmad]

Duplicate of #501 already merged in #502 and coming in the next release

Not quite duplicate. #501 and #502 are about in syntax, which is __contains__ for. But my case is accessing variable[key], so DynaconfConfig.__getitem__ is used, which calls DynaconfConfig.get which calls flask.config.Config.get with default value set to None. But accessing real flask.config.Config via [] does not uses default value.

[rochacbruno]

@shsmad you are right, I just added #522 to fix this problem, the new release must come monday at morning. (I usually don't do releases on friday)