Package: python-decouple
Python Decouple: Strict separation of settings from codeDecouplehelps you to organize your settings so that you can
change parameters without having to redeploy your app.It also makes it easy for you to:store parameters ininior.envfiles;define comprehensive default values;properly convert values to the correct data type;haveonly oneconfiguration module to rule all your instances.It was originally designed for Django, but became an independent generic tool
for separating settings from code.SummaryPython Decouple: Strict separation of settings from codeWhy?Why not just use environment variables?UsageEncodingsWhere is the settings data stored?Ini fileEnv fileExample: How do I use it with Django?Attention withundefinedparametersOverriding config files with environment variablesHow does it work?Understanding the CAST argumentBuilt in Csv HelperBuilt in Choices helperContributeLicenseWhy?The settings files in web frameworks store many different kinds of parameters:Locale and i18n;Middlewares and Installed Apps;Resource handles to the database, Memcached, and other backing services;Credentials to external services such as Amazon S3 or Twitter;Per-deploy values such as the canonical hostname for the instance.The first 2 areproject settingsand the last 3 areinstance settings.You should be able to changeinstance settingswithout redeploying your app.Why not just use environment variables?Envvarsworks, but sinceos.environonly returns strings, it’s tricky.Let’s say you have anenvvarDEBUG=False. If you run:ifos.environ['DEBUG']:printTrueelse:printFalseIt will printTrue, becauseos.environ['DEBUG']returns thestring"False".
Since it’s a non-empty string, it will be evaluated as True.Decoupleprovides a solution that doesn’t look like a workaround:config('DEBUG',cast=bool).UsageInstall:pip install python-decoupleThen use it on yoursettings.py.Import theconfigobject:fromdecoupleimportconfigRetrieve the configuration parameters:SECRET_KEY=config('SECRET_KEY')DEBUG=config('DEBUG',default=False,cast=bool)EMAIL_HOST=config('EMAIL_HOST',default='localhost')EMAIL_PORT=config('EMAIL_PORT',default=25,cast=int)EncodingsDecouple’s default encoding isUTF-8.But you can specify your preferred encoding.Sinceconfigis lazy and only opens the configuration file when it’s first needed, you have the chance to change
its encoding right after import.fromdecoupleimportconfigconfig.encoding='cp1251'SECRET_KEY=config('SECRET_KEY')If you wish to fall back to your system’s default encoding use:importlocalefromdecoupleimportconfigconfig.encoding=locale.getpreferredencoding(False)SECRET_KEY=config('SECRET_KEY')Where is the settings data stored?Decouplesupports both.iniand.envfiles.Ini fileSimply create asettings.ininext to your configuration module in the form:[settings]DEBUG=TrueTEMPLATE_DEBUG=%(DEBUG)sSECRET_KEY=ARANDOMSECRETKEYDATABASE_URL=mysql://myuser:mypassword@myhost/mydatabasePERCENTILE=90%%#COMMENTED=42Note: SinceConfigParsersupportsstring interpolation, to represent the character%you need to escape it as%%.Env fileSimply create a.envtext file in your repository’s root directory in the form:DEBUG=True
TEMPLATE_DEBUG=True
SECRET_KEY=ARANDOMSECRETKEY
DATABASE_URL=mysql://myuser:mypassword@myhost/mydatabase
PERCENTILE=90%#COMMENTED=42Example: How do I use it with Django?Given that I have a.envfile in my repository’s root directory, here is a snippet of mysettings.py.I also recommend usingpathlibanddj-database-url.# coding: utf-8fromdecoupleimportconfigfromunipathimportPathfromdj_database_urlimportparseasdb_urlBASE_DIR=Path(__file__).parentDEBUG=config('DEBUG',default=False,cast=bool)TEMPLATE_DEBUG=DEBUGDATABASES={'default':config('DATABASE_URL',default='sqlite:///'+BASE_DIR.child('db.sqlite3'),cast=db_url)}TIME_ZONE='America/Sao_Paulo'USE_L10N=TrueUSE_TZ=TrueSECRET_KEY=config('SECRET_KEY')EMAIL_HOST=config('EMAIL_HOST',default='localhost')EMAIL_PORT=config('EMAIL_PORT',default=25,cast=int)EMAIL_HOST_PASSWORD=config('EMAIL_HOST_PASSWORD',default='')EMAIL_HOST_USER=config('EMAIL_HOST_USER',default='')EMAIL_USE_TLS=config('EMAIL_USE_TLS',default=False,cast=bool)# ...Attention withundefinedparametersIn the above example, all configuration parameters exceptSECRET_KEY =config('SECRET_KEY')have a default value in case it does not exist in the.envfile.IfSECRET_KEYis not present in the.env,decouplewill raise anUndefinedValueError.Thisfail fastpolicy helps you avoid chasing misbehaviours when you eventually forget a parameter.Overriding config files with environment variablesSometimes you may want to change a parameter value without having to edit the.inior.envfiles.Since version 3.0,decouplerespects theunix way.
Therefore environment variables have precedence over config files.To override a config parameter you can simply do:DEBUG=True python manage.pyHow does it work?Decouplealways searches forOptionsin this order:Environment variables;Repository: ini or .env file;Default argument passed to config.There are 4 classes doing the magic:ConfigCoordinates all the configuration retrieval.RepositoryIniCan read values fromos.environand ini files, in that order.Note:Since version 3.0decouplerespects unix precedence of environment variablesoverconfig files.RepositoryEnvCan read values fromos.environand.envfiles.Note:Since version 3.0decouplerespects unix precedence of environment variablesoverconfig files.AutoConfigThis is alazyConfigfactory that detects which configuration repository you’re using.It recursively searches up your configuration module path looking for asettings.inior a.envfile.Optionally, it acceptssearch_pathargument to explicitly define
where the search starts.Theconfigobject is an instance ofAutoConfigthat instantiates aConfigwith the properRepositoryon the first time it is used.Understanding the CAST argumentBy default, all values returned bydecouplearestrings, after all they are
read fromtext filesor theenvvars.However, your Python code may expect some other value type, for example:Django’sDEBUGexpects a booleanTrueorFalse.Django’sEMAIL_PORTexpects aninteger.Django’sALLOWED_HOSTSexpects alistof hostnames.Django’sSECURE_PROXY_SSL_HEADERexpects atuplewith two elements, the name of the header to look for and the required value.To meet this need, theconfigfunction accepts acastargument which
receives anycallable, that will be used totransformthe string value
into something else.Let’s see some examples for the above mentioned cases:>>>os.environ['DEBUG']='False'>>>config('DEBUG',cast=bool)False>>>os.environ['EMAIL_PORT']='42'>>>config('EMAIL_PORT',cast=int)42>>>os.environ['ALLOWED_HOSTS']='.localhost, .herokuapp.com'>>>config('ALLOWED_HOSTS',cast=lambdav:[s.strip()forsinv.split(',')])['.localhost','.herokuapp.com']>>>os.environ['SECURE_PROXY_SSL_HEADER']='HTTP_X_FORWARDED_PROTO, https'>>>config('SECURE_PROXY_SSL_HEADER',cast=Csv(post_process=tuple))('HTTP_X_FORWARDED_PROTO','https')As you can see,castis very flexible. But the last example got a bit complex.Built in Csv HelperTo address the complexity of the last example,Decouplecomes with an extensibleCsv helper.Let’s improve the last example:>>>fromdecoupleimportCsv>>>os.environ['ALLOWED_HOSTS']='.localhost, .herokuapp.com'>>>config('ALLOWED_HOSTS',cast=Csv())['.localhost','.herokuapp.com']You can also have adefaultvalue that must be a string to be processed byCsv.>>>fromdecoupleimportCsv>>>config('ALLOWED_HOSTS',default='127.0.0.1',cast=Csv())['127.0.0.1']You can also parametrize theCsv Helperto return other types of data.>>>os.environ['LIST_OF_INTEGERS']='1,2,3,4,5'>>>config('LIST_OF_INTEGERS',cast=Csv(int))[1,2,3,4,5]>>>os.environ['COMPLEX_STRING']='%virtual_env%\t*important stuff*\ttrailing spaces   '>>>csv=Csv(cast=lambdas:s.upper(),delimiter='\t',strip=' %*')>>>csv(os.environ['COMPLEX_STRING'])['VIRTUAL_ENV','IMPORTANT STUFF','TRAILING SPACES']By defaultCsvreturns alist, but you can get atupleor whatever you want using thepost_processargument:>>>os.environ['SECURE_PROXY_SSL_HEADER']='HTTP_X_FORWARDED_PROTO, https'>>>config('SECURE_PROXY_SSL_HEADER',cast=Csv(post_process=tuple))('HTTP_X_FORWARDED_PROTO','https')Built in Choices helperAllows for cast and validation based on a list of choices. For example:>>>fromdecoupleimportconfig,Choices>>>os.environ['CONNECTION_TYPE']='usb'>>>config('CONNECTION_TYPE',cast=Choices(['eth','usb','bluetooth']))'usb'>>>os.environ['CONNECTION_TYPE']='serial'>>>config('CONNECTION_TYPE',cast=Choices(['eth','usb','bluetooth']))Traceback(mostrecentcalllast):...ValueError:Valuenotinlist:'serial';validvaluesare['eth','usb','bluetooth']You can also parametrizeChoices helperto cast to another type:>>>os.environ['SOME_NUMBER']='42'>>>config('SOME_NUMBER',cast=Choices([7,14,42],cast=int))42You can also use a Django-like choices tuple:>>>USB='usb'>>>ETH='eth'>>>BLUETOOTH='bluetooth'>>>>>>CONNECTION_OPTIONS=(...(USB,'USB'),...(ETH,'Ethernet'),...(BLUETOOTH,'Bluetooth'),)...>>>os.environ['CONNECTION_TYPE']=BLUETOOTH>>>config('CONNECTION_TYPE',cast=Choices(choices=CONNECTION_OPTIONS))'bluetooth'ContributeYour contribution is welcome.Setup your development environment:git clone git@github.com:henriquebastos/python-decouple.git
cd python-decouple
python -m venv .venv
source .venv/bin/activate
pip install -r requirements.txt
toxDecouplesupports both Python 2.7 and 3.6. Make sure you have both installed.I usepyenvto
manage multiple Python versions and I described my workspace setup on this article:The definitive guide to setup my Python workspaceYou can submit pull requests and issues for discussion. However I only
consider merging tested code.LicenseThe MIT License (MIT)Copyright (c) 2017 Henrique Bastos <henrique at bastos dot net>Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the “Software”), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.