environs: simplified environment variable parsing
environs is a Python library for parsing environment variables. It allows you to store configuration separate from your code, as per The Twelve-Factor App methodology.
Contents
- Features
- Install
- Basic usage
- Supported types
- Reading .env files
- Handling prefixes
- Variable expansion
- Validation
- Deferred validation
- Serialization
- Defining custom parser behavior
- Usage with Flask
- Usage with Django
- Why...?
- License
Features
- Type-casting
- Read
.envfiles intoos.environ(useful for local development) - Validation
- Define custom parser behavior
- Framework-agnostic, but integrates well with Flask and Django
Install
pip install environs
Basic usage
With some environment variables set...
export GITHUB_USER=sloria
export MAX_CONNECTIONS=100
export SHIP_DATE='1984-06-25'
export TTL=42
export ENABLE_LOGIN=true
export GITHUB_REPOS=webargs,konch,ped
export GITHUB_REPO_PRIORITY="webargs=2,konch=3"
export COORDINATES=23.3,50.0
export LOG_LEVEL=DEBUG
Parse them with environs...
from environs import Env
env = Env()
env.read_env() # read .env file, if it exists
# required variables
gh_user = env("GITHUB_USER") # => 'sloria'
secret = env("SECRET") # => raises error if not set
# casting
max_connections = env.int("MAX_CONNECTIONS") # => 100
ship_date = env.date("SHIP_DATE") # => datetime.date(1984, 6, 25)
ttl = env.timedelta("TTL") # => datetime.timedelta(0, 42)
log_level = env.log_level("LOG_LEVEL") # => logging.DEBUG
# providing a default value
enable_login = env.bool("ENABLE_LOGIN", False) # => True
enable_feature_x = env.bool("ENABLE_FEATURE_X", False) # => False
# parsing lists
gh_repos = env.list("GITHUB_REPOS") # => ['webargs', 'konch', 'ped']
coords = env.list("COORDINATES", subcast=float) # => [23.3, 50.0]
# parsing dicts
gh_repos_priorities = env.dict(
"GITHUB_REPO_PRIORITY", subcast_values=int
) # => {'webargs': 2, 'konch': 3}
Supported types
The following are all type-casting methods of Env:
env.strenv.boolenv.intenv.floatenv.decimalenv.list(accepts optionalsubcastanddelimiterkeyword arguments)env.dict(accepts optionalsubcast_keysandsubcast_valueskeyword arguments)env.jsonenv.datetimeenv.dateenv.timeenv.timedelta(assumes value is an integer in seconds)env.urlenv.uuidenv.log_levelenv.path(casts to apathlib.Path)env.enum(casts to any given enum type specified intypekeyword argument, accepts optionalignore_casekeyword argument)
Reading .env files
# .env
DEBUG=true
PORT=4567
Call Env.read_env before parsing variables.
from environs import Env
env = Env()
# Read .env into os.environ
env.read_env()
env.bool("DEBUG") # => True
env.int("PORT") # => 4567
Reading a specific file
By default, Env.read_env will look for a .env file in current directory and (if no .env exists in the CWD) recurse upwards until a .env file is found.
You can also read a specific file:
from environs import Env
with open(".env.test", "w") as fobj:
fobj.write("A=foo\n")
fobj.write("B=123\n")
env = Env()
env.read_env(".env.test", recurse=False)
assert env("A") == "foo"
assert env.int("B") == 123
Handling prefixes
# export MYAPP_HOST=lolcathost
# export MYAPP_PORT=3000
with env.prefixed("MYAPP_"):
host = env("HOST", "localhost") # => 'lolcathost'
port = env.int("PORT", 5000) # => 3000
# nested prefixes are also supported:
# export MYAPP_DB_HOST=lolcathost
# export MYAPP_DB_PORT=10101
with env.prefixed("MYAPP_"):
with env.prefixed("DB_"):
db_host = env("HOST", "lolcathost")
db_port = env.int("PORT", 10101)
Variable expansion
# export CONNECTION_URL=https://${USER:-sloria}:${PASSWORD}@${HOST:-localhost}/
# export PASSWORD=secret
# export YEAR=${CURRENT_YEAR:-2020}
from environs import Env
env = Env(expand_vars=True)
connection_url = env("CONNECTION_URL") # =>'https://sloria:secret@localhost'
year = env.int("YEAR") # =>2020
Validation
# export TTL=-2
# export NODE_ENV='invalid'
# export EMAIL='^_^'
from environs import Env
from marshmallow.validate import OneOf, Length, Email
env = Env()
# simple validator
env.int("TTL", validate=lambda n: n > 0)
# => Environment variable "TTL" invalid: ['Invalid value.']
# using marshmallow validators
env.str(
"NODE_ENV",
validate=OneOf(
["production", "development"], error="NODE_ENV must be one of: {choices}"
),
)
# => Environment variable "NODE_ENV" invalid: ['NODE_ENV must be one of: production, development']
# multiple validators
env.str("EMAIL", validate=[Length(min=4), Email()])
# => Environment variable "EMAIL" invalid: ['Shorter than minimum length 4.', 'Not a valid email address.']
Deferred validation
By default, a validation error is raised immediately upon calling a parser method for an invalid environment variable. To defer validation and raise an exception with the combined error messages for all invalid variables, pass eager=False to Env. Call env.seal() after all variables have been parsed.
# export TTL=-2
# export NODE_ENV='invalid'
# export EMAIL='^_^'
from environs import Env
from marshmallow.validate import OneOf, Email, Length, Range
env = Env(eager=False)
TTL = env.int("TTL", validate=Range(min=0, max=100))
NODE_ENV = env.str(
"NODE_ENV",
validate=OneOf(
["production", "development"], error="NODE_ENV must be one of: {choices}"
),
)
EMAIL = env.str("EMAIL", validate=[Length(min=4), Email()])
env.seal()
# environs.EnvValidationError: Environment variables invalid: {'TTL': ['Must be greater than or equal to 0 and less than or equal to 100.'], 'NODE_ENV': ['NODE_ENV must be one of: production, development'], 'EMAIL': ['Shorter than minimum length 4.', 'Not a valid email address.']}
env.seal() validates all parsed variables and prevents further parsing (calling a parser method will raise an error).
Serialization
# serialize to a dictionary of simple types (numbers and strings)
env.dump()
# {'COORDINATES': [23.3, 50.0],
# 'ENABLE_FEATURE_X': False,
# 'ENABLE_LOGIN': True,
# 'GITHUB_REPOS': ['webargs', 'konch', 'ped'],
# 'GITHUB_USER': 'sloria',
# 'MAX_CONNECTIONS': 100,
# 'MYAPP_HOST': 'lolcathost',
# 'MYAPP_PORT': 3000,
# 'SHIP_DATE': '1984-06-25',
# 'TTL': 42}
Defining custom parser behavior
# export DOMAIN='http://myapp.com'
# export COLOR=invalid
from furl import furl
# Register a new parser method for paths
@env.parser_for("furl")
def furl_parser(value):
return furl(value)
domain = env.furl("DOMAIN") # => furl('https://myapp.com')
# Custom parsers can take extra keyword arguments
@env.parser_for("choice")
def choice_parser(value, choices):
if value not in choices:
raise environs.EnvError("Invalid!")
return value
color = env.choice("COLOR", choices=["black"]) # => raises EnvError
Usage with Flask
# myapp/settings.py
from environs import Env
env = Env()
env.read_env()
# Override in .env for local development
DEBUG = env.bool("FLASK_DEBUG", default=False)
# SECRET_KEY is required
SECRET_KEY = env.str("SECRET_KEY")
Load the configuration after you initialize your app.
# myapp/app.py
from flask import Flask
app = Flask(__name__)
app.config.from_object("myapp.settings")
For local development, use a .env file to override the default configuration.
# .env
DEBUG=true
SECRET_KEY="not so secret"
Note: Because environs depends on python-dotenv, the flask CLI will automatically read .env and .flaskenv files.
Usage with Django
environs includes a number of helpers for parsing connection URLs. To install environs with django support:
pip install environs[django]
Use env.dj_db_url, env.dj_cache_url and env.dj_email_url to parse the DATABASE_URL, CACHE_URL and EMAIL_URL environment variables, respectively.
For more details on URL patterns, see the following projects that environs is using for converting URLs.
Basic example:
# myproject/settings.py
from environs import Env
env = Env()
env.read_env()
# Override in .env for local development
DEBUG = env.bool("DEBUG", default=False)
# SECRET_KEY is required
SECRET_KEY = env.str("SECRET_KEY")
# Parse database URLs, e.g. "postgres://localhost:5432/mydb"
DATABASES = {"default": env.dj_db_url("DATABASE_URL")}
# Parse email URLs, e.g. "smtp://"
email = env.dj_email_url("EMAIL_URL", default="smtp://")
EMAIL_HOST = email["EMAIL_HOST"]
EMAIL_PORT = email["EMAIL_PORT"]
EMAIL_HOST_PASSWORD = email["EMAIL_HOST_PASSWORD"]
EMAIL_HOST_USER = email["EMAIL_HOST_USER"]
EMAIL_USE_TLS = email["EMAIL_USE_TLS"]
# Parse cache URLS, e.g "redis://localhost:6379/0"
CACHES = {"default": env.dj_cache_url("CACHE_URL")}
For local development, use a .env file to override the default configuration.
# .env
DEBUG=true
SECRET_KEY="not so secret"
For a more complete example, see django_example.py in the examples/ directory.
Why...?
Why envvars?
See The 12-factor App section on configuration.
Why not os.environ?
While os.environ is enough for simple use cases, a typical application will need a way to manipulate and validate raw environment variables. environs abstracts common tasks for handling environment variables.
environs will help you
- cast envvars to the correct type
- specify required envvars
- define default values
- validate envvars
- parse list and dict values
- parse dates, datetimes, and timedeltas
- parse expanded variables
- serialize your configuration to JSON, YAML, etc.
Why another library?
There are many great Python libraries for parsing environment variables. In fact, most of the credit for environs' public API goes to the authors of envparse and django-environ.
environs aims to meet three additional goals:
- Make it easy to extend parsing behavior and develop plugins.
- Leverage the deserialization and validation functionality provided by a separate library (marshmallow).
- Clean up redundant API.
See this GitHub issue which details specific differences with envparse.
License
MIT licensed. See the LICENSE file for more details.
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