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.

Features

• Type-casting
• Read .env files into os.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

# 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 COORDINATES=23.3,50.0

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)

# 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]

Supported types

The following are all type-casting methods of Env:

• env.str
• env.bool
• env.int
• env.float
• env.decimal
• env.list (accepts optional subcast keyword argument)
• env.dict (accepts optional subcast keyword argument)
• env.json
• env.datetime
• env.date
• env.timedelta (assumes value is an integer in seconds)
• env.url
• env.uuid

Reading .env files

# myapp/.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

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)

Proxied variables

# export MAILGUN_LOGIN=sloria
# export SMTP_LOGIN={{MAILGUN_LOGIN}}

smtp_login = env("SMTP_LOGIN")  # =>'sloria'

Validation

# export TTL=-2
# export NODE_ENV='invalid'
# export EMAIL='^_^'


# simple validator
env.int("TTL", validate=lambda n: n > 0)
# => Environment variable "TTL" invalid: ['Invalid value.']

# using marshmallow validators
from marshmallow.validate import OneOf

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
from marshmallow.validate import Length, Email

env.str("EMAIL", validate=[Length(min=4), Email()])
# => Environment variable "EMAIL" invalid: ['Shorter than minimum length 4.', 'Not a valid email address.']

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("enum")
def enum_parser(value, choices):
    if value not in choices:
        raise environs.EnvError("Invalid!")
    return value


color = env.enum("COLOR", choices=["black"])  # => raises EnvError

Note: Environment variables parsed with a custom parser function will be serialized by Env.dump without any modification. To define special serialization behavior, use Env.parser_from_field instead (see next section).

Marshmallow integration

# export STATIC_PATH='app/static'

# Custom parsers can be defined as marshmallow Fields
import pathlib

import marshmallow as ma


class PathField(ma.fields.Field):
    def _deserialize(self, value, *args, **kwargs):
        return pathlib.Path(value)

    def _serialize(self, value, *args, **kwargs):
        return str(value)


env.add_parser_from_field("path", PathField)

static_path = env.path("STATIC_PATH")  # => PosixPath('app/static')
env.dump()["STATIC_PATH"]  # => 'app/static'

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"

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 and env.dj_email_url to parse the DATABASE_URL and EMAIL_URL environment variables, respectively.

# 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"]

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 proxied 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:

1. Make it easy to extend parsing behavior and develop plugins.
2. Leverage the deserialization and validation functionality provided by a separate library (marshmallow).
3. Clean up redundant API.

See this GitHub issue which details specific differences with envparse.

License

MIT licensed. See the LICENSE file for more details.