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
  • Reading a specific file
• Handling prefixes
• Variable expansion
• Validation
• Deferred validation
• URL schemes
• Serialization
• Defining custom parser behavior
• Usage with Flask
• Usage with Django
• Why...?
  • Why envvars?
  • Why not os.environ?
  • Why another library?
• License

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

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.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(seconds=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.str
• env.bool
• env.int
• env.float
• env.decimal
• env.list (accepts optional subcast and delimiter keyword arguments)
• env.dict (accepts optional subcast_keys, subcast_values and delimiter keyword arguments)
• env.json
• env.datetime
• env.date
• env.time
• env.timedelta (assumes value is an integer in seconds, or an ordered duration string like 7h7s or 7w 7d 7h 7m 7s 7ms 7us)
• env.url
  • This returns a urllib.parse.ParseResult and therefore expects a ParseResult for its default.

from urllib.parse import urlparse

from environs import env

MY_API_URL = env.url(
    "MY_API_URL",
    default=urlparse("http://api.example.com"),
)

If you want the return value to be a string, use env.str with validate.URL instead.

from environs import env, validate

MY_API_URL = env.str(
    "MY_API_URL",
    default="http://api.example.com",
    validate=validate.URL(),
)

• env.uuid
• env.log_level
• env.path (casts to a pathlib.Path)
• env.enum (casts to any given enum type specified in enum keyword argument)
  • Pass by_value=True to parse and validate by the Enum's values.

Reading .env files

# .env
DEBUG=true
PORT=4567

Call Env.read_env before parsing variables.

from environs import 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.read_env(".env.test", recurse=False)

assert env("A") == "foo"
assert env.int("B") == 123

Handling prefixes

Pass prefix to the constructor if all your environment variables have the same prefix.

from environs import Env

# export MYAPP_HOST=lolcathost
# export MYAPP_PORT=3000


env = Env(prefix="MYAPP_")

host = env("HOST", "localhost")  # => 'lolcathost'
port = env.int("PORT", 5000)  # => 3000

Alternatively, you can use the prefixed context manager.

from environs import env

# 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, validate, ValidationError


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


# custom validator
def validator(n):
    if n <= 0:
        raise ValidationError("Invalid value.")


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

environs.validate is equivalent to marshmallow.validate, so you can use any of the validators provided by that module.

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).

URL schemes

env.url() supports non-standard URL schemes via the schemes argument.

from urllib.parse import urlparse

REDIS_URL = env.url(
    "REDIS_URL", urlparse("redis://redis:6379"), schemes=["redis"], require_tld=False
)

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.

• dj-database-url
• django-cache-url
• dj-email-url

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:

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.