pydantic/docs/usage/settings.md

One of pydantic's most useful applications is to define default settings, and allow them to be overridden by environment variables or keyword arguments (e.g. in unit tests).

{!./examples/settings.py!}

(This script is complete, it should run "as is")

The following rules apply when finding and interpreting environment variables:

• When no custom environment variable name(s) are given, the environment variable name is built using the field name and prefix, eg to override special_function use export my_prefix_special_function='foo.bar', the default prefix is an empty string. aliases are ignored for building the environment variable name.
• Custom environment variable names can be set using with Config.fields.[field name].env or Field(..., env=...), in the above example auth_key and api_key's environment variable setups are the equivalent.
• In these cases env can either be a string or a list of strings. When a list of strings order is important: in the case of redis_dsn service_redis_dsn would take precedence over redis_url.

!!! warning Since v1.0 pydantic does not consider field aliases when finding environment variables to populate settings models, use env instead as described above.

To aid the transition from aliases to `env`, a warning will be raised when aliases are used on settings models
without a custom env var name. If you really mean to use aliases, either ignore the warning or set `env` to
suppress it.

By default BaseSettings considers field values in the following priority (where 3. has the highest priority and overrides the other two):

1. The default values set in your Settings class.
2. Environment variables, e.g. my_prefix_special_function as described above.
3. Arguments passed to the Settings class on initialisation.

Complex types like list, set, dict and sub-models can be set by using JSON environment variables.

Case-sensitivity can be turned on through Config:

{!./examples/settings_case_sensitive.py!}

When case_sensitive is True, the environment variable must be in all-caps, so in this example redis_host could only be modified via export REDIS_HOST.

!!! note On Windows, python's os module always treats environment variables as case-insensitive, so the case_sensitive config setting will have no effect -- settings will always be updated ignoring case.