User Manual

Fulll Documentation
https://config-resolver.readthedocs.org/en/latest/
Repository
https://github.com/exhuma/config_resolver
PyPI
https://pypi.python.org/pypi/config_resolver

Rationale

Many of the larger frameworks (not only web frameworks) offer their own configuration management. But it looks different everywhere. Both in code and in usage later on. Additionally, the operating system usually has some default, predictable place to look for configuration values. On Linux, this is /etc and the XDG Base Dir Spec.

The code for finding these config files is always the same. But finding config files can be more interesting than that:

  • If config files contain passwords, the application should issue appropriate warnings if it encounters an insecure file and refuse to load it.
  • The expected structure in the config file can be versioned (think: schema). If an application is upgraded and expects new values to exist in an old version file, it should notify the user.
  • It should be possible to override the configuration per installed instance, even per execution.

config_resolver tackles all these challenges in a simple-to-use drop-in module. The module uses no additional external modules (no additional dependencies, pure Python) so it can be used in any application without adding unnecessary bloat.

One last thing that config_resolver provides, is a better handling of default values than instances of SafeConfigParser of the standard library. The stdlib config parser can only specify defaults for options without associating them to a section! This means that you cannot have two options with the same name in multiple sections with different default values. config_resolver handles default values at the time you call .get(), which makes it independent of the section.

Description / Usage

The module provides two main classes:

  • Config: This is the default class.
  • SecuredConfig: This is a subclass of Config which refuses to load files which a readable by other people than the owner.

The simple usage for both is identical. The only difference is the above mentioned decision to load files or not:

from config_resolver imoprt Config
cfg = Config('acmecorp', 'bird_feeder')

This will look for config files in (in that order):

  • /etc/acmecorp/bird_feeder/app.ini
  • /etc/xdg/acmecorp/bird_feeder/app.ini
  • ~/.acmecorp/bird_feeder/app.ini – This will be deprecated (no longer loaded) in config_resolver 5.0
  • ~/.config/acmecorp/bird_feeder/app.ini
  • ./.acmecorp/bird_feeder/app.ini

If all files exist, one which is loaded later, will override the values of an earlier file. No values will be removed, this means you can put system-wide defaults in /etc and specialise/override from there.

The Freedesktop XDG standard

freedesktop.org standardises the location of configuration files in the XDG specification Since version 4.1.0, config_resolver reads these paths as well, and honors the defined environment variables. To ensure backwards compatibility, those paths have only been added to the resolution order. They have a higher precedence than the old locations though. So the following applies:

XDG item overrides
/etc/xdg/<group>/<app> /etc/<group>/<app>
~/.config/<group>/</app> ~/.<group>/<app>
$XDG_DATA_HOME $GROUP_APP_PATH
$XDG_CONFIG_DIRS $GROUP_APP_PATH

Tip

If a config file is found at ~/.<group>/<app>, a log message with a warning is issued since config_resolver 4.1.0 encouraging the end-user to move the config file to ~/.config/<group>/<app>.

Files are parsed using the default Python configparser.ConfigParser (i.e. ini files).

Advanced Usage

Versioning

It is pretty much always useful to keep track of the expected “schema” of a config file. If in a later version of your application, you decide to change a configuration value’s name, remove a variable, or require a new one the end-user needs to be notified.

For this use-case, you can create versioned config_resolver.Config instances in your application:

cfg = Config('group', 'app', version='2.1')

Config file example:

[meta]
version=2.1

[database]
dsn=foobar

If you don’t specify a version number in the construcor, an unversioned file is assumed.

Only “major” and “minor” numbers are supported. If the application encounters a file with a different “major” value, it will raise a config_resolver.IncompatibleVersion exception. Differences in minor numbers are only logged with a “warning” level.

Rule of thumb: If your application accepts a new config value, but can function just fine with default values, increment the minor number. If on the other hand, something has changed, and the user needs to change the config file, increment the major number.

Requiring files (bail out if no config is found)

Since version 3.3.0, you have a bit more control about how files are loaded. The config_resolver.Config class takes a new argument: require_load. If this is set to True, an OSError is raised if no config file was loaded. Alternatively, and, purely a matter of taste, you can leave this on it’s default False value and inspect the loaded_files attribute on the config_resolver.Config instance. If it’s empty, nothing has been loaded.

Overriding internal defaults

Both the search path and the basename of the file (app.ini) can be overridden by the application developer via the API and by the end-user via environment variables.

By the application developer

Apart from the “group name” and “application name”, the config_resolver.Config class accepts search_path and filename as arguments. search_path controls to what folders are searched for config files, filename controls the basename of the config file. filename is especially useful if you want to separate different concepts into different files:

app_cfg = Config('acmecorp', 'bird_feeder')
db_cfg = Config('acmecorp', 'bird_feeder', filename='db.ini')

By the end-user

The end-user has access to two environment variables:

  • <GROUP_NAME>_<APP_NAME>_PATH overrides the default search path.
  • XDG_CONFIG_HOME overrides the path considered as “home” locations for config files (default=``~/.config``)
  • XDG_CONFIG_DIRS overrides additional path elements as recommended by the freedesktop.org XDG basedir spec. Paths are separated by : and are sorted with descending precedence (leftmost is the most important one).
  • <GROUP_NAME>_<APP_NAME>_FILENAME overrides the default basename of the config file (default=``app.ini``).

Logging

All operations are logged using the default logging package with a logger with the name config_resolver. All operational logs (opening/reading file) are logged with the INFO level. The log messages include the absolute names of the loaded files. If a file is not loadable, a WARNING message is emitted. It also contains a couple of DEBUG messages. If you want to see those messages on-screen you could do the following:

import logging
from config_resolver import Config
logging.basicConfig(level=logging.DEBUG)
conf = Config('mycompany', 'myapplication')

If you want to use the INFO level in your application, but silence only the config_resolver logs, add the following to your code:

logging.getLogger('config_resolver').setLevel(logging.WARNING)

As of version 4.2.0, all log messages are prefixed with the group and application name. This helps identifying log messages if multiple packages in your application use config_resolver. The prefix filter can be accessed via the instance member _prefix_filter if you want to change or remove it:

from config_resolver import Config
conf = Config('mycompany', 'myapplication')
print conf._prefix_filter

More detailed information about logging is out of the scope of this document. Consider reading the logging tutorial of the official Python docs.

Environment Variables

The resolver can also be manipulated using environment variables to allow different values for different running instances. The variable names are all upper-case and are prefixed with both group- and application-name.

<group_name>_<app_name>_PATH

The search path for config files. You can specify multiple paths by separating it by the system’s path separator default (: on Linux).

If the path is prefixed with +, then the path elements are appended to the default search path.

<group_name>_<app_name>_FILENAME
The file name of the config file. Note that this should not be given with leading path elements. It should simply be a file basename (f.ex.: my_config.ini)
XDG_CONFIG_HOME and XDG_CONFIG_DIRS
See the XDG specification

Difference to ConfigParser

There is one major difference to the default Python ConfigParser: the get() method accepts a “default” parameter. If specified, that value is returned in case ConfigParser does not return a value. Remember that the ConfigParser instance supports defaults as well if specified in the constructor.

Using the default parameter on get(), you can now have two options with the same name in two sections with different values. Imagine the following:

[database1]
dsn=sqlite:///tmp/db.sqlite3

[database2]
dsn=sqlite:///tmp/db2.sqlite3

In the core ConfigParser you could not specify two different default values! The default parameter makes this possible.

Note

AGAIN: The core ConfigParser default mechanism still takes precedence!

Debugging

Creating an instance of Config will not raise an error (except if explicitly asked to do so). Instead it will always return a valid, (but possibly empty) instance. So errors can be hard to see sometimes.

The idea behind this, is to encourage you to have sensible default values, so that the application can run, even without configuration. For “development-time” exceptions, consider calling get() without a default value.

Your first stop should be to configure logging and look at the emitted messages.

In order to determine whether any config file was loaded, you can look into the loaded_files instance variable. It contains a list of all the loaded files, in the order of loading. If that list is empty, no config has been found. Also remember that the order is important. Later elements will override values from earlier elements.

Additionally, another instance variable named active_path represents the search path after processing of environment variables and runtime parameters. This may also be useful to display informtation to the end-user.

Examples

A simple config instance (with logging):

import logging
from config_resolver import Config

logging.basicConfig(level=logging.DEBUG)
cfg = Config("acmecorp", "bird_feeder")
print cfg.get('section', 'var')

An instance which will not load unsecured files:

import logging
from config_resolver import SecuredConfig

logging.basicConfig(level=logging.DEBUG)
cfg = SecuredConfig("acmecorp", "bird_feeder")
print cfg.get('section', 'var')

Loading a versioned config file:

import logging
from config_resolver import Config

logging.basicConfig(level=logging.DEBUG)
cfg = Config("acmecorp", "bird_feeder", version="1.0")
print cfg.get('section', 'var')

Default values:

import logging
from config_resolver import Config

logging.basicConfig(level=logging.DEBUG)
cfg = Config("acmecorp", "bird_feeder", version="1.0")

# This will not raise an error (but emit a DEBUG log entry).
print cfg.get('section', 'example_non_existing_option_name', default=10)

# this may raise a "NoOptionError"
print cfg.get('section', 'example_non_existing_option_name')

# this may raise a "NoSectionError"
print cfg.get('example_non_existing_section_name', 'varname')

Indices and tables