Configuration system (astropy.config)

Introduction

The astropy configuration system is designed to give users control of various parameters used in astropy or affiliated packages without delving into the source code to make those changes.

Note

The configuration system got a major overhaul in astropy 0.4 as part of APE3. See Configuration transition for information about updating code to use the new API.

Getting Started

The Astropy configuration options are most easily set by modifying the configuration file. It will be automatically generated with all the default values commented out the first time you import Astropy. You can find the exact location by doing:

>>> from astropy.config import get_config_dir
>>> get_config_dir()

And you should see the location of your configuration directory. The standard scheme generally puts your configuration directory in $HOME/.astropy/config, but if you’ve set the environment variable XDG_CONFIG_HOME and the $XDG_CONFIG_HOME/astropy directory exists, it will instead be there.

Note

This is a slight variation from the behavior of most Linux applications with respect to $XDG_CONFIG_HOME, where the default, if $XDG_CONFIG_HOME is not defined, would be to put configuration in ~/.config/astropy.

Once you’ve found the configuration file, open it with your favorite editor. It should have all of the sections you might want, with descriptions and the type of the value that is accepted. Feel free to edit this as you wish, and any of these changes will be reflected when you next start Astropy. Or, if you want to see your changes immediately in your current Astropy session, just do:

>>> from astropy.config import reload_config
>>> reload_config()

Note

If for whatever reason your $HOME/.astropy directory is not accessible (i.e., you have astropy running somehow as root but you are not the root user), the best solution is to set the XDG_CONFIG_HOME and XDG_CACHE_HOME environment variables pointing to directories, and create an astropy directory inside each of those. Both the configuration and data download systems will then use those directories and never try to access the $HOME/.astropy directory.

Using astropy.config

Accessing Values

By convention, configuration parameters live inside of objects called conf at the root of each subpackage. For example, configuration parameters related to data files live in astropy.utils.data.conf. This object has properties for getting and setting individual configuration parameters. For instance to get the default URL for astropy remote data do:

>>> from astropy.utils.data import conf
>>> conf.dataurl
'http://data.astropy.org/'

Changing Values at Run-time

Changing configuration values persistently is done by editing the configuration file as described above. Values can also, however, be modified in an active python session by setting any of the properties on a conf object.

For example, if there is a part of your configuration file that looks like:

[utils.data]

# URL for astropy remote data site.
dataurl = http://data.astropy.org/

# Time to wait for remote data query (in seconds).
remote_timeout = 3.0

You should be able to modify the values at run-time this way:

>>> from astropy.utils.data import conf
>>> conf.dataurl
'http://data.astropy.org/'
>>> conf.dataurl = 'http://astropydata.mywebsite.com'
>>> conf.dataurl
'http://astropydata.mywebsite.com'
>>> conf.remote_timeout
3.0
>>> conf.remote_timeout = 4.5
>>> conf.remote_timeout
4.5

Reloading Configuration

Instead of modifying the variables in python, you can also modify the configuration files and then reload them. For example, if you modify the configuration file to say:

[utils.data]

# URL for astropy remote data site.
dataurl = http://myotherdata.mywebsite.com/

# Time to wait for remote data query (in seconds).
remote_timeout = 6.3

And then run the following commands:

>>> conf.reload('dataurl')
>>> conf.reload('remote_timeout')

This should update the variables with the values from the configuration file:

>>> conf.dataurl
'http://myotherdata.mywebsite.com/'
>>> conf.remote_timeout
6.3

You can reload all configuration parameters of a conf object at once by calling reload with no parameters:

>>> conf.reload()

Or if you want to reload all astropy configuration at once, use the reload_config function:

>>> config.reload_config('astropy')

You can also reset a configuration parameter back to its default value. Note that this is the default value defined in the Python code, and has nothing to do with the configuration file on disk:

>>> conf.reset('dataurl')
>>> conf.dataurl
'http://data.astropy.org/'

Upgrading astropy

Each time you upgrade to a new major version of astropy, the configuration parameters may have changed.

If you never edited your configuration file, there is nothing for you to do. It will automatically be replaced with a configuration file template for the newly installed version of astropy.

If you did customize your configuration file, it will not be touched. Instead, a new configuration file template will be installed alongside it with the version number in the filename, for example astropy.0.4.cfg. You can compare this file to your astropy.cfg file to see what needs to be changed or updated.

Adding new configuration items

Configuration items should be used wherever an option or setting is needed that is either tied to a system configuration or should persist across sessions of astropy or an affiliated package. Options that may affect the results of science calculations should not be configuration items, but should instead be astropy.utils.state.ScienceState, so it’s possible to reproduce science results without them being affected by configuration parameters set in a particular environment. Admittedly, this is only a guideline, as the precise cases where a configuration item is preferred over, say, a keyword option for a function is somewhat personal preference. It is the preferred form of persistent configuration, however, and astropy packages must all use it (and it is recommended for affiliated packages).

The reference guide below describes the interface for creating a conf object with a number of configuration parameters. They should be defined at the top level, i.e. in the __init__.py of each subpackage that has configuration items:

""" This is the docstring at the beginning of a module
"""
from astropy import config as _config

class Conf(_config.ConfigNamespace):
    """
    Configuration parameters for my subpackage.
    """
    some_setting = _config.ConfigItem(
        1, 'Description of some_setting')
    another_setting = _config.ConfigItem(
        'string value', 'Description of another_setting')
# Create an instance for the user
conf = Conf()

... implementation ...
def some_func():
    #to get the value of these options, I might do:
    something = conf.some_setting + 2
    return conf.another_setting + ' Also, I added text.'

The configuration items also need to be added to the config file template. For astropy, this file is in astropy/astropy.cfg. For an affiliated package called, for example, packagename, the file is in packagename/packagename.cfg. For the example above, the following content would be added to the config file template:

[subpackage]
## Description of some_setting
# some_setting = 1

## Description of another_setting
# another_setting = foo

Note that the key/value pairs are commented out. This will allow for changing the default values in a future version of astropy without requiring the user to edit their configuration file to take advantage of the new defaults. By convention, the descriptions of each parameter are in comment lines starting with two hash characters (##) to distinguish them from commented out key/value pairs.

Item Types and Validation

If not otherwise specified, a ConfigItem gets its type from the type of the defaultvalue it is given when it is created. The item can only be set to be an object of this type. Hence:

some_setting = ConfigItem(1, 'A description.')
...
conf.some_setting = 1.2

will fail, because 1.2 is a float and 1 is an int.

Note that if you want the configuration item to be limited to a particular set of options, you should pass in a list as the defaultvalue option. The first entry in the list will be taken as the default, and the list as a whole gives all the valid options. For example:

an_option = ConfigItem(
    ['a', 'b', 'c'],
    "This option can be 'a', 'b', or 'c'")
...
conf.an_option = 'b'  # succeeds
conf.an_option = 'c'  # succeeds
conf.an_option = 'd'  # fails!
conf.an_option = 6    # fails!

Finally, a ConfigItem can be explicitly given a type via the cfgtype option:

an_int_setting = ConfigItem(
    1, 'A description.', cfgtype='integer')
...
conf.an_int_setting = 3     # works fine
conf.an_int_setting = 4.2   # fails!

If the default value’s type doesn’t match cfgtype, the ConfigItem cannot be created:

an_int_setting = ConfigItem(
    4.2, 'A description.', cfgtype='integer')

In summary, the default behavior (of automatically determining cfgtype) is usually what you want. The main exception is when you want your configuration item to be a list. The default behavior will treat that as a list of options unless you explicitly tell it that the ConfigItem itself is supposed to be a list:

a_list_setting = ConfigItem([1, 2, 3], 'A description.')

a_list_setting = ConfigItem([1, 2, 3], 'A description.', cfgtype='list')

Details of all the valid cfgtype items can be found in the validation section of the configobj manual. Below is a list of the valid values here for quick reference:

  • ‘integer’
  • ‘float’
  • ‘boolean’
  • ‘string’
  • ‘ip_addr’
  • ‘list’
  • ‘tuple’
  • ‘int_list’
  • ‘float_list’
  • ‘bool_list’
  • ‘string_list’
  • ‘ip_addr_list’
  • ‘mixed_list’
  • ‘option’
  • ‘pass’

Usage Tips

Keep in mind is that ConfigItem objects can be changed at runtime by users. So it is always recommended to read their values immediately before use instead of just storing their initial value to some other variable (or used as a default for a function). For example, the following will work, but is incorrect usage:

def some_func(val=conf.some_setting):
    return val + 2

This works fine as long as the user doesn’t change its value during runtime, but if they do, the function won’t know about the change:

>>> some_func()
3
>>> conf.some_setting = 3
>>> some_func()  # naively should return 5, because 3 + 2 = 5
3

There are two ways around this. The typical/intended way is:

def some_func():
    """
    The `SOME_SETTING` configuration item influences this output
    """
    return conf.some_setting + 2

Or, if the option needs to be available as a function parameter:

def some_func(val=None):
    """
    If not specified, `val` is set by the `SOME_SETTING` configuration item.
    """
    return (conf.some_setting if val is None else val) + 2

Reference/API

astropy.config Package

This module contains configuration and setup utilities for the Astropy project. This includes all functionality related to the affiliated package index.

Functions

get_cache_dir() Determines the Astropy cache directory name and creates the directory if it doesn’t exist.
get_config([packageormod, reload]) Gets the configuration object or section associated with a particular package or module.
get_config_dir([create]) Determines the Astropy configuration directory name and creates the directory if it doesn’t exist.
reload_config([packageormod]) Reloads configuration settings from a configuration file for the root package of the requested package/module.

Classes

ConfigItem([defaultvalue, description, …]) A setting and associated value stored in a configuration file.
ConfigNamespace A namespace of configuration items.
ConfigurationMissingWarning A Warning that is issued when the configuration directory cannot be accessed (usually due to a permissions problem).
InvalidConfigurationItemWarning A Warning that is issued when the configuration value specified in the astropy configuration file does not match the type expected for that configuration value.
set_temp_cache([path, delete]) Context manager to set a temporary path for the Astropy download cache, primarily for use with testing (though there may be other applications for setting a different cache directory, for example to switch to a cache dedicated to large files).
set_temp_config([path, delete]) Context manager to set a temporary path for the Astropy config, primarily for use with testing.

Class Inheritance Diagram

Inheritance diagram of astropy.config.configuration.ConfigItem, astropy.config.configuration.ConfigNamespace, astropy.config.configuration.ConfigurationMissingWarning, astropy.config.configuration.InvalidConfigurationItemWarning, astropy.config.paths.set_temp_cache, astropy.config.paths.set_temp_config