author: Michael Droettboom
date-created: 2013 December 10
date-last-revised: 2014 April 30
date-accepted: 2014 April 30
type: Standard Track
status: Accepted
We need a new approach to configuration that makes it easier to modify and extend configuration over time, plays well with other tools in the Python ecosystem, and most importantly, encourages the reproducibility of science results.
The most important goal of the configuration system should be the reproducibility of scientific results. Some of the existing configuration parameters relate to peculiarities of platforms and environments (let's call this "platform configuration"), and others relate to actual scientific data processing (let's call this "science state"). This APE argues that the existing configuration system is well-suited to platform configuration (providing that some modest enhancements are made), whereas scientific state should be handled in more directly in code in a way that keeps those settings closer to a given application and not in user- or machine-global settings.
For example, utils.console.use_color configures whether ANSI color codes will be output to the terminal. This might be changed to False if the user's system does not support them, but that would have no bearing on the output of science processing. In contrast, cosmology.core.default_cosmology changes the default cosmology to use, which could put sources in different locations, changing the ultimate science result.
Global state of any kind (not just in a config file) that relates to scientific results should be discouraged wherever possible, but there are cases where it is just too convenient. An example might be the list of URLs to use in a vo query (the configuration item vo.validator.validate.cs_urls). Another example of global state that doesn't currently use the configuration system is the set of active units (astropy.units.set_enabled_units). These sorts of things should be settable from Python code, and ideally have context managers, but should not use the configuration system, since storing them in user- and machine-global files hinders reproducibility. We should instead encourage the use of Python code, and the various facilities that Python has to reuse code, rather than config files.
The current configuration system stores uses the ConfigObj library to store configuration in a file in the user's home directory (~/.astropy or ~/.config/astropy, depending on platform and configuration). The configuration items are defined within each subpackage using declarations that look like:
SESAME_URL = ConfigurationItem("sesame_url",
["http://cdsweb.u-strasbg.fr/cgi-bin/nph-sesame/",
"http://vizier.cfa.harvard.edu/viz-bin/nph-sesame/"],
"The URL to Sesame's web-queryable database.",
cfgtype='string_list')At build time, all of these configuration items are collated into a template config file that is copied to the user's home directory (if it doesn't already exist) upon first import of astropy.
Configuration items may be set and saved to disk directly from Python:
In [1]: from astropy.coordinates import name_resolve
In [2]: name_resolve.NAME_RESOLVE_TIMEOUT.set(42)
In [3]: name_resolve.NAME_RESOLVE_TIMEOUT.save()The config file may also be edited by hand. It provides a convenient summary and description of each of the available options.
There are some things about this system that would be nice to retain:
- It's easy to save values to the file from Python.
- It's easy to edit the config file by hand.
- It can be loaded very early on during the import of astropy so it can affect things, such as logging, that are not very dynamic.
However, there are a number of shortcomings of this approach:
- The entire astropy package must be imported to determine what the config items are, and generate the template configuration file. This is slow, and can place otherwise unnecessary restrictions on the code in order to make it importable at build time.
- The template config file is copied to the user's home directory with all values uncommented. This means we don't know which values the user has explicitly changed, and can't change default values in future versions of astropy.
- We don't know the version of astropy that the config file was created with, so we can't update it when astropy is updated.
Unrelated to the system itself, but to how it's been used thus far, it is difficult to find the configuration items using introspection in Python, since many are defined rather "deep" within the subpackages. For example, we have cosmology.core.DEFAULT_COSMOLOGY instead of cosmology.DEFAULT_COSMOLOGY. All of these configuration items should be moved to the config namespace within each subpackage. To use this example, cosmology.config.DEFAULT_COSMOLOGY.
The following improvements to the base configuration system will be made:
- Since, as a result of this APE, the number of configuration options will be much smaller, we will just include a template config file directly in the source code repository, install that alongside the source code, and copy that into the user's configuration directory upon first import (see below for the details of how and when it is copied to the user's config directory). Obviously, it will require some care to keep it in sync with the declaration of the configuration options in the source code. However, it is the simplest possible thing that can work and resolves the issues with generating the config file template at build time.
- The template config file will have all key/value pairs commented out by default. This way, default values can be changed in astropy and will only be overridden by the config file if the user explicitly does so.
- Store a version string in the config file. This, at a later date, but not as part of this APE, may allow for graceful automatic upgrades of the config file. This version should correspond to the release version of astropy, and not include a githash.
- Upgrading of the config file will roughly follow the Debian configuration file guidelines, which strikes a good balance between safety and simplicity. It prevents the user's config file changes from being accidentally overwritten, but doesn't try to be too clever about automatically updating the content. In short:
- If there is no user config file, copy the one from astropy's current version.
- If there is a user config file, and it is entirely commented out, the file is overwritten.
- If there is a user config file and its contents match exactly those of a stock astropy release prior to this APE (prior to astropy 0.4), it is overwritten.
- If the user config file is different from the config file template of a previous astropy version, don't touch it. Install alongside it
astropy.cfg.verwhereveris the current version. Optionally, install aastropy.cfg.ver.diffwhich is a diff of the user's config file and the current config file template. Display a warning that the config file has changed and the user may want to manually resolve the differences between their file and the new one. This warning should only be displayed once (whenastropy.cfg.verdoesn't already exist) so that users who frequently switch between versions of astropy are not bombarded with warnings.
Once doing that, each existing configuration item will be determined to be either "platform" or "science".
For "platform" configuration items:
- Include the item within the new config file template in the source repository.
- Move the configuration item to the
subpackage.confnamespace, which is a subclass of a base class for managing configuration items. - For backward compatibility, keep special delegation objects that delegate from the existing location to the new location and raise deprecation warnings when used.
- The configuration items may still be set by their old keys in the config file for one major release cycle, but a deprecation warning will be shown.
- The configuration item should be documented in the subpackages documentation in a standardized section ("Configuration").
For "science" configuration items:
Define a standard Python context manager for setting the global state associated with each configuration item. For example, this should work:
from astropy import cosmology with cosmology.set_default_cosmology('WMAP9'): # do something # This also works, but doesn't automatically "reset" itself # at the end of the block cosmology.set_default_cosmology('WMAP9')- These context managers will be documented in the API section of the subpackage in the standard way along with the rest of the API.
- Retain special delegation objects at the existing location of the configuration items that call these new Python context managers. These will raise deprecation warnings describing how to update code.
- When these configuration items are found in the config file, deprecation warnings will be shown, but only if they are different from the defaults as specified in astropy 0.3. Doing this without checking against the defaults would give everyone a warning, since all users currently have an astropy 0.3 config file with all values set.
To support the new way of dealing with scientific configuration, ways of conveniently running Python code at the start of every script should be documented. This should include, in order of increasing "broadness":
- Making state changes at the top of your script.
- Having a Python module that all your scripts explicitly import.
- Using IPython's "profiles"
- Using Python's "sitecustomize" (though this would be the least desirable, as it has many of the reproducibility problems that plague the current configuration system).
For a subsequent release, we will remove all of the deprecated backward-compatibility delegation objects.
A set of guidelines about the difference between "platform configuration" and "science state" will be added to the relevant developer documentation.
As an optional follow-on to all of the above, a astropy.reset_science_defaults() function may be added, that will reset all of the science state to their defaults. If all of the science state context managers inherit from the same base class, presumably providing that should be fairly automatic and straightforward.
The following configuration items have been moved/renamed or converted to science state:
Renamed configuration parameters| Old config file location | Old Python location | New config file location | New Python location |
|---|---|---|---|
[] unicode_output |
UNICODE_OUTPUT |
unchanged | conf.unicode_output |
[coordinates.name_resolve] name_resolve_timeout |
coordinates.name_resolve.NAME_RESOLVE_TIMEOUT |
[astropy.utils.data] remote_timeout |
astropy.utils.data.conf.remote_timeout |
[coordinates.name_resolve] sesame_url |
coordinates.name_resolve.SESAME_URL |
removed | coordinates.name_resolve.sesame_url.get/set |
[coordinates.name_resolve] sesame_database |
coordinates.name_resolve.SESAME_DATABASE |
removed | coordinates.name_resolve.sesame_database.get/set |
[cosmology.core] default_cosmology |
cosmology.core.DEFAULT_COSMOLOGY |
removed | cosmology.default_cosmology.get/set |
[io.fits] enable_record_valued_keyword_cards |
io.fits.ENABLE_RECORD_VALUED_KEYWORD_CARDS |
unchanged | io.fits.conf.enable_record_valued_keyword_cards |
[io.fits] extension_name_case_sensitive |
io.fits.EXTENSION_NAME_CASE_SENSITIVE |
unchanged | io.fits.conf.extension_name_case_sensitive |
[io.fits] strip_header_whitespace |
io.fits.STRIP_HEADER_WHITESPACE |
unchanged | io.fits.conf.strip_header_whitespace |
[io.fits] use_memmap |
io.fits.USE_MEMMAP |
unchanged | io.fits.conf.use_memmap |
[io.votable.table] pedantic |
io.votable.table.PEDANTIC |
[io.votable] pedantic |
io.votable.conf.pedantic |
[logger] log_exceptions |
logger.LOG_EXCEPTIONS |
unchanged | logger.conf.log_exceptions |
[logger] log_file_format |
logger.LOG_FILE_FORMAT |
unchanged | logger.conf.log_file_format |
[logger] log_file_level |
logger.LOG_FILE_LEVEL |
unchanged | logger.conf.log_file_level |
[logger] log_file_path |
logger.LOG_FILE_PATH |
unchanged | logger.conf.log_file_path |
[logger] log_level |
logger.LOG_LEVEL |
unchanged | logger.conf.log_level |
[logger] log_to_file |
logger.LOG_TO_FILE |
unchanged | logger.conf.log_to_file |
[logger] log_warnings |
logger.LOG_WARNINGS |
unchanged | logger.conf.log_warnings |
[logger] use_color |
logger.USE_COLOR |
[] use_color |
conf.use_color |
[nddata.nddata] warn_unsupported_correlated |
nddata.nddata.WARN_UNSUPPORTED_CORRELATED |
[nddata] warn_unsupported_correlated |
nddata.conf.warn_unsupported_correlated |
[table.column] auto_colname |
table.column.AUTO_COLNAME |
[table] auto_colname |
table.conf.auto_colname |
[table.pprint] max_lines |
table.pprint.MAX_LINES |
[table] max_lines |
table.conf.max_lines |
[table.pprint] max_width |
table.pprint.MAX_WIDTH |
[table] max_width |
table.conf.max_width |
[utils.console] use_color |
utils.console.USE_COLOR |
[] use_color |
conf.use_color |
[utils.data] compute_hash_block_size |
astropy.utils.data.COMPUTE_HASH_BLOCK_SIZE |
unchanged | astropy.utils.data.conf.compute_hash_block_size |
[utils.data] dataurl |
astropy.utils.data.DATAURL |
unchanged | astropy.utils.data.conf.dataurl |
[utils.data] delete_temporary_downloads_at_exit |
astropy.utils.data.DELETE_TEMPORARY_DOWNLOADS_AT_EXIT |
unchanged | astropy.utils.data.conf.delete_temporary_downloads_at_exit |
[utils.data] download_cache_block_size |
astropy.utils.data.DOWNLOAD_CACHE_BLOCK_SIZE |
unchanged | astropy.utils.data.conf.download_cache_block_size |
[utils.data] download_cache_lock_attempts |
astropy.utils.data.download_cache_lock_attempts |
unchanged | astropy.utils.data.conf.download_cache_lock_attempts |
[utils.data] remote_timeout |
astropy.utils.data.REMOTE_TIMEOUT |
unchanged | astropy.utils.data.conf.remote_timeout |
[vo.client.conesearch] conesearch_dbname |
vo.client.conesearch.CONESEARCH_DBNAME |
[vo] conesearch_dbname |
vo.conf.conesearch_dbname |
[vo.client.vos_catalog] vos_baseurl |
vo.client.vos_catalog.BASEURL |
[vo] vos_baseurl |
vo.conf.vos_baseurl |
[vo.samp.utils] use_internet |
vo.samp.utils.ALLOW_INTERNET |
[vo.samp] use_internet |
vo.samp.conf.use_internet |
[vo.validator.validate] cs_mstr_list |
vo.validator.validate.CS_MSTR_LIST |
[vo.validator] conesearch_master_list |
vo.validator.conf.conesearch_master_list |
[vo.validator.validate] cs_urls |
vo.validator.validate.CS_URLS |
[vo.validator] conesearch_urls |
vo.validator.conf.conesearch_urls |
[vo.validator.validate] noncrit_warnings |
vo.validator.validate.noncrit_warnings |
[vo.validator] noncritical_warnings |
vo.validator.conf.noncritical_warnings |
The delegation objects should retain backward compatibility for at least one release.
In an earlier draft of this proposal, I proposed that we throw out the existing configuration system altogether. By doing so, however, we would lose the ability to easily update and save values to the file from Python. We also would require users to use IPython profiles (which are a fairly advanced feature) just to do basic things. It also makes it harder to convey to the user which things are recommended for user- or machine-wide configuration and which things may lead to scripts and applications not being portable. In the present proposal it's obvious: the config file is for user-global stuff; everything else is just Python code.
The first draft of this APE surmised an elaborate automatic configuration file update system. This draft proposes much like package upgrading on Debian, which preserves the state of a user-modified configuration file, while otherwise staying out of the way and providing manual intervention to upgrade the config file. That should probably be "good enough" and is far simpler, and presumable less prone to errors or surprising bugs.
No objections have been raised to this APE in its current form, and it has therefore been accepted on April 30th 2014.