INI File Inheritance

Adds INI-file inheritance to ConfigParser. Note that although it effectively behaves very similarly to passing multiple files to ConfigParser's read() method, that requires changing the code at that point. If that is not feasible, or the INI files should dictate inheritance themselves, then the iniherit package is a better alternative.

Oh, iniherit also adds support for environment variable expansion via %(ENV:VARNAME)s. This really shouldn't be here, but since iniherit supports the %(SUPER)s expansion, it was "just too easy" to add envvar expansion as well...

Project Info

• Project Page: https://github.com/cadithealth/iniherit
• Bug Tracking: https://github.com/cadithealth/iniherit/issues

TL;DR

Given the following two files, base.ini:

[app:main]
name = My Application Name

and config.ini:

[DEFAULT]
# the following will cause both "base.ini" and "override.ini" to be
# inherited. the "?" indicates that "override.ini" will be ignored
# if not found; otherwise an error would occur.
%inherit = base.ini ?override.ini

Then the following code will pass the assert:

import iniherit
cfg = iniherit.SafeConfigParser()
cfg.read('config.ini')

assert cfg.get('app:main', 'name') == 'My Application Name'

Alternatively, the global ConfigParser can be altered to support inheritance:

import iniherit
iniherit.mixin.install_globally()

import ConfigParser
cfg = ConfigParser.SafeConfigParser()
cfg.read('config.ini')

assert cfg.get('app:main', 'name') == 'My Application Name'

Note that the call to install_globally() must be invoked before any other module imports ConfigParser for the global override to have an effect.

The command-line program iniherit allows flattening of INI files (i.e. collapsing all inheritance rules), optionally in "watch" mode:

$ iniherit --watch --interval 2 --verbose input.ini output.ini
INFO:iniherit.cli:"source.ini" changed; updating output...
INFO:iniherit.cli:"inherited-file.ini" changed; updating output...
^C

Installation

Install iniherit via your favorite PyPI package manager works as follows:

# if using python 3+, upgrade your `distribute` package *first*
$ pip install "distribute>=0.7.3"

# then istall with pip:
$ pip install iniherit

Inheritance Mechanism

INI file inheritance with the iniherit package:

• To add inheritance to an INI file, add an %inherit option to the "DEFAULT" section of the INI file to inherit all sections of the specified files. Example:

  [DEFAULT]
  %inherit  = base.ini
  def_var   = Overrides the "def_var" setting, if present,
    in the "DEFAULT" section of "base.ini".
  
  [my-app]
  sect_var  = Overrides the "sect_var" setting, if present,
    in the "my-app" section of "base.ini". Other sections in
    "base.ini" will also be inherited, even if not specified
    here.

• The %inherit option points to a space-separated, URL-encoded, list of files to inherit values from, whose values are loaded depth-first, left-to-right. For example:

  [DEFAULT]
  %inherit = base.ini file-with%20space.ini

• To inherit only a specific section, add the %inherit option directly to the applicable section. By default, the section by the same name will be loaded from the other files, unless the filename is suffixed with square-bracket enclosed ("[" ... "]"), URL-encoded, section name. Example:

  [section]
  %inherit = base.ini override.ini[other%20section]

  In this case, the "section" section in "base.ini" will be inherited, followed by the "other section" from "override.ini".

  Note that if the inherited section includes interpolation references to the DEFAULT section, these will NOT be carried over! In other words, inheritance currently ONLY inherits the actual values, not the interpreted values. Be warned, as this can lead to surprising results!

  If a filename has "[" in the actual name, it can be URL-encoded.

• Filenames, if specified relatively, are taken to be relative to the current INI file.

• If a filename is prefixed with "?", then it will be loaded optionally: i.e. if the file does not exist, it will be silently ignored. If the file does NOT have a "?" prefixed and cannot be found, then an IOError will be raised. Note that this is unlike standard ConfigParser.read() behavior, which silently ignores any files that cannot be found.

  If a filename has "?" as its first character, it can be URL-encoded.

• Note that the actual name of the inherit option can be changed by changing either iniherit.parser.DEFAULT_INHERITTAG for a global effect, or ConfigParser.IM_INHERITTAG for a per-instance effect.

Substitutions

The iniherit package adds the following additional substitution options:

• %(SUPER[:-DEFAULT])s

  Evaluates to the inherited value of the current section/key value. If the inherited INI does not specify a value and no default is provided, then an InterpolationMissingSuperError is raised. The "inherited value" is evaluated depth-first. Note that "SUPER" must be all upper case.

  For example, given the following INI files:

  # base.ini
  [loggers]
  keys = root, app

  # config.ini
  [DEFAULT]
  %inherit = base.ini
  [loggers]
  keys = %(SUPER)s, auth
  wdef = %(SUPER:-more)s or less
  nada = %(SUPER)s boom!

  Then the following Python will result:

  import iniherit
  iniherit.mixin.install_globally()
  import ConfigParser
  cfg = ConfigParser.SafeConfigParser()
  cfg.read('config.ini')
  
  cfg.get('loggers', 'keys')  # ==> 'root, app, auth'
  cfg.get('loggers', 'wdef')  # ==> 'more or less'
  cfg.get('loggers', 'nada')  # ==> raises InterpolationMissingSuperError

  As with standard interpolation errors, the InterpolationMissingSuperError exception is only raised if/when the value is requested from the config (with raw set to falsy).

• %(ENV:VARNAME[:-DEFAULT])s

  Evaluates to the value of the environment variable name "VARNAME". If the environment variable is not defined and no default is provided, then an InterpolationMissingEnvError is raised. Note that environment variable names are always case sensitive.

  For example, given the following INI file:

  # config.ini
  [section]
  home = %(ENV:HOME)s
  rdir = %(ENV:RDIR:-/var/run)s
  nada = %(ENV:RDIR)s

  Then the following Python will result:

  import iniherit
  iniherit.mixin.install_globally()
  import ConfigParser
  cfg = ConfigParser.SafeConfigParser()
  cfg.read('config.ini')
  
  import os
  os.environ['home'] = '/home/user'  # ensure "HOME" envvar exists
  os.environ.pop('RDIR', None)       # ensure "RDIR" envvar does NOT exist
  
  cfg.get('section', 'home')  # ==> '/home/user'
  cfg.get('section', 'rdir')  # ==> '/var/run'
  cfg.get('section', 'nada')  # ==> raises InterpolationMissingEnvError

  As with standard interpolation errors, the InterpolationMissingEnvError exception is only raised if/when the value is requested from the config (with raw set to falsy).

Gotchas

• After an inherit-enabled INI file is loaded, the ConfigParser no longer has knowledge of where a particular option was loaded from or how it was derived. For this reason, when the write method is called, the ConfigParser generates an INI file without inheritance. In other words, it flattens the inheritance tree.