How to use variables already defined in ConfigParser

Question

I'm using ConfigParser in Python

config.ini is

[general]
name: my_name
base_dir: /home/myhome/exp

exe_dir: ${base_dir}/bin

Here I want exp_dir becomes /home/myhome/exp/bin not ${base_dir}/bin.

It means ${base_dir} would be substituted to /home/myhome/exp automatically.

Answer 1

You can use ConfigParser interpolation

On top of the core functionality, SafeConfigParser supports interpolation. This means values can contain format strings which refer to other values in the same section, or values in a special DEFAULT section. Additional defaults can be provided on initialization.

For example:

[My Section] 
foodir: %(dir)s/whatever 
dir=frob 
long: this value continues    
    in the next line 

would resolve the %(dir)s to the value of dir (frob in this case). All reference expansions are done on demand.

Your example becomes:

[general]
name: my_name
base_dir: /home/myhome/exp

exe_dir: %(base_dir)s/bin

Answer 2

Instead of "${foo}", write "%(foo)s". (See http://docs.python.org/library/configparser.html and search for "interpolation". This works for either an ordinary ConfigParser or a SafeConfigParser.)

Answer 3

In Python 3, you can use ${base_dir}/bin, and the extended interpolation allows you to use variables from other sections. Example:

[Common]
home_dir: /Users
library_dir: /Library
system_dir: /System
macports_dir: /opt/local

[Frameworks]
Python: 3.2
path: ${Common:system_dir}/Library/Frameworks/

[Arthur]
nickname: Two Sheds
last_name: Jackson
my_dir: ${Common:home_dir}/twosheds
my_pictures: ${my_dir}/Pictures
python_dir: ${Frameworks:path}/Python/Versions/${Frameworks:Python}