[New-bugs-announce] [issue29333] ConfigParser calls Interpolation.before_read after reading
report at bugs.python.org
Fri Jan 20 06:50:36 EST 2017
New submission from Gereon Kaiping:
In its current implementation, `ConfigParser` calls its Interpolation's `before_read` method in the very last step of `_read`, when all properties have already been overwritten by their new uninterpolated values.
I am developing a program with modular config files: It is possible to supply several configuration files on the command line, and they are all fed through the `.read` method.
Now it would be amazing to use `read` time interpolation instead of `get` time interpolation to construct things like
outputfilename = %(outputfilename)s_extension_from_this_ini_module
By looking at the `Interpolation` class, it seems that behaviour like this should be supported by supplying a `before_read` as follows.
def before_read(self, parser, section, option, value):
L = 
interpolations = parser[section]
parser, option, L, value, section, interpolations, 1)
However, this is not possible, because `before_read` is only called *after* all values in the config file have been read and all old values in the ConfigParser object have been overridden.
The attached file contains a subclass of `BasicInterpolation` and a subclass of `ConfigParser` which in concert allow me to write recursive property definitions as given above.
The downside of this change is that (a) interpolation values can't be defined *after* they are used any more, and (b) it is not possible to hack this parser to accept multi-line option names as interpolations. (To me personally, both of these don't sound like useful features to begin with.)
title: ConfigParser calls Interpolation.before_read after reading
Added file: http://bugs.python.org/file46355/earlyinterpolation.py
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