[Python-Dev] configparser 1.1 - one last bug fix needed (original) (raw)
Łukasz Langa lukasz at langa.pl
Mon Dec 13 23:22:29 CET 2010
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Hi there.
There's one last thing that needs to be done with configparser for 3.2. Raymond, Fred, Michael and Georg already expressed their approval on that so unless anybody finds a flaw in the idea expressed below, I'm going to make the change for 3.2b2:
the ConfigParser class will be removed
the SafeConfigParser class will be renamed to ConfigParser
2to3 will rename SafeConfigParser classes to ConfigParser
2to3 will warn on the subtle behaviour change when ConfigParser classes are found
What's the difference?
Both ConfigParser and SafeConfigParser implement interpolation, e.g. option values can contain special tokens similar to those implemented by Python's string formatting: %(example)s. These tokens are replaced during get() operations by values from respective keys (either from the current section or from the DEFAULT section).
SafeConfigParser was originally introduced to fix a couple of ConfigParser problems:
when a token didn't match the %(name)s syntax, it was simply treated as a raw value. This caused configuration errors like %var or %(no_closing_s) to be missed.
if someone actually wanted to store arbitrary strings in values, including Python formatting strings, there was no way to escape %(name)s in the configuration. The programmer had to know in advance that some value may hold %(name)s and only get() values from that option using
get('section', 'option', raw=True)
Then however, that option could not use interpolation anymore.
set() originally allowed to store non-string values in the parser. This was not meant to be a feature and caused trouble when the user tried to save the configuration to a file or get the stored values back using typed get() methods.
SafeConfigParser solves these problems by validating interpolation syntax (only %(name)s or %% are allowed, the latter being an escaped percent sign) and raising exceptions on syntax errors, and validating type on set() operations so that no non-string values can be passed to the parser using the API.
Why change that?
When ConfigParser was left alone, it remained the default choice for most end users, including our own distutils and logging libs. This was a very weak choice, and most current ConfigParser users are not aware of the interpolation quirks. I had to close a couple of issues related to people trying to store non-string values internally in the parser.
The current situation needlessly complicates the documentation. Explaining all the above quirks to each new user who only wants to parse an INI file is weak at best. Moreover, users trust Python to do the right thing by default and according to their intuition. In this case, going for the default configparser.ConfigParser class without consulting the documentation is clearly a suboptimal choice.
One last argument is that SafeConfigParser is an awkward name. It implicates the other parsers are somehow unsafe, or that this specific parser protects users from something. This is generally considered a naming antipattern.
When?
You might ask whether this can be done for 3.2 (e.g. is that a feature or a bugfix). In Raymond's words, the beta process should be used to flesh out the APIs, test whether they work as expected and fix suboptimal decisions before we hit the release candidate stage. He consideres this essentially a bugfix and I agree.
You might ask why do that now and not for 3.3. We believe that 3.2 is the last possible moment of introducing a change like that. The adoption rate is currently still low and application authors porting projects from 2.x expect incompatibilities. When they are non-issues, handled by 2to3, there's nothing to be afraid of.
But isn't that... INCOMPATIBLE?!
Yes, it is. Thanks to the low py3k adoption rate now's the only moment where there's marginal risk of introducing silent incompatibility (there are hardly any py3k projects out there). Projects ported from Python 2.x will be informed by 2to3 of the change. We believe that this will fix more bugs than it introduces.
Support for bare % signs would be the single case where ConfigParser might have appeared a more natural solution. In those cases we expect that users will rather choose to turn off interpolation whatsoever.
Summary
If you have any strong, justified arguments against this bugfix, speak up. Otherwise the change will be made on Thursday.
-- Interpolating regards, Łukasz Langa tel. +48 791 080 144 WWW http://lukasz.langa.pl/
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