ConfigParser.get has an optional argument vars that -
according to the docstring- "which must be a dictionary 
whose contents overrides any pre-existing defaults."
I understand that it overrides the default values, but 
should not override an actual value existing in the file. 
That is, given this contents:

[DEFAULT]
key1=default1

[section]
key2=value2

vars={'key1:vars1','key2:vars2'}

cfg.get('section','key1',0,vars) gives 'vars1' (ok)

cfg.get('section','key2',0,vars) gives 'vars2' (wrong, 
should be 'value2', since key2 is actually in the section 
and no default is needed).

To correct this behavior, simply move this two lines of 
code up in ConfigParser.get (ConfigParser.py), just below 
the first line and before the first try/except:

        # Update with the entry specific variables
        if vars is not None:
            d.update(vars)

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This is indeed what happens.  However, changing this could
break existing code, so an alternative fix would be to
change the documentation to "overrides any pre-existing
values.".  I am not sure what the desired behaviour is - if
it is what is current, then recommend updating the
documentation.  If the desired behaviour is what the
documentation currently says, then recommend applying the
above patch (although the code is not exactly the same
anymore, the effect is).

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I believe the current behaviour is there ever since revision
1.6, when the vars parameter was first introduced. The
commit message says

date: 1999/01/26 19:29:25;  author: guido;  state: Exp; 
lines: +17 -6
Patch by Chris Petrilli (not really tested since I don't
know this
module myself) to accept an option keyword argument (vars)
that is
substituted on top of the defaults that were setup in
__init__.  The
patch also fixes the problem where you can't have recusive
references
inside your configuration file.

We might want to ask Petrilli what the intention of this
feature was, or just proceed with changing the documentation
(and docstrings); contributions in that direction are
welcome. I agree with  anadelonbrin that the proposed change
is probably not adequate.

The following two statements from ConfigParser document clearly mention
that what is passed in 'vars' are defaults and defaults come into
picture only when values are not explicitly set.

"Default values can be specified by passing them into the ConfigParser
constructor as a dictionary. Additional defaults may be passed into the
get() method which will override all others."

"ConfigParser.get(section, option[, raw[, vars]])¶
    Get an option value for the named section. All the '%'
interpolations are expanded in the return values, based on the defaults
passed into the constructor, as well as the options vars provided,
unless the raw argument is true."

If we can not change the behaviour (as it will break existing code), we
should explicitly document this fact. Basically, get() looks for options
in the following order:

1) in 'vars'.
2) in the actual section
3) in default section

The patch "cfgdoc.diff" contains changes to rst as well as to the doc
string. Can some one please review it?