Adding an ImmutableConfigValue class

[michaelbynum]

Changes proposed in this PR:

This PR adds a class derived from ConfigValue which can be made temporarily immutable by setting the _mutable attribute to False.

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[michaelbynum]

@jsiirola Please let me know what you think of this implementation. The idea is that you just set the _mutable flag to False when you want the config value to be immutable.

[coveralls]

Coverage increased (+0.1%) to 61.194% when pulling 9cc57f0 on michaelbynum:immutable-config into ad6043c on PyUtilib:master.

[jsiirola]

What about this:

class ImmutableConfigValue(ConfigValue):
    def set_value(self, value):
        raise RuntimeError(str(self) + ' is currently immutable.')

class MarkImmutable(object):
    def __init__(self, *args):
        self._locked = []
        try:
            for arg in args:
                if type(arg) is not ConfigValue:
                    raise ValueError()
                arg.__class__ = ImmutableConfigValue
                self._locked.append(arg)
        except:
            self.release_lock()

    def release_lock(self):
        for arg in self._locked:
            arg.__class__ = ConfigValue
        self._locked = []

...this would allow you to lock configurations that you didn't actually create. With a little more work, we could make it so that the locks are either exclusive (or support nested).

[michaelbynum]

@jsiirola Good suggestion.

[codecov-io]

Codecov Report

Merging #95 into master will increase coverage by 0.08%.
The diff coverage is 86.66%.

@@            Coverage Diff             @@
##           master      #95      +/-   ##
==========================================
+ Coverage   63.13%   63.21%   +0.08%     
==========================================
  Files          87       87              
  Lines        8788     8818      +30     
==========================================
+ Hits         5548     5574      +26     
- Misses       3240     3244       +4     

Impacted Files	Coverage Δ
pyutilib/misc/config.py	92.27% <86.66%> (-0.24%)	⬇️

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