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POHow can I apply a prefix to dictionary access?
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copied!<p>I'm imitating the behavior of the <code>ConfigParser</code> module to write a highly specialized parser that exploits some well-defined structure in the configuration files for a particular application I work with. Several sections of the config file contain hundreds of variable and routine mappings prefixed with either <code>Variable_</code> or <code>Routine_</code>, like this:</p> <pre><code>[Map.PRD] Variable_FOO=LOC1 Variable_BAR=LOC2 Routine_FOO=LOC3 Routine_BAR=LOC4 ... [Map.SHD] Variable_FOO=LOC1 Variable_BAR=LOC2 Routine_FOO=LOC3 Routine_BAR=LOC4 ... </code></pre> <p>I'd like to maintain the basic structure of <code>ConfigParser</code> where each section is stored as a single dictionary, so users would still have access to the classic syntax:</p> <pre><code>config.content['Mappings']['Variable_FOO'] = 'LOC1' </code></pre> <p>but also be able to use a simplified API that drills down to this section:</p> <pre><code>config.vmapping('PRD')['FOO'] = 'LOC1' config.vmapping('PRD')['BAR'] = 'LOC2' config.rmapping('PRD')['FOO'] = 'LOC3' config.rmapping('PRD')['BAR'] = 'LOC4' </code></pre> <p>Currently I'm implementing this by storing the section in a special subclass of <code>dict</code> to which I've added a <code>prefix</code> attribute. The <code>variable</code> and <code>routine</code> properties of the parser set the <code>prefix</code> attribute of the <code>dict</code>-like object to <code>'Variable_'</code> or <code>'Routine_'</code> and then modified <code>__getitem__</code> and <code>__setitem__</code> attributes of the <code>dict</code> handle gluing the prefix together with the key to access the appropriate item. It's working, but involves a lot of boilerplate to implement all the associated niceties like supporting iteration.</p> <p>I suppose my ideal solution would be do dispense with the subclassed <code>dict</code> and have have the <code>variable</code> and <code>routine</code> properties somehow present a "view" of the plain <code>dict</code> object underneath without the prefixes.</p> <h2>Update</h2> <p>Here's the solution I implemented, largely based on @abarnet's answer:</p> <pre><code>class MappingDict(object): def __init__(self, prefix, d): self.prefix, self.d = prefix, d def prefixify(self, name): return '{}_{}'.format(self.prefix, name) def __getitem__(self, name): name = self.prefixify(name) return self.d.__getitem__(name) def __setitem__(self, name, value): name = self.prefixify(name) return self.d.__setitem__(name, value) def __delitem__(self, name): name = self.prefixify(name) return self.d.__delitem__(name) def __iter__(self): return (key.partition('_')[-1] for key in self.d if key.startswith(self.prefix)) def __repr__(self): return 'MappingDict({})'.format(dict.__repr__(self)) class MyParser(object): SECTCRE = re.compile(r'\[(?P<header>[^]]+)\]') def __init__(self, filename): self.filename = filename self.content = {} lines = [x.strip() for x in open(filename).read().splitlines() if x.strip()] for line in lines: match = re.match(self.SECTCRE, line) if match: section = match.group('header') self.content[section] = {} else: key, sep, value = line.partition('=') self.content[section][key] = value def write(self, filename): fp = open(filename, 'w') for section in sorted(self.content, key=sectionsort): fp.write("[%s]\n" % section) for key in sorted(self.content[section], key=cpfsort): value = str(self.content[section][key]) fp.write("%s\n" % '='.join([key,value])) fp.write("\n") fp.close() def vmapping(self, nsp): section = 'Map.{}'.format(nsp) return MappingDict('Variable', self.content[section]) def rmapping(self, nsp): section = 'Map.{}'.format(nsp) return MappingDict('Routine', self.content[section]) </code></pre> <p>It's used like this:</p> <pre><code>config = MyParser('myfile.cfg') vmap = config.vmapping('PRD') vmap['FOO'] = 'LOC5' vmap['BAR'] = 'LOC6' config.write('newfile.cfg') </code></pre> <p>The resulting <code>newfile.cfg</code> reflects the <code>LOC5</code> and <code>LOC6</code> changes.</p>

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