bashdict.py

import re


class BashDict(dict):
    variable_regex = r'(\$\{([\w\d\_]+)\})'
    default_return = ''

    def _parse_variables(self, value):
        if '${' in value:
            def replace(match):
                variable_name = match.group(2)
                parsed = self.get(variable_name)
                if not parsed:
                    return self.default_return
                return parsed
            return re.sub(self.variable_regex,
                          replace,
                          value)
        return value

    def __getitem__(self, key):
        value = super(BashDict, self).__getitem__(key)
        return self._parse_variables(value)

    def get(self, key):
        value = super(BashDict, self).get(key)
        return self._parse_variables(value)
    

d = BashDict()
d['KEY'] = 'foo'
d['VAR1'] = '${KEY}pene'
d['VAR2'] = '${VAR1}polla'
d['VAR3'] = '${VAR1}${VAR2} hahah'

print(d['VAR1'])
print(d.get('VAR1'))
print(d['VAR2'])
print(d.get('VAR2'))
print(d['VAR3'])
print(d.get('VAR3'))