"""
Expression evaluation support.
For the moment this depends on python's eval. In the future it should be
replaced with a "safe" parser.
"""
from collections.abc import MutableMapping
from itertools import chain
[docs]class ExpressionContext(MutableMapping):
[docs] def __init__(self, dict, parent=None):
"""
Create a new expression context that looks for values in the
container object 'dict', and falls back to 'parent'
"""
self.dict = dict
self.parent = parent
def __delitem__(self, key):
if key in self.dict:
del self.dict[key]
elif self.parent is not None and key in self.parent:
del self.parent[key]
def __iter__(self):
return chain(iter(self.dict), iter(self.parent or []))
def __len__(self):
return len(self.dict) + len(self.parent or [])
def __getitem__(self, key):
if key in self.dict:
return self.dict[key]
if self.parent is not None and key in self.parent:
return self.parent[key]
raise KeyError(key)
def __setitem__(self, key, value):
self.dict[key] = value
def __contains__(self, key):
if key in self.dict:
return True
if self.parent is not None and key in self.parent:
return True
return False
def __str__(self):
return str(self.dict)
def __bool__(self):
if not self.dict and not self.parent:
return False
return True
__nonzero__ = __bool__