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A class to access dicts using attribute syntax
I've written a class that allows to access dictionary entries of arbitrary dicts with appropriate string keys through attribute access syntax on an instance of the class.
My questions are:
-
Is this class a good idea to begin with?
-
Is there anything that should be done differently?
Here's the class:
class DictProxy:
"""
Allow to access a dictionary through attribute access syntax.
Obviously only dictionary entries whose keys are strings
conforming to identifier rules can be accessed this way; also,
identifiers starting with underscore are not delegated to the
dictionary (thus dictionary entries whose key starts with
underscore cannot be accessed this way).
"""
def __init__(self, dictionary):
"""
Initialize the DictProxy with a dictionary
"""
self._dictionary = dictionary
def __setattr__(self, name, value):
"""
Redirect non-underscore attribute assignments to dictionary updates
"""
if name[0] == "_":
super().__setattr__(name, value)
else:
self._dictionary[name] = value
def __getattr__(self, name):
"""
Redirect non-underscore attribute reads to dictionary reads
"""
if name[0] == "_":
return super().__getattr__(name)
else:
try:
return self._dictionary[name]
except KeyError as error:
# hasattr fails if the exception isn't an AttributeError
raise AttributeError(error)
def __delattr__(self, name):
"""
Redirect non-underscore attribute deletes to dictionary deletes
"""
# the actual code
if name[0] == "_":
return super().__delete__(name)
else:
del self._dictionary[name]
def test():
dictionary = { "a": 1, "b": 2, "c": 3 }
proxy = DictProxy(dictionary)
assert(hasattr(proxy, "a"))
assert(hasattr(proxy, "b"))
assert(hasattr(proxy, "c"))
assert(not hasattr(proxy, "d"))
assert(proxy.a == 1)
assert(proxy.b == 2)
assert(proxy.c == 3)
proxy.a = 4
assert(proxy.a == 4)
assert(dictionary["a"] == 4)
proxy.d = 5
assert(hasattr(proxy, "d"))
assert(proxy.d == 5)
assert("d" in dictionary)
assert(dictionary["d"] == 5)
del proxy.a
assert(not hasattr(proxy, "a"))
assert(not "a" in dictionary)
delattr(proxy, "b")
assert(not hasattr(proxy, "b"))
assert(not "b" in dictionary)
dictionary["a"] = 6
assert(hasattr(proxy, "a"))
assert(proxy.a == 6)
del dictionary["c"]
assert(not hasattr(proxy, "c"))
if __name__ == '__main__':
test()
2 answers
The following users marked this post as Works for me:
User | Comment | Date |
---|---|---|
celtschk | (no comment) | Apr 14, 2022 at 03:56 |
-
It is useful sometimes, for example consider a program accepting input in the form of templates. An input of
"Value: {obj.field}"
is more readable than"Value: {obj['field']}"
.- Such an example is an app processing JSON and evaluating expressions on it, like
jq
or a similar Python app,pjy
:pjy d.item.subitem
instead ofpjy "d['item']['subitem']"
- Such an example is an app processing JSON and evaluating expressions on it, like
-
There are other ways to do it:
- Subclass dict and implement
def __getattr__(self, attr): return self[attr]
- Use
self.__dict__
instead ofself._dictionary
- Subclass dict and implement
0 comment threads
I like the idea, but more as an exercise or a demonstration what can be done with Python. I also like that the code comes with a set of test cases which are written in a way that they nicely serve as user documentation. In fact, I could even imagine this example being used in a tutorial about the __setattr__
etc. methods.
However, unless there is a real use-case where such a feature is needed for some reason, I would rather not start using it only for the syntactic difference: For readers of code, such tricks increase the cognitive complexity.
Having one idiomatic way of doing something makes things easier in most cases (https://peps.python.org/pep-0020):
There should be one -- and preferably only one -- obvious way to do it.
1 comment thread