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How to use a dot “.” to access members of dictionary?

Posted by: admin November 1, 2017 Leave a comment

Questions:

How do I make Python dictionary members accessible via a dot “.”?

For example, instead of writing mydict['val'], I’d like to write mydict.val.

Also I’d like to access nested dicts this way. For example

mydict.mydict2.val 

would refer to

mydict = { 'mydict2': { 'val': ... } }
Answers:

You can do it using this class I just made. With this class you can use the Map object like another dictionary(including json serialization) or with the dot notation. I hope to help you:

class Map(dict):
    """
    Example:
    m = Map({'first_name': 'Eduardo'}, last_name='Pool', age=24, sports=['Soccer'])
    """
    def __init__(self, *args, **kwargs):
        super(Map, self).__init__(*args, **kwargs)
        for arg in args:
            if isinstance(arg, dict):
                for k, v in arg.iteritems():
                    self[k] = v

        if kwargs:
            for k, v in kwargs.iteritems():
                self[k] = v

    def __getattr__(self, attr):
        return self.get(attr)

    def __setattr__(self, key, value):
        self.__setitem__(key, value)

    def __setitem__(self, key, value):
        super(Map, self).__setitem__(key, value)
        self.__dict__.update({key: value})

    def __delattr__(self, item):
        self.__delitem__(item)

    def __delitem__(self, key):
        super(Map, self).__delitem__(key)
        del self.__dict__[key]

Usage examples:

m = Map({'first_name': 'Eduardo'}, last_name='Pool', age=24, sports=['Soccer'])
# Add new key
m.new_key = 'Hello world!'
# Or
m['new_key'] = 'Hello world!'
print m.new_key
print m['new_key']
# Update values
m.new_key = 'Yay!'
# Or
m['new_key'] = 'Yay!'
# Delete key
del m.new_key
# Or
del m['new_key']

Questions:
Answers:

I’ve always kept this around in a util file. You can use it as a mixin on your own classes too.

>>> class dotdict(dict):
...     """dot.notation access to dictionary attributes"""
...     __getattr__ = dict.get
...     __setattr__ = dict.__setitem__
...     __delattr__ = dict.__delitem__
... 
>>> mydict = {'val':'it works'}
>>> nested_dict = {'val':'nested works too'}
>>> mydict = dotdict(mydict)
>>> mydict.val
'it works'
>>> mydict.nested = dotdict(nested_dict)
>>> mydict.nested.val
'nested works too'

Questions:
Answers:

Derive from dict and and implement __getattr__ and __setattr__.

Or you can use Bunch which is very similar.

I don’t think it’s possible to monkeypatch built-in dict class.

Questions:
Answers:

Install dotmap via pip

pip install dotmap

It does everything you want it to do and subclasses dict, so it operates like a normal dictionary:

from dotmap import DotMap

m = DotMap()
m.hello = 'world'
m.hello
m.hello += '!'
# m.hello and m['hello'] now both return 'world!'
m.val = 5
m.val2 = 'Sam'

On top of that, you can convert it to and from dict objects:

d = m.toDict()
m = DotMap(d) # automatic conversion in constructor

This means that if something you want to access is already in dict form, you can turn it into a DotMap for easy access:

import json
jsonDict = json.loads(text)
data = DotMap(jsonDict)
print data.location.city

Finally, it automatically creates new child DotMap instances so you can do things like this:

m = DotMap()
m.people.steve.age = 31

Comparison to Bunch

Full disclosure: I am the creator of the DotMap. I created it because Bunch was missing these features

  • remembering the order items are added and iterating in that order
  • automatic child DotMap creation, which saves time and makes for cleaner code when you have a lot of hierarchy
  • constructing from a dict and recursively converting all child dict instances to DotMap
Questions:
Answers:

I tried this:

class dotdict(dict):
    def __getattr__(self, name):
        return self[name]

you can try __getattribute__ too.

make every dict a type of dotdict would be good enough, if you want to init this from a multi-layer dict, try implement __init__ too.

Questions:
Answers:

Building on Kugel’s answer and taking Mike Graham’s words of caution into consideration, what if we make a wrapper?

class DictWrap(object):
  """ Wrap an existing dict, or create a new one, and access with either dot 
    notation or key lookup.

    The attribute _data is reserved and stores the underlying dictionary.
    When using the += operator with create=True, the empty nested dict is 
    replaced with the operand, effectively creating a default dictionary
    of mixed types.

    args:
      d({}): Existing dict to wrap, an empty dict is created by default
      create(True): Create an empty, nested dict instead of raising a KeyError

    example:
      >>>dw = DictWrap({'pp':3})
      >>>dw.a.b += 2
      >>>dw.a.b += 2
      >>>dw.a['c'] += 'Hello'
      >>>dw.a['c'] += ' World'
      >>>dw.a.d
      >>>print dw._data
      {'a': {'c': 'Hello World', 'b': 4, 'd': {}}, 'pp': 3}

  """

  def __init__(self, d=None, create=True):
    if d is None:
      d = {}
    supr = super(DictWrap, self)  
    supr.__setattr__('_data', d)
    supr.__setattr__('__create', create)

  def __getattr__(self, name):
    try:
      value = self._data[name]
    except KeyError:
      if not super(DictWrap, self).__getattribute__('__create'):
        raise
      value = {}
      self._data[name] = value

    if hasattr(value, 'items'):
      create = super(DictWrap, self).__getattribute__('__create')
      return DictWrap(value, create)
    return value

  def __setattr__(self, name, value):
    self._data[name] = value  

  def __getitem__(self, key):
    try:
      value = self._data[key]
    except KeyError:
      if not super(DictWrap, self).__getattribute__('__create'):
        raise
      value = {}
      self._data[key] = value

    if hasattr(value, 'items'):
      create = super(DictWrap, self).__getattribute__('__create')
      return DictWrap(value, create)
    return value

  def __setitem__(self, key, value):
    self._data[key] = value

  def __iadd__(self, other):
    if self._data:
      raise TypeError("A Nested dict will only be replaced if it's empty")
    else:
      return other

Questions:
Answers:

Don’t. Attribute access and indexing are separate things in Python, and you shouldn’t want them to perform the same. Make a class (possibly one made by namedtuple) if you have something that should have accessible attributes and use [] notation to get an item from a dict.

Questions:
Answers:

The language itself doesn’t support this, but sometimes this is still a useful requirement. Besides the Bunch recipe, you can also write a little method which can access a dictionary using a dotted string:

def get_var(input_dict, accessor_string):
    """Gets data from a dictionary using a dotted accessor-string"""
    current_data = input_dict
    for chunk in accessor_string.split('.'):
        current_data = current_data.get(chunk, {})
    return current_data

which would support something like this:

>> test_dict = {'thing': {'spam': 12, 'foo': {'cheeze': 'bar'}}}
>> output = get_var(test_dict, 'thing.spam.foo.cheeze')
>> print output
'bar'
>>

Questions:
Answers:

If you want to pickle your modified dictionary, you need to add few state methods to above answers:

class dotdict(dict):
    """dot.notation access to dictionary attributes"""
    def __getattr__(self, attr):
        return self.get(attr)
    __setattr__= dict.__setitem__
    __delattr__= dict.__delitem__

    def __getstate__(self):
        return self

    def __setstate__(self, state):
        self.update(state)
        self.__dict__ = self

Questions:
Answers:

Fabric has a really nice, minimal implementation. Extending that to allow for nested access, we can use a defaultdict, and the result looks something like this:

from collections import defaultdict

class AttributeDict(defaultdict):
    def __init__(self):
        super(AttributeDict, self).__init__(AttributeDict)

    def __getattr__(self, key):
        try:
            return self[key]
        except KeyError:
            raise AttributeError(key)

    def __setattr__(self, key, value):
        self[key] = value

Make use of it as follows:

keys = AttributeDict()
keys.abc.xyz.x = 123
keys.abc.xyz.a.b.c = 234

That elaborates a bit on Kugel’s answer of “Derive from dict and and implement __getattr__ and __setattr__“. Now you know how!

Questions:
Answers:
def dict_to_object(dick):
    # http://stackoverflow.com/a/1305663/968442

    class Struct:
        def __init__(self, **entries):
            self.__dict__.update(entries)

    return Struct(**dick)

If one decides to permanently convert that dict to object this should do. You can create a throwaway object just before accessing.

d = dict_to_object(d)

Questions:
Answers:

I ended up trying BOTH the AttrDict and the Bunch libraries and found them to be way to slow for my uses. After a friend and I looked into it, we found that the main method for writing these libraries results in the library aggressively recursing through a nested object and making copies of the dictionary object throughout. With this in mind, we made two key changes. 1) We made attributes lazy-loaded 2) instead of creating copies of a dictionary object, we create copies of a light-weight proxy object. This is the final implementation. The performance increase of using this code is incredible. When using AttrDict or Bunch, these two libraries alone consumed 1/2 and 1/3 respectively of my request time(what!?). This code reduced that time to almost nothing(somewhere in the range of 0.5ms). This of course depends on your needs, but if you are using this functionality quite a bit in your code, definitely go with something simple like this.

class DictProxy(object):
    def __init__(self, obj):
        self.obj = obj

    def __getitem__(self, key):
        return wrap(self.obj[key])

    def __getattr__(self, key):
        try:
            return wrap(getattr(self.obj, key))
        except AttributeError:
            try:
                return self[key]
            except KeyError:
                raise AttributeError(key)

    # you probably also want to proxy important list properties along like
    # items(), iteritems() and __len__

class ListProxy(object):
    def __init__(self, obj):
        self.obj = obj

    def __getitem__(self, key):
        return wrap(self.obj[key])

    # you probably also want to proxy important list properties along like
    # __iter__ and __len__

def wrap(value):
    if isinstance(value, dict):
        return DictProxy(value)
    if isinstance(value, (tuple, list)):
        return ListProxy(value)
    return value

See the original implementation here by https://stackoverflow.com/users/704327/michael-merickel.

The other thing to note, is that this implementation is pretty simple and doesn’t implement all of the methods you might need. You’ll need to write those as required on the DictProxy or ListProxy objects.

Questions:
Answers:

To build upon epool’s answer, this version allows you to access any dict inside via the dot operator:

foo = {
    "bar" : {
        "baz" : [ {"boo" : "hoo"} , {"baba" : "loo"} ]
    }
}

For instance, foo.bar.baz[1].baba returns "loo".

class Map(dict):
    def __init__(self, *args, **kwargs):
        super(Map, self).__init__(*args, **kwargs)
        for arg in args:
            if isinstance(arg, dict):
                for k, v in arg.iteritems():
                    if isinstance(v, dict):
                        v = Map(v)
                    if isinstance(v, list):
                        self.__convert(v)
                    self[k] = v

        if kwargs:
            for k, v in kwargs.iteritems():
                if isinstance(v, dict):
                    v = Map(v)
                elif isinstance(v, list):
                    self.__convert(v)
                self[k] = v

    def __convert(self, v):
        for elem in xrange(0, len(v)):
            if isinstance(v[elem], dict):
                v[elem] = Map(v[elem])
            elif isinstance(v[elem], list):
                self.__convert(v[elem])

    def __getattr__(self, attr):
        return self.get(attr)

    def __setattr__(self, key, value):
        self.__setitem__(key, value)

    def __setitem__(self, key, value):
        super(Map, self).__setitem__(key, value)
        self.__dict__.update({key: value})

    def __delattr__(self, item):
        self.__delitem__(item)

    def __delitem__(self, key):
        super(Map, self).__delitem__(key)
        del self.__dict__[key]

Questions:
Answers:

This solution is a refinement upon the one offered by epool to address the requirement of the OP to access nested dicts in a consistent manner. The solution by epool did not allow for accessing nested dicts.

class YAMLobj(dict):
def __init__(self, args):
    super(YAMLobj, self).__init__(args)
    if isinstance(args, dict):
        for k, v in args.iteritems():
            if not isinstance(v, dict):
                self[k] = v
            else:
                self.__setattr__(k, YAMLobj(v))


def __getattr__(self, attr):
    return self.get(attr)

def __setattr__(self, key, value):
    self.__setitem__(key, value)

def __setitem__(self, key, value):
    super(YAMLobj, self).__setitem__(key, value)
    self.__dict__.update({key: value})

def __delattr__(self, item):
    self.__delitem__(item)

def __delitem__(self, key):
    super(YAMLobj, self).__delitem__(key)
    del self.__dict__[key]

With this class, one can now do something like: A.B.C.D.

Questions:
Answers:

I’d like to throw my own solution into the ring:

https://github.com/skorokithakis/jsane

It allows you to parse JSON into something you can access with.attribute.lookups.like.this.r(), mostly because I hadn’t seen this answer before starting to work on it.

Questions:
Answers:

Not a direct answer to the OP’s question, but inspired by and perhaps useful for some.. I’ve created an object-based solution using the internal __dict__ (In no way optimized code)

payload = {
    "name": "John",
    "location": {
        "lat": 53.12312312,
        "long": 43.21345112
    },
    "numbers": [
        {
            "role": "home",
            "number": "070-12345678"
        },
        {
            "role": "office",
            "number": "070-12345679"
        }
    ]
}


class Map(object):
    """
    Dot style access to object members, access raw values
    with an underscore e.g.

    class Foo(Map):
        def foo(self):
            return self.get('foo') + 'bar'

    obj = Foo(**{'foo': 'foo'})

    obj.foo => 'foobar'
    obj._foo => 'foo'

    """

    def __init__(self, *args, **kwargs):
        for arg in args:
            if isinstance(arg, dict):
                for k, v in arg.iteritems():
                    self.__dict__[k] = v
                    self.__dict__['_' + k] = v

        if kwargs:
            for k, v in kwargs.iteritems():
                self.__dict__[k] = v
                self.__dict__['_' + k] = v

    def __getattribute__(self, attr):
        if hasattr(self, 'get_' + attr):
            return object.__getattribute__(self, 'get_' + attr)()
        else:
            return object.__getattribute__(self, attr)

    def get(self, key):
        try:
            return self.__dict__.get('get_' + key)()
        except (AttributeError, TypeError):
            return self.__dict__.get(key)

    def __repr__(self):
        return u"<{name} object>".format(
            name=self.__class__.__name__
        )


class Number(Map):
    def get_role(self):
        return self.get('role')

    def get_number(self):
        return self.get('number')


class Location(Map):
    def get_latitude(self):
        return self.get('lat') + 1

    def get_longitude(self):
        return self.get('long') + 1


class Item(Map):
    def get_name(self):
        return self.get('name') + " Doe"

    def get_location(self):
        return Location(**self.get('location'))

    def get_numbers(self):
        return [Number(**n) for n in self.get('numbers')]


# Tests

obj = Item({'foo': 'bar'}, **payload)

assert type(obj) == Item
assert obj._name == "John"
assert obj.name == "John Doe"
assert type(obj.location) == Location
assert obj.location._lat == 53.12312312
assert obj.location._long == 43.21345112
assert obj.location.latitude == 54.12312312
assert obj.location.longitude == 44.21345112

for n in obj.numbers:
    assert type(n) == Number
    if n.role == 'home':
        assert n.number == "070-12345678"
    if n.role == 'office':
        assert n.number == "070-12345679"