Instead of using a dict to store and pass data we are going completely OOPS approach of storing the data as class attributes and call the get methods defined according to need.
In Java i was able to achieve this but having some trouble in Python. Any Solution would be helpful.
import json
class InputModel:
def __init__(self, input_payload):
self.id1 = input_payload["id1"]
self.route = RouteModel(input_payload["route"])
self.id2 = input_payload["id2"]
self.id3 = input_payload["id3"]
self.id4 = input_payload["id4"]
self.id5 = input_payload["id5"]
def get_id1(self):
return self.id1
#similar for other ids
class RouteModel:
def __init__(self, input_payload_route):
self.id6 = input_payload_route["id6"]
self.id7 = input_payload_route["id7"]
def get_id6(self):
return self.id6
#similar for other ids
json_str = '{"id1":"string","route":{"id6":"string","id7":"string"},"id2": "string","id3": "string","id4": "string","id5": "string"}'
json_dict = json.loads(json_str)
im = InputModel(json_dict)
print(im.get_id1())
print(im.get_id6())
not able to access the nested class attributes
Seems like you went for 1 extra indent in your class methods, thus you couldn't reach them.
Also, to reach id6 of RouteModel, you had to refer to 'route' first:
import json
class InputModel:
def __init__(self, input_payload):
self.id1 = input_payload["id1"]
self.route = RouteModel(input_payload["route"])
self.id2 = input_payload["id2"]
self.id3 = input_payload["id3"]
self.id4 = input_payload["id4"]
self.id5 = input_payload["id5"]
def get_id1(self):
return self.id1
#similar for other ids
class RouteModel:
def __init__(self, input_payload_route):
self.id6 = input_payload_route["id6"]
self.id7 = input_payload_route["id7"]
def get_id6(self):
return self.id6
#similar for other ids
json_str = '{"id1":"string","route":{"id6":"string","id7":"string"},"id2": "string","id3": "string","id4": "string","id5": "string"}'
json_dict = json.loads(json_str)
im = InputModel(json_dict)
print(im.get_id1())
print(im.route.get_id6())
Output:
string
string
The problem is that you are only defining get_id* in your local scope, you need to assign it to the instance if you insist on defining it inside the __init__ method.
I minimized your code example to isolate your issue.
class RouteModel:
def __init__(self):
self.id6 = "foo"
def get_id6(self_=self):
return self_.id6
self.get_id6 = get_id6
rm = RouteModel()
print(rm.get_id6())
>>> "foo"
If I understand your question correctly, you want to be able to access the ids directly as attributes, no matter how deep they are nested in the dictionary.
This solution creates the attributes recursively:
import json
class InputModel:
def __init__(self, payload):
self.create_attrs(payload)
def create_attrs(self, d):
for key, value in d.items():
# if the value is a dict, call create_attrs recursively
if isinstance(value, dict):
self.create_attrs(value)
else:
# create an attribute key=value, e.g. id1="string"
setattr(self, key, value)
json_str = '{"id1":"string","route":{"id6":"string","id7":"string"},"id2": "string","id3": "string","id4": "string","id5": "string"}'
json_dict = json.loads(json_str)
im = InputModel(json_dict)
print(im.id1)
print(im.id6)
After going through answers provided, mostly have defined instance attributes and not class attributes.
Correct me if I'm wrong here but I think this is how class attributes are defined right?
import json
class InputModel:
def __init__(self, input_payload):
InputModel.id1 = input_payload["id1"]
InputModel.route = RouteModel(input_payload["route"])
InputModel.id2 = input_payload["id2"]
InputModel.id3 = input_payload["id3"]
InputModel.id4 = input_payload["id4"]
InputModel.id5 = input_payload["id5"]
def get_id1():
return InputModel.id1
#OR
##classmethod
#def get_id1(cls):
# return cls.id1
#similar for other ids
class RouteModel:
def __init__(self, input_payload_route):
RouteModel.id6 = input_payload_route["id6"]
RouteModel.id7 = input_payload_route["id7"]
def get_id6():
return RouteModel.id6
#similar for other ids
json_str = '{"id1":"string","route":{"id6":"string","id7":"string"},"id2": "string","id3": "string","id4": "string","id5": "string"}'
json_dict = json.loads(json_str)
InputModel(json_dict)
print(InputModel.get_id1())
print(InputModel.route.get_id6())
print(RouteModel.get_id6())
I would like to consult some piece of code with you. I have:
if tuple_type == Operation.START_SERVER:
dictionary = ServersDictionary()
dictionary.start(some_param)
elif tuple_type == Operation.STOP_SERVER:
dictionary = ServersDictionary()
dictionary.stop(some_param)
(...)
elif tuple_type == Operation.START_APP:
dictionary = AppsDictionary()
dictionary.start(some_param)
elif ...
(....)
And there I have 27 if / elifs. Normally, I would go into map - function dispatcher, but after every if / elif I have two lines of code with same dictionary reference. Would you suggest me some clean solution to replace those ugly constructions?
Creating 27 classes for applying polymorphism or 27 functions doesn't sound good... what do you think?
You're right, a mapping is the way to go. Use getattr to access a method from its name:
mapping = {Operation.START_SERVER: (ServerDictionary, 'start', some_param),
Operation.STOP_SERVER: (ServerDictionary, 'stop', some_param),
Operation.START_APP: (AppsDictionary, 'start', some_param)}
...
cls, method, param = mapping[tuple_type]
dictionary = cls()
getattr(dictionary, method)(param)
You can enclose the meta info into your enums, if that is ok for you client code, meaning that you own the enums. Here is an example:
class Operation(Enum):
START_SERVER = (0, "start", ServersDictionary)
STOP_SERVER = (1, "stop", ServersDictionary)
START_APP = (1, "start", AppsDictionary)
And then have a single function to handle your operations:
def handle_operation(operation, some_param):
klass = operation.klass
dictionary = klass()
fn = getattr(dictionary, operation.value)
fn(some_param)
This is assuming you are using the Enum you had in one of your questions. In that case, you will need to add one line there:
class Enum(object):
__metaclass__ = EnumMeta
def __init__(self, value):
super(Enum, self).__init__()
self.value, self.repr, self.klass = value[0], value[1], value[2]
def __repr__(self):
return str(self.repr)
Then you will not need any case checks, simply:
handle_operation(tuple_type)
Maybe you can represent the operation with a dict or tupple, like
op = {'target': 'Servers', 'action': 'start', 'params': (arg1, arg2)}
then you can access it like
obj = globals()[op['target']+'Dictionary']()
getattr(obj, op['action'])(*op['params'])
I have this dictionary:
{"id":3,"name":"MySQL","description":"MySQL Database Server - Fedora 21 - medium","image":"","flavor":""}
And I have this object:
class Record():
id = None
name = None
description = None
image = None
flavor = None
How can I assign values from the dictionary to their corresponding class fields?
Take a dict object as the parameter of init function:
class Record(object):
def __init__(self,record_dict):
try:
self.id = record_dict['id']
self.name = record_dict['name']
self.description = record_dict['description']
self.image = record_dict['image']
self.flavor = record_dict['flavor']
except KeyError:
print 'KeyError'
def get_name(self):
return self.name
adict = {"id":3,"name":"MySQL","description":"MySQL Database Server - Fedora 21 - medium","image":"","flavor":""}
one_obj = Record(adict)
print one_obj
print one_obj.get_name()
output:
<__main__.Record object at 0x022E4C90>
MySQL
works for me...
You probably want something like this:
class Record:
def __init__(self, myDict):
self.id = myDict[“id”]
self.name = myDict[“name”]
self.description = myDict[“description”]
self.image = myDict[“image”]
self.flavor = myDict[“flavor”]
And call it:
rec = Record(myDict)
See here to understand the difference between class and instance variables.
Long story short, class variables have a single value for every instance of the class while instance variables values are unique to each instance.
A class variable is defined like this:
class myClass:
Classvar = ‘something’
An instance variable is defined like this:
class myClass:
def __init__():
Self.instanceVar = ‘something else’
This has already been answered here:
Convert Python dict to object?
My favorite method is this one: x.__dict__.update(d)
You can assign them as follows, assuming your dictionary name is input
id = input['id']
name = input['name']
description = input['description']
image = input['image']
flavor = input['flavor']
Try this method in which you grab the attributes of the object:
r = Record()
attributes = [i for i in dir(r) if not i.startswith('_')]
Basically, there are a bunch of background attributes that contain a bunch of underscores. The dir method gets all the attributes and we create a list of the ones that we want. At this point:
# attributes = ['count', 'description', 'flavor', 'id', 'image', 'index', 'name']
So now we use __setattr__ to set the attributes we just grabbed according to the my_dict
for i in attributes:
r.__setattr__(i, my_dict[i])
See the code run online here.
When you create the Record, pass in the dictionary. Then map the key to the value.
Another simple method, see the code here
r = Record()
for k, v in my_dict.items():
exec('r.' + k + '="' + str(v) + '"')
This question already has answers here:
Is it possible to index nested lists using tuples in python?
(7 answers)
Closed 7 months ago.
There are a lot of good getattr()-like functions for parsing nested dictionary structures, such as:
Finding a key recursively in a dictionary
Suppose I have a python dictionary , many nests
https://gist.github.com/mittenchops/5664038
I would like to make a parallel setattr(). Essentially, given:
cmd = 'f[0].a'
val = 'whatever'
x = {"a":"stuff"}
I'd like to produce a function such that I can assign:
x['f'][0]['a'] = val
More or less, this would work the same way as:
setattr(x,'f[0].a',val)
to yield:
>>> x
{"a":"stuff","f":[{"a":"whatever"}]}
I'm currently calling it setByDot():
setByDot(x,'f[0].a',val)
One problem with this is that if a key in the middle doesn't exist, you need to check for and make an intermediate key if it doesn't exist---ie, for the above:
>>> x = {"a":"stuff"}
>>> x['f'][0]['a'] = val
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
KeyError: 'f'
So, you first have to make:
>>> x['f']=[{}]
>>> x
{'a': 'stuff', 'f': [{}]}
>>> x['f'][0]['a']=val
>>> x
{'a': 'stuff', 'f': [{'a': 'whatever'}]}
Another is that keying for when the next item is a lists will be different than the keying when the next item is a string, ie:
>>> x = {"a":"stuff"}
>>> x['f']=['']
>>> x['f'][0]['a']=val
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'str' object does not support item assignment
...fails because the assignment was for a null string instead of a null dict. The null dict will be the right assignment for every non-list in dict until the very last one---which may be a list, or a value.
A second problem, pointed out in the comments below by #TokenMacGuy, is that when you have to create a list that does not exist, you may have to create an awful lot of blank values. So,
setattr(x,'f[10].a',val)
---may mean the algorithm will have to make an intermediate like:
>>> x['f']=[{},{},{},{},{},{},{},{},{},{},{}]
>>> x['f'][10]['a']=val
to yield
>>> x
{"a":"stuff","f":[{},{},{},{},{},{},{},{},{},{},{"a":"whatever"}]}
such that this is the setter associated with the getter...
>>> getByDot(x,"f[10].a")
"whatever"
More importantly, the intermediates should /not/ overwrite values that already exist.
Below is the junky idea I have so far---I can identify the lists versus dicts and other data types, and create them where they do not exist. However, I don't see (a) where to put the recursive call, or (b) how to 'build' the deep object as I iterate through the list, and (c) how to distinguish the /probing/ I'm doing as I construct the deep object from the /setting/ I have to do when I reach the end of the stack.
def setByDot(obj,ref,newval):
ref = ref.replace("[",".[")
cmd = ref.split('.')
numkeys = len(cmd)
count = 0
for c in cmd:
count = count+1
while count < numkeys:
if c.find("["):
idstart = c.find("[")
numend = c.find("]")
try:
deep = obj[int(idstart+1:numend-1)]
except:
obj[int(idstart+1:numend-1)] = []
deep = obj[int(idstart+1:numend-1)]
else:
try:
deep = obj[c]
except:
if obj[c] isinstance(dict):
obj[c] = {}
else:
obj[c] = ''
deep = obj[c]
setByDot(deep,c,newval)
This seems very tricky because you kind of have to look-ahead to check the type of the /next/ object if you're making place-holders, and you have to look-behind to build a path up as you go.
UPDATE
I recently had this question answered, too, which might be relevant or helpful.
I have separated this out into two steps. In the first step, the query string is broken down into a series of instructions. This way the problem is decoupled, we can view the instructions before running them, and there is no need for recursive calls.
def build_instructions(obj, q):
"""
Breaks down a query string into a series of actionable instructions.
Each instruction is a (_type, arg) tuple.
arg -- The key used for the __getitem__ or __setitem__ call on
the current object.
_type -- Used to determine the data type for the value of
obj.__getitem__(arg)
If a key/index is missing, _type is used to initialize an empty value.
In this way _type provides the ability to
"""
arg = []
_type = None
instructions = []
for i, ch in enumerate(q):
if ch == "[":
# Begin list query
if _type is not None:
arg = "".join(arg)
if _type == list and arg.isalpha():
_type = dict
instructions.append((_type, arg))
_type, arg = None, []
_type = list
elif ch == ".":
# Begin dict query
if _type is not None:
arg = "".join(arg)
if _type == list and arg.isalpha():
_type = dict
instructions.append((_type, arg))
_type, arg = None, []
_type = dict
elif ch.isalnum():
if i == 0:
# Query begins with alphanum, assume dict access
_type = type(obj)
# Fill out args
arg.append(ch)
else:
TypeError("Unrecognized character: {}".format(ch))
if _type is not None:
# Finish up last query
instructions.append((_type, "".join(arg)))
return instructions
For your example
>>> x = {"a": "stuff"}
>>> print(build_instructions(x, "f[0].a"))
[(<type 'dict'>, 'f'), (<type 'list'>, '0'), (<type 'dict'>, 'a')]
The expected return value is simply the _type (first item) of the next tuple in the instructions. This is very important because it allows us to correctly initialize/reconstruct missing keys.
This means that our first instruction operates on a dict, either sets or gets the key 'f', and is expected to return a list. Similarly, our second instruction operates on a list, either sets or gets the index 0 and is expected to return a dict.
Now let's create our _setattr function. This gets the proper instructions and goes through them, creating key-value pairs as necessary. Finally, it also sets the val we give it.
def _setattr(obj, query, val):
"""
This is a special setattr function that will take in a string query,
interpret it, add the appropriate data structure to obj, and set val.
We only define two actions that are available in our query string:
.x -- dict.__setitem__(x, ...)
[x] -- list.__setitem__(x, ...) OR dict.__setitem__(x, ...)
the calling context determines how this is interpreted.
"""
instructions = build_instructions(obj, query)
for i, (_, arg) in enumerate(instructions[:-1]):
_type = instructions[i + 1][0]
obj = _set(obj, _type, arg)
_type, arg = instructions[-1]
_set(obj, _type, arg, val)
def _set(obj, _type, arg, val=None):
"""
Helper function for calling obj.__setitem__(arg, val or _type()).
"""
if val is not None:
# Time to set our value
_type = type(val)
if isinstance(obj, dict):
if arg not in obj:
# If key isn't in obj, initialize it with _type()
# or set it with val
obj[arg] = (_type() if val is None else val)
obj = obj[arg]
elif isinstance(obj, list):
n = len(obj)
arg = int(arg)
if n > arg:
obj[arg] = (_type() if val is None else val)
else:
# Need to amplify our list, initialize empty values with _type()
obj.extend([_type() for x in range(arg - n + 1)])
obj = obj[arg]
return obj
And just because we can, here's a _getattr function.
def _getattr(obj, query):
"""
Very similar to _setattr. Instead of setting attributes they will be
returned. As expected, an error will be raised if a __getitem__ call
fails.
"""
instructions = build_instructions(obj, query)
for i, (_, arg) in enumerate(instructions[:-1]):
_type = instructions[i + 1][0]
obj = _get(obj, _type, arg)
_type, arg = instructions[-1]
return _get(obj, _type, arg)
def _get(obj, _type, arg):
"""
Helper function for calling obj.__getitem__(arg).
"""
if isinstance(obj, dict):
obj = obj[arg]
elif isinstance(obj, list):
arg = int(arg)
obj = obj[arg]
return obj
In action:
>>> x = {"a": "stuff"}
>>> _setattr(x, "f[0].a", "test")
>>> print x
{'a': 'stuff', 'f': [{'a': 'test'}]}
>>> print _getattr(x, "f[0].a")
"test"
>>> x = ["one", "two"]
>>> _setattr(x, "3[0].a", "test")
>>> print x
['one', 'two', [], [{'a': 'test'}]]
>>> print _getattr(x, "3[0].a")
"test"
Now for some cool stuff. Unlike python, our _setattr function can set unhashable dict keys.
x = []
_setattr(x, "1.4", "asdf")
print x
[{}, {'4': 'asdf'}] # A list, which isn't hashable
>>> y = {"a": "stuff"}
>>> _setattr(y, "f[1.4]", "test") # We're indexing f with 1.4, which is a list!
>>> print y
{'a': 'stuff', 'f': [{}, {'4': 'test'}]}
>>> print _getattr(y, "f[1.4]") # Works for _getattr too
"test"
We aren't really using unhashable dict keys, but it looks like we are in our query language so who cares, right!
Finally, you can run multiple _setattr calls on the same object, just give it a try yourself.
>>> class D(dict):
... def __missing__(self, k):
... ret = self[k] = D()
... return ret
...
>>> x=D()
>>> x['f'][0]['a'] = 'whatever'
>>> x
{'f': {0: {'a': 'whatever'}}}
You can hack something together by fixing two problems:
List that automatically grows when accessed out of bounds (PaddedList)
A way to delay the decision of what to create (list of dict) until you accessed it by the first time (DictOrList)
So the code will look like this:
import collections
class PaddedList(list):
""" List that grows automatically up to the max index ever passed"""
def __init__(self, padding):
self.padding = padding
def __getitem__(self, key):
if isinstance(key, int) and len(self) <= key:
self.extend(self.padding() for i in xrange(key + 1 - len(self)))
return super(PaddedList, self).__getitem__(key)
class DictOrList(object):
""" Object proxy that delays the decision of being a List or Dict """
def __init__(self, parent):
self.parent = parent
def __getitem__(self, key):
# Type of the structure depends on the type of the key
if isinstance(key, int):
obj = PaddedList(MyDict)
else:
obj = MyDict()
# Update parent references with the selected object
parent_seq = (self.parent if isinstance(self.parent, dict)
else xrange(len(self.parent)))
for i in parent_seq:
if self == parent_seq[i]:
parent_seq[i] = obj
break
return obj[key]
class MyDict(collections.defaultdict):
def __missing__(self, key):
ret = self[key] = DictOrList(self)
return ret
def pprint_mydict(d):
""" Helper to print MyDict as dicts """
print d.__str__().replace('defaultdict(None, {', '{').replace('})', '}')
x = MyDict()
x['f'][0]['a'] = 'whatever'
y = MyDict()
y['f'][10]['a'] = 'whatever'
pprint_mydict(x)
pprint_mydict(y)
And the output of x and y will be:
{'f': [{'a': 'whatever'}]}
{'f': [{}, {}, {}, {}, {}, {}, {}, {}, {}, {}, {'a': 'whatever'}]}
The trick consist on creating a defaultdict of objects that can be either a dict or a list depending how you access it.
So when you have the assigment x['f'][10]['a'] = 'whatever' it will work the following way:
Get X['f']. It wont exist so it will return a DictOrList object for the index 'f'
Get X['f'][10]. DictOrList.getitem will be called with an integer index. The DictOrList object will replace itself in the parent collection by a PaddedList
Access the 11th element in the PaddedList will grow it by 11 elements and will return the MyDict element in that position
Assign "whatever" to x['f'][10]['a']
Both PaddedList and DictOrList are bit hacky, but after all the assignments there is no more magic, you have an structure of dicts and lists.
It is possible to synthesize recursively setting items/attributes by overriding __getitem__ to return a return a proxy that can set a value in the original function.
I happen to be working on a library that does a few things similar to this, so I was working on a class that can dynamically assign its own subclasses at instantiation. It makes working with this sort of thing easier, but if that kind of hacking makes you squeamish, you can get similar behavior by creating a ProxyObject similar to the one I create and by creating the individual classes used by the ProxyObject dynamically in the a function. Something like
class ProxyObject(object):
... #see below
def instanciateProxyObjcet(val):
class ProxyClassForVal(ProxyObject,val.__class__):
pass
return ProxyClassForVal(val)
You can use dictionary like I've used in FlexibleObject below would make that implementation significantly more efficient if this is the way you implement it. The code I will providing uses the FlexibleObject though. Right now it only supports classes that, like almost all of Python's builtin classes are capable of being generated by taking an instance of themselves as their sole argument to their __init__/__new__. In the next week or two, I'll add support for anything pickleable, and link to a github repository that contains it. Here's the code:
class FlexibleObject(object):
""" A FlexibleObject is a baseclass for allowing type to be declared
at instantiation rather than in the declaration of the class.
Usage:
class DoubleAppender(FlexibleObject):
def append(self,x):
super(self.__class__,self).append(x)
super(self.__class__,self).append(x)
instance1 = DoubleAppender(list)
instance2 = DoubleAppender(bytearray)
"""
classes = {}
def __new__(cls,supercls,*args,**kws):
if isinstance(supercls,type):
supercls = (supercls,)
else:
supercls = tuple(supercls)
if (cls,supercls) in FlexibleObject.classes:
return FlexibleObject.classes[(cls,supercls)](*args,**kws)
superclsnames = tuple([c.__name__ for c in supercls])
name = '%s%s' % (cls.__name__,superclsnames)
d = dict(cls.__dict__)
d['__class__'] = cls
if cls == FlexibleObject:
d.pop('__new__')
try:
d.pop('__weakref__')
except:
pass
d['__dict__'] = {}
newcls = type(name,supercls,d)
FlexibleObject.classes[(cls,supercls)] = newcls
return newcls(*args,**kws)
Then to use this to use this to synthesize looking up attributes and items of a dictionary-like object you can do something like this:
class ProxyObject(FlexibleObject):
#classmethod
def new(cls,obj,quickrecdict,path,attribute_marker):
self = ProxyObject(obj.__class__,obj)
self.__dict__['reference'] = quickrecdict
self.__dict__['path'] = path
self.__dict__['attr_mark'] = attribute_marker
return self
def __getitem__(self,item):
path = self.__dict__['path'] + [item]
ref = self.__dict__['reference']
return ref[tuple(path)]
def __setitem__(self,item,val):
path = self.__dict__['path'] + [item]
ref = self.__dict__['reference']
ref.dict[tuple(path)] = ProxyObject.new(val,ref,
path,self.__dict__['attr_mark'])
def __getattribute__(self,attr):
if attr == '__dict__':
return object.__getattribute__(self,'__dict__')
path = self.__dict__['path'] + [self.__dict__['attr_mark'],attr]
ref = self.__dict__['reference']
return ref[tuple(path)]
def __setattr__(self,attr,val):
path = self.__dict__['path'] + [self.__dict__['attr_mark'],attr]
ref = self.__dict__['reference']
ref.dict[tuple(path)] = ProxyObject.new(val,ref,
path,self.__dict__['attr_mark'])
class UniqueValue(object):
pass
class QuickRecursiveDict(object):
def __init__(self,dictionary={}):
self.dict = dictionary
self.internal_id = UniqueValue()
self.attr_marker = UniqueValue()
def __getitem__(self,item):
if item in self.dict:
val = self.dict[item]
try:
if val.__dict__['path'][0] == self.internal_id:
return val
else:
raise TypeError
except:
return ProxyObject.new(val,self,[self.internal_id,item],
self.attr_marker)
try:
if item[0] == self.internal_id:
return ProxyObject.new(KeyError(),self,list(item),
self.attr_marker)
except TypeError:
pass #Item isn't iterable
return ProxyObject.new(KeyError(),self,[self.internal_id,item],
self.attr_marker)
def __setitem__(self,item,val):
self.dict[item] = val
The particulars of the implementation will vary depending on what you want. It's obviously significantly easier to just override __getitem__ in the proxy than it is to override both __getitem__ and __getattribute__ or __getattr__. The syntax you are using in setbydot makes it look like you would be happiest with some solution that overrides a mixture of the two.
If you are just using the dictionary to compare values, using =,<=,>= etc. Overriding __getattribute__ works really nicely. If you are wanting to do something more sophisticated, you will probably be better off overriding __getattr__ and doing some checks in __setattr__ to determine whether you want to be synthesizing setting the attribute by setting a value in the dictionary or whether you want to be actually setting the attribute on the item you've obtained. Or you might want to handle it so that if your object has an attribute, __getattribute__ returns a proxy to that attribute and __setattr__ always just sets the attribute in the object (in which case, you can completely omit it). All of these things depend on exactly what you are trying to use the dictionary for.
You also may want to create __iter__ and the like. It takes a little bit of effort to make them, but the details should follow from the implementation of __getitem__ and __setitem__.
Finally, I'm going to briefly summarize the behavior of the QuickRecursiveDict in case it's not immediately clear from inspection. The try/excepts are just shorthand for checking to see whether the ifs can be performed. The one major defect of synthesizing the recursive setting rather than find a way to do it is that you can no longer be raising KeyErrors when you try to access a key that hasn't been set. However, you can come pretty close by returning a subclass of KeyError which is what I do in the example. I haven't tested it so I won't add it to the code, but you may want to pass in some human-readable representation of the key to KeyError.
But aside from all that it works rather nicely.
>>> qrd = QuickRecursiveDict
>>> qrd[0][13] # returns an instance of a subclass of KeyError
>>> qrd[0][13] = 9
>>> qrd[0][13] # 9
>>> qrd[0][13]['forever'] = 'young'
>>> qrd[0][13] # 9
>>> qrd[0][13]['forever'] # 'young'
>>> qrd[0] # returns an instance of a subclass of KeyError
>>> qrd[0] = 0
>>> qrd[0] # 0
>>> qrd[0][13]['forever'] # 'young'
One more caveat, the things being returned is not quite what it looks like. It's a proxy to what it looks like. If you want the int 9, you need int(qrd[0][13]) not qrd[0][13]. For ints this doesn't matter much since, +,-,= and all that bypass __getattribute__ but for lists, you would lose attributes like append if you didn't recast them. (You'd keep len and other builtin methods, just not attributes of list. You lose __len__.)
So that's it. The code's a little bit convoluted, so let me know if you have any questions. I probably can't answer them until tonight unless the answer's really brief. I wish I saw this question sooner, it's a really cool question, and I'll try to update a cleaner solution soon. I had fun trying to code a solution into the wee hours of last night. :)
Apologies if this question has already been asked but I do not think I know the correct terminology to search for an appropriate solution through google.
I would like to select an object from a list of objects by the value of it's attribute, for example:
class Example():
def __init__(self):
self.pList = []
def addPerson(self,name,number):
self.pList.append(Person(self,name,number))
class Person():
def __init__(self,name,number):
self.nom = name
self.num = number
a = Example()
a.addPerson('dave',123)
a.addPerson('mike',345)
a.pList #.... somehow select dave by giving the value 123
in my case the number will always be unique
Thanks for the help
One option is to use the next() built-in:
dave = next(person for person in a.pList if person.num == 123)
This will throw StopIteration if nothing is found. You can use the two-argument form of next() to provide a default value for that case:
dave = next(
(person for person in a.pList if person.num == 123),
None,
)
A slightly more verbose alternative is a for loop:
for person in a.pList:
if person.num == 123:
break
else:
print "Not found."
person = None
dave = person
If those nom's are unique keys, and all you are ever going to do is access your persons using this unique key you should indeed rather use a dictionary.
However if you want to add more attributes over time and if you like to be able to retrieve one or more person by any of those attributes, you might want to go with a more complex solution:
class Example():
def __init__(self):
self.__pList = []
def addPerson(self,name,number):
self.__pList.append(Person(name,number))
def findPerson(self, **kwargs):
return next(self.__iterPerson(**kwargs))
def allPersons(self, **kwargs):
return list(self.__iterPerson(**kwargs))
def __iterPerson(self, **kwargs):
return (person for person in self.__pList if person.match(**kwargs))
class Person():
def __init__(self,name,number):
self.nom = name
self.num = number
def __repr__(self):
return "Person('%s', %d)" % (self.nom, self.num)
def match(self, **kwargs):
return all(getattr(self, key) == val for (key, val) in kwargs.items())
So let's assume we got one Mike and two Dave's
a = Example()
a.addPerson('dave',123)
a.addPerson('mike',345)
a.addPerson('dave',678)
Now you can find persons by number:
>>> a.findPerson(num=345)
Person('mike', 345)
Or by name:
>>> a.allPersons(nom='dave')
[Person('dave', 123), Person('dave', 678)]
Or both:
>>> a.findPerson(nom='dave', num=123)
Person('dave', 123)
The terminology you need is 'map' or 'dictionnary' : this will lead you to the right page in the python doc.
Extremely basic example:
>>> a = {123:'dave', 345:'mike'}
>>> a[123]
'dave'
The missing underscore makes plist a public property. I don't think that's what you want, since it does not encapsulate the functionality and you could call a.plist.append instead of a.addPerson.
class Example():
...
def filter(self, criteria):
for p in self.plist:
if criteria(p):
yield p
def getByNum(self, num):
return self.filter(lambda p: p.num == num)
dave = next(a.getByNum(123))
If the numbers are unique, you may also consider using a dictionary that maps from number to name or person instead of a list. But that's up to your implementation.