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Add value of the already existing key in dict in a form of container in Python

Is there any built-in function that would do the following?
dictionary = {‘a’:1, ‘b’:2, ‘c’:3}
dictionary.update(c=10)
# what happens
dictionary ---- {‘a’:1, ‘b’:2, ‘c’:10}
# what I want to happen:
dictionary ---- {‘a’:1, ‘b’:2, ‘c’:(3, 10)}
By default if keys are the same, later key would override earlier one.
If the key is already present in dict, the value of the new key: value pair would be added to already existing value in a form of container, like tuple, or list or set.
I can write a helper function to do so but I believe it should be something built-in for this matter.

You can do this
from collections import defaultdict
d = defaultdict(list)
d["a"].append(1)
d["b"].append(2)
d["c"].append(3)
d["c"].append(10)
print(d)
Result
defaultdict(list, {'a': [1], 'b': [2], 'c': [3, 10]})

Your desired solution is not very elegant, so I am going to propose an alternative one.
Tuples are immutable. Let's use lists instead, because we can easily append to them.
The data type of the values should be consistent. Use lists in any case, even for single values.
Let's use a defaultdict such that we don't have to initialize lists manually.
Putting it together:
>>> from collections import defaultdict
>>> d = defaultdict(list)
>>> for v, k in enumerate('abc', 1):
... d[k].append(v)
...
>>> d
defaultdict(<class 'list'>, {'a': [1], 'b': [2], 'c': [3]})
>>> d['c'].append(10)
>>> d
defaultdict(<class 'list'>, {'a': [1], 'b': [2], 'c': [3, 10]})

You could rewrite the update function by creating a new class:
In Python bulitins.py:
def update(self, E=None, **F): # known special case of dict.update
"""
D.update([E, ]**F) -> None. Update D from dict/iterable E and F.
If E is present and has a .keys() method, then does: for k in E: D[k] = E[k]
If E is present and lacks a .keys() method, then does: for k, v in E: D[k] = v
In either case, this is followed by: for k in F: D[k] = F[k]
"""
pass
So I write this(Inherit from UserDict, suggested by #timgeb):
from collections import UserDict
class CustomDict(UserDict):
def __init__(self):
super().__init__()
def update(self, E=None, **F) -> None:
if E:
if isinstance(E, dict):
for k in E:
self[k] = E[k]
else:
for k, v in E:
self[k] = v
else:
if isinstance(F, dict):
for key in F:
if isinstance(self[key], list):
self[key].append(F[key])
else:
self[key] = [self[key], F[key]]
dictionary = CustomDict()
dictionary.update({'a': 1, 'b': 2, 'c': 3})
print(dictionary)
dictionary.update(a=3)
print(dictionary)
dictionary.update(a=4)
print(dictionary)
Result:
{'a': 1, 'b': 2, 'c': 3}
{'a': [1, 3], 'b': 2, 'c': 3}
{'a': [1, 3, 4], 'b': 2, 'c': 3}
Maybe there are some logic errors in my code,but welcome to point out.

Perhaps you could use something like:
dictionary = {'a':1, 'b':2, 'c':3}
dictionary.update({'c': 10 if not dictionary.get('c') else tuple([dictionary['c'],] + [10,])})
# {'a': 1, 'b': 2, 'c': (3, 10)}
But it should probably be wrapped into a function to make things clean. The general pattern would be (I suppose, based on your question):
dict = {...}
if 'a' not in dict:
do_this() # just add it to the dict?
else:
do_that() # build a tuple or list?
In your above question you're mixing types -- I'm not sure if you want that, a more pythonic approach might be to have all the values as list and use a defaultdict.

Create dict from local variables like JavaScript `{ var1, var2, ...}` with Python

For JavaScript, object can be created by
a=1;
obj={ a } // JSON.stringify(obj) == '{"a":1}'
this make the obj has key a and its value is 1
For Python code is:
a = 1
obj = { 'a': a }
Is it possible to create a function or class that make code like this?
a = 1
obj = func_or_class(a) # obj == {'a': 1}

It all depends on the way you need to work with the data, really. Your methods above are sound, especially the dict comprehension. So other than the old-fashioned way:
{"key": "value", ... }
I'd say you're doing it right. If you have a specific example, you can edit your question and I will try to work off that.
Hope that helps!

How about this?
var1 = 1
var2 = 2
var3 = 3
my_dict = {k: v for k, v in locals().items() if k.startswith('var')}
or
var1 = 1
var2 = 2
var3 = 3
var4 = "abc"
my_dict = {k: v for k, v in locals().items() if isinstance(v, int) or isinstance(v, str)}

Since all the built-in members of locals() starts and ends with __, you can filter them out by these prefix and suffix. In addition, you can filter out functions by using callable:
For example:
a = 1
b = 2
output = {k:v for k,v in locals().items() if not (k.startswith("__") and k.endswith("__")) and not callable(v)}
print(output) # output: {'a': 1, 'b': 2}

After trying a lot, the following code is nearly what I want:
class LocalDict(dict):
def __new__(cls, spaces, attrs):
return {k: v for k, v in spaces.items() if k in attrs.split()}
def foo():
a = 1
b = c = 2
my_dict = LocalDict(locals(), 'a b c')
return my_dict
print(foo()) # {'a': 1, 'b': 2, 'c': 2}
And I wonder whether there is some way that I don't need to pass locals to the LocalDict class.

How to add a list to a dictionary? [duplicate]

I would like to be able to get the name of a variable as a string but I don't know if Python has that much introspection capabilities. Something like:
>>> print(my_var.__name__)
'my_var'
I want to do that because I have a bunch of variables I'd like to turn into a dictionary like :
bar = True
foo = False
>>> my_dict = dict(bar=bar, foo=foo)
>>> print my_dict
{'foo': False, 'bar': True}
But I'd like something more automatic than that.
Python have locals() and vars(), so I guess there is a way.

As unwind said, this isn't really something you do in Python - variables are actually name mappings to objects.
However, here's one way to try and do it:
>>> a = 1
>>> for k, v in list(locals().iteritems()):
if v is a:
a_as_str = k
>>> a_as_str
a
>>> type(a_as_str)
'str'

I've wanted to do this quite a lot. This hack is very similar to rlotun's suggestion, but it's a one-liner, which is important to me:
blah = 1
blah_name = [ k for k,v in locals().iteritems() if v is blah][0]
Python 3+
blah = 1
blah_name = [ k for k,v in locals().items() if v is blah][0]

Are you trying to do this?
dict( (name,eval(name)) for name in ['some','list','of','vars'] )
Example
>>> some= 1
>>> list= 2
>>> of= 3
>>> vars= 4
>>> dict( (name,eval(name)) for name in ['some','list','of','vars'] )
{'list': 2, 'some': 1, 'vars': 4, 'of': 3}

This is a hack. It will not work on all Python implementations distributions (in particular, those that do not have traceback.extract_stack.)
import traceback
def make_dict(*expr):
(filename,line_number,function_name,text)=traceback.extract_stack()[-2]
begin=text.find('make_dict(')+len('make_dict(')
end=text.find(')',begin)
text=[name.strip() for name in text[begin:end].split(',')]
return dict(zip(text,expr))
bar=True
foo=False
print(make_dict(bar,foo))
# {'foo': False, 'bar': True}
Note that this hack is fragile:
make_dict(bar,
foo)
(calling make_dict on 2 lines) will not work.
Instead of trying to generate the dict out of the values foo and bar,
it would be much more Pythonic to generate the dict out of the string variable names 'foo' and 'bar':
dict([(name,locals()[name]) for name in ('foo','bar')])

This is not possible in Python, which really doesn't have "variables". Python has names, and there can be more than one name for the same object.

I think my problem will help illustrate why this question is useful, and it may give a bit more insight into how to answer it. I wrote a small function to do a quick inline head check on various variables in my code. Basically, it lists the variable name, data type, size, and other attributes, so I can quickly catch any mistakes I've made. The code is simple:
def details(val):
vn = val.__name__ # If such a thing existed
vs = str(val)
print("The Value of "+ str(vn) + " is " + vs)
print("The data type of " + vn + " is " + str(type(val)))
So if you have some complicated dictionary / list / tuple situation, it would be quite helpful to have the interpreter return the variable name you assigned. For instance, here is a weird dictionary:
m = 'abracadabra'
mm=[]
for n in m:
mm.append(n)
mydic = {'first':(0,1,2,3,4,5,6),'second':mm,'third':np.arange(0.,10)}
details(mydic)
The Value of mydic is {'second': ['a', 'b', 'r', 'a', 'c', 'a', 'd', 'a', 'b', 'r', 'a'], 'third': array([ 0., 1., 2., 3., 4., 5., 6., 7., 8., 9.]), 'first': [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]}
The data type of mydic is <type 'dict'>
details(mydic['first'])
The Value of mydic['first'] is (0, 1, 2, 3, 4, 5, 6)]
The data type of mydic['first'] is <type 'list'>
details(mydic.keys())
The Value of mydic.keys() is ['second', 'third', 'first']
The data type of mydic.keys() is <type 'tuple'>
details(mydic['second'][0])
The Value of mydic['second'][0] is a
The data type of mydic['second'][0] is <type 'str'>
I'm not sure if I put this in the right place, but I thought it might help. I hope it does.

I wrote a neat little useful function based on the answer to this question. I'm putting it here in case it's useful.
def what(obj, callingLocals=locals()):
"""
quick function to print name of input and value.
If not for the default-Valued callingLocals, the function would always
get the name as "obj", which is not what I want.
"""
for k, v in list(callingLocals.items()):
if v is obj:
name = k
print(name, "=", obj)
usage:
>> a = 4
>> what(a)
a = 4
>>|

I find that if you already have a specific list of values, that the way described by #S. Lotts is the best; however, the way described below works well to get all variables and Classes added throughout the code WITHOUT the need to provide variable name though you can specify them if you want. Code can be extend to exclude Classes.
import types
import math # mainly showing that you could import what you will before d
# Everything after this counts
d = dict(globals())
def kv_test(k,v):
return (k not in d and
k not in ['d','args'] and
type(v) is not types.FunctionType)
def magic_print(*args):
if len(args) == 0:
return {k:v for k,v in globals().iteritems() if kv_test(k,v)}
else:
return {k:v for k,v in magic_print().iteritems() if k in args}
if __name__ == '__main__':
foo = 1
bar = 2
baz = 3
print magic_print()
print magic_print('foo')
print magic_print('foo','bar')
Output:
{'baz': 3, 'foo': 1, 'bar': 2}
{'foo': 1}
{'foo': 1, 'bar': 2}

In python 3 this is easy
myVariable = 5
for v in locals():
if id(v) == id("myVariable"):
print(v, locals()[v])
this will print:
myVariable 5

Python3. Use inspect to capture the calling local namespace then use ideas presented here. Can return more than one answer as has been pointed out.
def varname(var):
import inspect
frame = inspect.currentframe()
var_id = id(var)
for name in frame.f_back.f_locals.keys():
try:
if id(eval(name)) == var_id:
return(name)
except:
pass

Here's the function I created to read the variable names. It's more general and can be used in different applications:
def get_variable_name(*variable):
'''gets string of variable name
inputs
variable (str)
returns
string
'''
if len(variable) != 1:
raise Exception('len of variables inputed must be 1')
try:
return [k for k, v in locals().items() if v is variable[0]][0]
except:
return [k for k, v in globals().items() if v is variable[0]][0]
To use it in the specified question:
>>> foo = False
>>> bar = True
>>> my_dict = {get_variable_name(foo):foo,
get_variable_name(bar):bar}
>>> my_dict
{'bar': True, 'foo': False}

In reading the thread, I saw an awful lot of friction. It's easy enough to give
a bad answer, then let someone give the correct answer. Anyway, here is what I found.
From: [effbot.org] (http://effbot.org/zone/python-objects.htm#names)
The names are a bit different — they’re not really properties of the object, and the object itself doesn't know what it’s called.
An object can have any number of names, or no name at all.
Names live in namespaces (such as a module namespace, an instance namespace, a function’s local namespace).
Note: that it says the object itself doesn’t know what it’s called, so that was the clue. Python objects are not self-referential. Then it says, Names live in namespaces. We have this in TCL/TK. So maybe my answer will help (but it did help me)
jj = 123
print eval("'" + str(id(jj)) + "'")
print dir()
166707048
['__builtins__', '__doc__', '__file__', '__name__', '__package__', 'jj']
So there is 'jj' at the end of the list.
Rewrite the code as:
jj = 123
print eval("'" + str(id(jj)) + "'")
for x in dir():
print id(eval(x))
161922920
['__builtins__', '__doc__', '__file__', '__name__', '__package__', 'jj']
3077447796
136515736
3077408320
3077656800
136515736
161922920
This nasty bit of code id's the name of variable/object/whatever-you-pedantics-call-it.
So, there it is. The memory address of 'jj' is the same when we look for it directly, as when we do the dictionary look up in global name space. I'm sure you can make a function to do this. Just remember which namespace your variable/object/wypci is in.
QED.

I wrote the package sorcery to do this kind of magic robustly. You can write:
from sorcery import dict_of
my_dict = dict_of(foo, bar)

Maybe I'm overthinking this but..
str_l = next((k for k,v in locals().items() if id(l) == id(v)))
>>> bar = True
>>> foo = False
>>> my_dict=dict(bar=bar, foo=foo)
>>> next((k for k,v in locals().items() if id(bar) == id(v)))
'bar'
>>> next((k for k,v in locals().items() if id(foo) == id(v)))
'foo'
>>> next((k for k,v in locals().items() if id(my_dict) == id(v)))
'my_dict'

import re
import traceback
pattren = re.compile(r'[\W+\w+]*get_variable_name\((\w+)\)')
def get_variable_name(x):
return pattren.match( traceback.extract_stack(limit=2)[0][3]) .group(1)
a = 1
b = a
c = b
print get_variable_name(a)
print get_variable_name(b)
print get_variable_name(c)

I uploaded a solution to pypi. It's a module defining an equivalent of C#'s nameof function.
It iterates through bytecode instructions for the frame its called in, getting the names of variables/attributes passed to it. The names are found in the .argrepr of LOAD instructions following the function's name.

Most objects don't have a __name__ attribute. (Classes, functions, and modules do; any more builtin types that have one?)
What else would you expect for print(my_var.__name__) other than print("my_var")? Can you simply use the string directly?
You could "slice" a dict:
def dict_slice(D, keys, default=None):
return dict((k, D.get(k, default)) for k in keys)
print dict_slice(locals(), ["foo", "bar"])
# or use set literal syntax if you have a recent enough version:
print dict_slice(locals(), {"foo", "bar"})
Alternatively:
throw = object() # sentinel
def dict_slice(D, keys, default=throw):
def get(k):
v = D.get(k, throw)
if v is not throw:
return v
if default is throw:
raise KeyError(k)
return default
return dict((k, get(k)) for k in keys)

Well, I encountered the very same need a few days ago and had to get a variable's name which was pointing to the object itself.
And why was it so necessary?
In short I was building a plug-in for Maya. The core plug-in was built using C++ but the GUI is drawn through Python(as its not processor intensive). Since I, as yet, don't know how to return multiple values from the plug-in except the default MStatus, therefore to update a dictionary in Python I had to pass the the name of the variable, pointing to the object implementing the GUI and which contained the dictionary itself, to the plug-in and then use the MGlobal::executePythonCommand() to update the dictionary from the global scope of Maya.
To do that what I did was something like:
import time
class foo(bar):
def __init__(self):
super(foo, self).__init__()
self.time = time.time() #almost guaranteed to be unique on a single computer
def name(self):
g = globals()
for x in g:
if isinstance(g[x], type(self)):
if g[x].time == self.time:
return x
#or you could:
#return filter(None,[x if g[x].time == self.time else None for x in g if isinstance(g[x], type(self))])
#and return all keys pointing to object itself
I know that it is not the perfect solution in in the globals many keys could be pointing to the same object e.g.:
a = foo()
b = a
b.name()
>>>b
or
>>>a
and that the approach isn't thread-safe. Correct me if I am wrong.
At least this approach solved my problem by getting the name of any variable in the global scope which pointed to the object itself and pass it over to the plug-in, as argument, for it use internally.
I tried this on int (the primitive integer class) but the problem is that these primitive classes don't get bypassed (please correct the technical terminology used if its wrong). You could re-implement int and then do int = foo but a = 3 will never be an object of foo but of the primitive. To overcome that you have to a = foo(3) to get a.name() to work.

With python 2.7 and newer there is also dictionary comprehension which makes it a bit shorter. If possible I would use getattr instead eval (eval is evil) like in the top answer. Self can be any object which has the context your a looking at. It can be an object or locals=locals() etc.
{name: getattr(self, name) for name in ['some', 'vars', 'here]}

I was working on a similar problem. #S.Lott said "If you have the list of variables, what's the point of "discovering" their names?" And my answer is just to see if it could be done and if for some reason you want to sort your variables by type into lists. So anyways, in my research I came came across this thread and my solution is a bit expanded and is based on #rlotun solution. One other thing, #unutbu said, "This idea has merit, but note that if two variable names reference the same value (e.g. True), then an unintended variable name might be returned." In this exercise that was true so I dealt with it by using a list comprehension similar to this for each possibility: isClass = [i for i in isClass if i != 'item']. Without it "item" would show up in each list.
__metaclass__ = type
from types import *
class Class_1: pass
class Class_2: pass
list_1 = [1, 2, 3]
list_2 = ['dog', 'cat', 'bird']
tuple_1 = ('one', 'two', 'three')
tuple_2 = (1000, 2000, 3000)
dict_1 = {'one': 1, 'two': 2, 'three': 3}
dict_2 = {'dog': 'collie', 'cat': 'calico', 'bird': 'robin'}
x = 23
y = 29
pie = 3.14159
eee = 2.71828
house = 'single story'
cabin = 'cozy'
isClass = []; isList = []; isTuple = []; isDict = []; isInt = []; isFloat = []; isString = []; other = []
mixedDataTypes = [Class_1, list_1, tuple_1, dict_1, x, pie, house, Class_2, list_2, tuple_2, dict_2, y, eee, cabin]
print '\nMIXED_DATA_TYPES total count:', len(mixedDataTypes)
for item in mixedDataTypes:
try:
# if isinstance(item, ClassType): # use this for old class types (before 3.0)
if isinstance(item, type):
for k, v in list(locals().iteritems()):
if v is item:
mapping_as_str = k
isClass.append(mapping_as_str)
isClass = [i for i in isClass if i != 'item']
elif isinstance(item, ListType):
for k, v in list(locals().iteritems()):
if v is item:
mapping_as_str = k
isList.append(mapping_as_str)
isList = [i for i in isList if i != 'item']
elif isinstance(item, TupleType):
for k, v in list(locals().iteritems()):
if v is item:
mapping_as_str = k
isTuple.append(mapping_as_str)
isTuple = [i for i in isTuple if i != 'item']
elif isinstance(item, DictType):
for k, v in list(locals().iteritems()):
if v is item:
mapping_as_str = k
isDict.append(mapping_as_str)
isDict = [i for i in isDict if i != 'item']
elif isinstance(item, IntType):
for k, v in list(locals().iteritems()):
if v is item:
mapping_as_str = k
isInt.append(mapping_as_str)
isInt = [i for i in isInt if i != 'item']
elif isinstance(item, FloatType):
for k, v in list(locals().iteritems()):
if v is item:
mapping_as_str = k
isFloat.append(mapping_as_str)
isFloat = [i for i in isFloat if i != 'item']
elif isinstance(item, StringType):
for k, v in list(locals().iteritems()):
if v is item:
mapping_as_str = k
isString.append(mapping_as_str)
isString = [i for i in isString if i != 'item']
else:
for k, v in list(locals().iteritems()):
if v is item:
mapping_as_str = k
other.append(mapping_as_str)
other = [i for i in other if i != 'item']
except (TypeError, AttributeError), e:
print e
print '\n isClass:', len(isClass), isClass
print ' isList:', len(isList), isList
print ' isTuple:', len(isTuple), isTuple
print ' isDict:', len(isDict), isDict
print ' isInt:', len(isInt), isInt
print ' isFloat:', len(isFloat), isFloat
print 'isString:', len(isString), isString
print ' other:', len(other), other
# my output and the output I wanted
'''
MIXED_DATA_TYPES total count: 14
isClass: 2 ['Class_1', 'Class_2']
isList: 2 ['list_1', 'list_2']
isTuple: 2 ['tuple_1', 'tuple_2']
isDict: 2 ['dict_1', 'dict_2']
isInt: 2 ['x', 'y']
isFloat: 2 ['pie', 'eee']
isString: 2 ['house', 'cabin']
other: 0 []
'''

you can use easydict
>>> from easydict import EasyDict as edict
>>> d = edict({'foo':3, 'bar':{'x':1, 'y':2}})
>>> d.foo
3
>>> d.bar.x
1
>>> d = edict(foo=3)
>>> d.foo
3
another example:
>>> d = EasyDict(log=False)
>>> d.debug = True
>>> d.items()
[('debug', True), ('log', False)]

On python3, this function will get the outer most name in the stack:
import inspect
def retrieve_name(var):
"""
Gets the name of var. Does it from the out most frame inner-wards.
:param var: variable to get name from.
:return: string
"""
for fi in reversed(inspect.stack()):
names = [var_name for var_name, var_val in fi.frame.f_locals.items() if var_val is var]
if len(names) > 0:
return names[0]
It is useful anywhere on the code. Traverses the reversed stack looking for the first match.

While this is probably an awful idea, it is along the same lines as rlotun's answer but it'll return the correct result more often.
import inspect
def getVarName(getvar):
frame = inspect.currentframe()
callerLocals = frame.f_back.f_locals
for k, v in list(callerLocals.items()):
if v is getvar():
callerLocals.pop(k)
try:
getvar()
callerLocals[k] = v
except NameError:
callerLocals[k] = v
del frame
return k
del frame
You call it like this:
bar = True
foo = False
bean = False
fooName = getVarName(lambda: foo)
print(fooName) # prints "foo"

should get list then return
def get_var_name(**kwargs):
"""get variable name
get_var_name(var = var)
Returns:
[str] -- var name
"""
return list(kwargs.keys())[0]

It will not return the name of variable but you can create dictionary from global variable easily.
class CustomDict(dict):
def __add__(self, other):
return CustomDict({**self, **other})
class GlobalBase(type):
def __getattr__(cls, key):
return CustomDict({key: globals()[key]})
def __getitem__(cls, keys):
return CustomDict({key: globals()[key] for key in keys})
class G(metaclass=GlobalBase):
pass
x, y, z = 0, 1, 2
print('method 1:', G['x', 'y', 'z']) # Outcome: method 1: {'x': 0, 'y': 1, 'z': 2}
print('method 2:', G.x + G.y + G.z) # Outcome: method 2: {'x': 0, 'y': 1, 'z': 2}

With python-varname you can easily do it:
pip install python-varname
from varname import Wrapper
foo = Wrapper(True)
bar = Wrapper(False)
your_dict = {val.name: val.value for val in (foo, bar)}
print(your_dict)
# {'foo': True, 'bar': False}
Disclaimer: I'm the author of that python-varname library.

>>> a = 1
>>> b = 1
>>> id(a)
34120408
>>> id(b)
34120408
>>> a is b
True
>>> id(a) == id(b)
True
this way get varname for a maybe 'a' or 'b'.

Dictionary initializing in python

Is there any way to refer to the dict keys in the initialization body using one line and using dict keys "a" and "b"
Example:
def func(a,b)
return {"a":longComputation1(), "b":longComputation2(), sum_a_b:?????}
Please don't change semanthics of code. This just an example.

Use the function parameters:
>>> def func(a, b):
... return {"a": a, "b": b, "sum_a_b": a + b}
...
>>> func(1, 2)
{'a': 1, 'b': 2, 'sum_a_b': 3}
UPDATE Question changed after I posted the above code; Use jonrsharpe's solution.

Short answer: no.
This would have to be done over multiple lines:
def func():
d = {"a": longComputation1(),
"b": longComputation2()}
d.update(sum_a_b = d['a'] + d['b'])
return d

Use a function to create the dict and define the names of the keys for the sum in a key named sum:
def sum_dict(**kwargs):
result = {}
total = 0
sum_keys = kwargs["sum"]
del kwargs["sum"]
for key, value in kwargs.items():
val = value()
result[key] = val
if key in sum_keys:
total += val
result["sum_" + "_".join(sum_keys)] = total
return result
print(sum_dict(a=lambda: 3,b=lambda: 2,c=lambda: 14, sum=["a", "b"]))
# {'a': 3, 'c': 14, 'b': 2, 'sum_a_b': 5}
To access the keys from a not created dict is not possible.
Another way would be to create a own dict class.

I wonder what the practical application of this is, but if you mean that the key is dynamically constructed at initialisation time from other keys already present in the dictionary:
d = {"a":longComputation1(), "b":longComputation2()}
d['_'.join(['sum'] + d.keys())] = sum(d.values()) # assumes that keys are strings
>>> d
{'a': 10, 'b': 20, 'sum_a_b': 30} # assuming that longComputation1() == 10 and longComputation2() = 20
Sorry that it is not a single line (why the constraint??), but AFAIK you can't refer to the dict's keys during initialisation.

Convert a list into a nested dictionary

For example I have
x = ['a','b','c']
I need to convert it to:
y['a']['b']['c'] = ''
Is that possible?
For the background, I have a config file which contains dotted notation that points to a place in some json data. I'd like to use the dotted notation string to access that specific data in the json file. For example, in the config:
path_to_data = "user.name.first_name"
I'd like my script to recognize that as:
json_data["user"]["name"]["first_name"]
so I can get the value of the first_name field. I converted the original string into a list, and now I don't know how to convert it to a nested dict.
EDIT: There is an existing data structure that I need to apply the dict with. Let's say:
m = {'a': {'b': {'c': 'lolcat'}}}
so that
m['a']['b']['c']
gives me 'lolcat'. If I get the right dictionary structure (as some of the replies did), I would still need to apply this to the existing dictionary 'm'.
So, again, I get this from a config file:
c = 'a.b.c'
That I converted to a list, thinking this will make things easier:
x = ['a','b','c']
Now I have a json-like data structure:
m = {'a': {'b': {'c': 'lolcat'}}}
So the nested dict generated from 'x' should be able to traverse 'm' so that
m['a']['b']['c']
gets me the cat.

li = ['a','b','c']
d = reduce(lambda x, y: {y:x}, reversed(li+['']))
print(d)
print(d['a']['b']['c'])

I guess you also want to include a value in the end. This works for that too:
def get_value(d, l):
if len(l) > 1:
return get_value(d[l[0]], l[1:])
return d[l[0]]
def add_keys(d, l, c=None):
if len(l) > 1:
d[l[0]] = _d = {}
d[l[0]] = d.get(l[0], {})
add_keys(d[l[0]], l[1:], c)
else:
d[l[0]] = c
def main():
d = {}
l1 = ['a', 'b', 'c', 'd']
c1 = 'letters'
l2 = [42, "42", (42,)]
c2 = 42
add_keys(d, l1, c1)
print d
add_keys(d, l2, c2)
print d
if __name__ == '__main__':
main()
It prints:
{'a': {'b': {'c': {'d': 'letters'}}}}
{'a': {'b': {'c': {'d': 'letters'}}}, 42: {'42': {(42,): 42}}}
letters
42
So it surely works. Recursion for the win.

>>> x = ['a','b','c']
>>> y={}
>>> y[x[-1]]=""
>>> x.pop(-1)
'c'
>>> for i in x[::-1]:
... y={i:y}
...
>>> y
{'a': {'b': {'c': ''}}}
>>> y['a']['b']['c']
''

This will work.
#!/usr/bin/python2
from __future__ import print_function
x = ['a','b','c']
def ltod(l):
rv = d = {}
while l:
i = l.pop(0)
d[i] = {}
d = d[i]
return rv
d = ltod(x)
print(d)
print(d["a"]["b"]["c"])
d["a"]["b"]["c"] = "text"
print(d["a"]["b"]["c"])
Outputs:
{'a': {'b': {'c': {}}}}
{}
text

Find below sample that is not very beautiful but quite simple:
path_to_data = "user.name.first_name"
keys = path_to_data.split('.')
t = []
for key in keys[::-1]: # just to iterate in reversed order
if not t:
t.append({k:{}})
else:
t[-1] = ({k: t[-1]})
#t[0] will contain your dictionary

A general solution would be to use collections.defaultdict to create a nested dictionary. Then override __setitem__ for whatever behavior you'd like. This example will do the string parsing as well.
from collections import defaultdict
class nesteddict(defaultdict):
def __init__(self):
defaultdict.__init__(self, nesteddict)
def __setitem__(self, key, value):
keys = key.split('.')
for key in keys[:-1]:
self = self[key]
defaultdict.__setitem__(self, keys[-1], value)
nd = nesteddict()
nd['a.b.c'] = 'lolcat'
assert nd['a']['b']['c'] == 'lolcat'