Related
I have a dictionary where key is string and value is list.
Now while adding a value associated with given key, I always have to check if there is any list yet, otherwise I have to initialize as empty list somewhat like following snippet:
if not k in myDict:
myDict[k] = []
myDict[k].append(v)
I am wondering if there is any way to combine these steps into single one in python 3.7.
There are at least three ways:
Use dict.setdefault
>>> data = {}
>>> data.setdefault('foo', []).append(42)
>>> data
{'foo': [42]}
Use defaultdict, which unlike .setdefault, takes a callable:
>>> from collections import defaultdict
>>> data = defaultdict(list)
>>> data
defaultdict(<class 'list'>, {})
>>> data['foo'].append(42)
>>> data
defaultdict(<class 'list'>, {'foo': [42]})
Finally, subclass dict and implement __missing__:
>>> class MyDict(dict):
... def __missing__(self, key):
... self[key] = value = []
... return value
...
>>> data = MyDict()
>>> data['foo'].append(42)
>>> data
{'foo': [42]}
Note, you can think of the last one as the most flexible, you have access to the actual key that's missing when you deal with it. defaultdict is a class factory, and it generates a subclass of dict as well. But, the callable is not passed any arguments, nevertheless, it is sufficient for most needs.
Further, note that the defaultdict and __missing__ approaches will keep the default behavior, this may be undesirable after you create your data structure, you probably want a KeyError usually, or at least, you don't want mydict[key] to add a key anymore.
In both cases, you can just create a regular dict from the dict subclasses, e.g. dict(data). This should generally be very fast, even for large dict objects, especially if it is a one-time cost. For defaultdict, you can also set the default_factory to None and the old behavior returns:
>>> data = defaultdict(list)
>>> data
defaultdict(<class 'list'>, {})
>>> data['foo']
[]
>>> data['bar']
[]
>>> data
defaultdict(<class 'list'>, {'foo': [], 'bar': []})
>>> data.default_factory = None
>>> data['baz']
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
KeyError: 'baz'
>>>
You can use dict.setdefault:
>>> d = {}
>>> d.setdefault('k', []).append(1)
>>> d
{'k': [1]}
>>> d.setdefault('k', []).append(2)
>>> d
{'k': [1, 2]}
Help on method_descriptor in dict:
dict.setdefault = setdefault(...)
D.setdefault(k[,d]) -> D.get(k,d), also set D[k]=d if k not in D
How do I add a key to an existing dictionary? It doesn't have an .add() method.
You create a new key/value pair on a dictionary by assigning a value to that key
d = {'key': 'value'}
print(d) # {'key': 'value'}
d['mynewkey'] = 'mynewvalue'
print(d) # {'key': 'value', 'mynewkey': 'mynewvalue'}
If the key doesn't exist, it's added and points to that value. If it exists, the current value it points to is overwritten.
I feel like consolidating info about Python dictionaries:
Creating an empty dictionary
data = {}
# OR
data = dict()
Creating a dictionary with initial values
data = {'a': 1, 'b': 2, 'c': 3}
# OR
data = dict(a=1, b=2, c=3)
# OR
data = {k: v for k, v in (('a', 1), ('b',2), ('c',3))}
Inserting/Updating a single value
data['a'] = 1 # Updates if 'a' exists, else adds 'a'
# OR
data.update({'a': 1})
# OR
data.update(dict(a=1))
# OR
data.update(a=1)
Inserting/Updating multiple values
data.update({'c':3,'d':4}) # Updates 'c' and adds 'd'
Python 3.9+:
The update operator |= now works for dictionaries:
data |= {'c':3,'d':4}
Creating a merged dictionary without modifying originals
data3 = {}
data3.update(data) # Modifies data3, not data
data3.update(data2) # Modifies data3, not data2
Python 3.5+:
This uses a new feature called dictionary unpacking.
data = {**data1, **data2, **data3}
Python 3.9+:
The merge operator | now works for dictionaries:
data = data1 | {'c':3,'d':4}
Deleting items in dictionary
del data[key] # Removes specific element in a dictionary
data.pop(key) # Removes the key & returns the value
data.clear() # Clears entire dictionary
Check if a key is already in dictionary
key in data
Iterate through pairs in a dictionary
for key in data: # Iterates just through the keys, ignoring the values
for key, value in d.items(): # Iterates through the pairs
for key in d.keys(): # Iterates just through key, ignoring the values
for value in d.values(): # Iterates just through value, ignoring the keys
Create a dictionary from two lists
data = dict(zip(list_with_keys, list_with_values))
To add multiple keys simultaneously, use dict.update():
>>> x = {1:2}
>>> print(x)
{1: 2}
>>> d = {3:4, 5:6, 7:8}
>>> x.update(d)
>>> print(x)
{1: 2, 3: 4, 5: 6, 7: 8}
For adding a single key, the accepted answer has less computational overhead.
"Is it possible to add a key to a Python dictionary after it has been created? It doesn't seem to have an .add() method."
Yes it is possible, and it does have a method that implements this, but you don't want to use it directly.
To demonstrate how and how not to use it, let's create an empty dict with the dict literal, {}:
my_dict = {}
Best Practice 1: Subscript notation
To update this dict with a single new key and value, you can use the subscript notation (see Mappings here) that provides for item assignment:
my_dict['new key'] = 'new value'
my_dict is now:
{'new key': 'new value'}
Best Practice 2: The update method - 2 ways
We can also update the dict with multiple values efficiently as well using the update method. We may be unnecessarily creating an extra dict here, so we hope our dict has already been created and came from or was used for another purpose:
my_dict.update({'key 2': 'value 2', 'key 3': 'value 3'})
my_dict is now:
{'key 2': 'value 2', 'key 3': 'value 3', 'new key': 'new value'}
Another efficient way of doing this with the update method is with keyword arguments, but since they have to be legitimate python words, you can't have spaces or special symbols or start the name with a number, but many consider this a more readable way to create keys for a dict, and here we certainly avoid creating an extra unnecessary dict:
my_dict.update(foo='bar', foo2='baz')
and my_dict is now:
{'key 2': 'value 2', 'key 3': 'value 3', 'new key': 'new value',
'foo': 'bar', 'foo2': 'baz'}
So now we have covered three Pythonic ways of updating a dict.
Magic method, __setitem__, and why it should be avoided
There's another way of updating a dict that you shouldn't use, which uses the __setitem__ method. Here's an example of how one might use the __setitem__ method to add a key-value pair to a dict, and a demonstration of the poor performance of using it:
>>> d = {}
>>> d.__setitem__('foo', 'bar')
>>> d
{'foo': 'bar'}
>>> def f():
... d = {}
... for i in xrange(100):
... d['foo'] = i
...
>>> def g():
... d = {}
... for i in xrange(100):
... d.__setitem__('foo', i)
...
>>> import timeit
>>> number = 100
>>> min(timeit.repeat(f, number=number))
0.0020880699157714844
>>> min(timeit.repeat(g, number=number))
0.005071878433227539
So we see that using the subscript notation is actually much faster than using __setitem__. Doing the Pythonic thing, that is, using the language in the way it was intended to be used, usually is both more readable and computationally efficient.
dictionary[key] = value
If you want to add a dictionary within a dictionary you can do it this way.
Example: Add a new entry to your dictionary & sub dictionary
dictionary = {}
dictionary["new key"] = "some new entry" # add new dictionary entry
dictionary["dictionary_within_a_dictionary"] = {} # this is required by python
dictionary["dictionary_within_a_dictionary"]["sub_dict"] = {"other" : "dictionary"}
print (dictionary)
Output:
{'new key': 'some new entry', 'dictionary_within_a_dictionary': {'sub_dict': {'other': 'dictionarly'}}}
NOTE: Python requires that you first add a sub
dictionary["dictionary_within_a_dictionary"] = {}
before adding entries.
The conventional syntax is d[key] = value, but if your keyboard is missing the square bracket keys you could also do:
d.__setitem__(key, value)
In fact, defining __getitem__ and __setitem__ methods is how you can make your own class support the square bracket syntax. See Dive Into Python, Classes That Act Like Dictionaries.
You can create one:
class myDict(dict):
def __init__(self):
self = dict()
def add(self, key, value):
self[key] = value
## example
myd = myDict()
myd.add('apples',6)
myd.add('bananas',3)
print(myd)
Gives:
>>>
{'apples': 6, 'bananas': 3}
This popular question addresses functional methods of merging dictionaries a and b.
Here are some of the more straightforward methods (tested in Python 3)...
c = dict( a, **b ) ## see also https://stackoverflow.com/q/2255878
c = dict( list(a.items()) + list(b.items()) )
c = dict( i for d in [a,b] for i in d.items() )
Note: The first method above only works if the keys in b are strings.
To add or modify a single element, the b dictionary would contain only that one element...
c = dict( a, **{'d':'dog'} ) ## returns a dictionary based on 'a'
This is equivalent to...
def functional_dict_add( dictionary, key, value ):
temp = dictionary.copy()
temp[key] = value
return temp
c = functional_dict_add( a, 'd', 'dog' )
Let's pretend you want to live in the immutable world and do not want to modify the original but want to create a new dict that is the result of adding a new key to the original.
In Python 3.5+ you can do:
params = {'a': 1, 'b': 2}
new_params = {**params, **{'c': 3}}
The Python 2 equivalent is:
params = {'a': 1, 'b': 2}
new_params = dict(params, **{'c': 3})
After either of these:
params is still equal to {'a': 1, 'b': 2}
and
new_params is equal to {'a': 1, 'b': 2, 'c': 3}
There will be times when you don't want to modify the original (you only want the result of adding to the original). I find this a refreshing alternative to the following:
params = {'a': 1, 'b': 2}
new_params = params.copy()
new_params['c'] = 3
or
params = {'a': 1, 'b': 2}
new_params = params.copy()
new_params.update({'c': 3})
Reference: What does `**` mean in the expression `dict(d1, **d2)`?
There is also the strangely named, oddly behaved, and yet still handy dict.setdefault().
This
value = my_dict.setdefault(key, default)
basically just does this:
try:
value = my_dict[key]
except KeyError: # key not found
value = my_dict[key] = default
E.g.,
>>> mydict = {'a':1, 'b':2, 'c':3}
>>> mydict.setdefault('d', 4)
4 # returns new value at mydict['d']
>>> print(mydict)
{'a':1, 'b':2, 'c':3, 'd':4} # a new key/value pair was indeed added
# but see what happens when trying it on an existing key...
>>> mydict.setdefault('a', 111)
1 # old value was returned
>>> print(mydict)
{'a':1, 'b':2, 'c':3, 'd':4} # existing key was ignored
This question has already been answered ad nauseam, but since my
comment
gained a lot of traction, here it is as an answer:
Adding new keys without updating the existing dict
If you are here trying to figure out how to add a key and return a new dictionary (without modifying the existing one), you can do this using the techniques below
Python >= 3.5
new_dict = {**mydict, 'new_key': new_val}
Python < 3.5
new_dict = dict(mydict, new_key=new_val)
Note that with this approach, your key will need to follow the rules of valid identifier names in Python.
If you're not joining two dictionaries, but adding new key-value pairs to a dictionary, then using the subscript notation seems like the best way.
import timeit
timeit.timeit('dictionary = {"karga": 1, "darga": 2}; dictionary.update({"aaa": 123123, "asd": 233})')
>> 0.49582505226135254
timeit.timeit('dictionary = {"karga": 1, "darga": 2}; dictionary["aaa"] = 123123; dictionary["asd"] = 233;')
>> 0.20782899856567383
However, if you'd like to add, for example, thousands of new key-value pairs, you should consider using the update() method.
Here's another way that I didn't see here:
>>> foo = dict(a=1,b=2)
>>> foo
{'a': 1, 'b': 2}
>>> goo = dict(c=3,**foo)
>>> goo
{'c': 3, 'a': 1, 'b': 2}
You can use the dictionary constructor and implicit expansion to reconstruct a dictionary. Moreover, interestingly, this method can be used to control the positional order during dictionary construction (post Python 3.6). In fact, insertion order is guaranteed for Python 3.7 and above!
>>> foo = dict(a=1,b=2,c=3,d=4)
>>> new_dict = {k: v for k, v in list(foo.items())[:2]}
>>> new_dict
{'a': 1, 'b': 2}
>>> new_dict.update(newvalue=99)
>>> new_dict
{'a': 1, 'b': 2, 'newvalue': 99}
>>> new_dict.update({k: v for k, v in list(foo.items())[2:]})
>>> new_dict
{'a': 1, 'b': 2, 'newvalue': 99, 'c': 3, 'd': 4}
>>>
The above is using dictionary comprehension.
First to check whether the key already exists:
a={1:2,3:4}
a.get(1)
2
a.get(5)
None
Then you can add the new key and value.
Add a dictionary (key,value) class.
class myDict(dict):
def __init__(self):
self = dict()
def add(self, key, value):
#self[key] = value # add new key and value overwriting any exiting same key
if self.get(key)!=None:
print('key', key, 'already used') # report if key already used
self.setdefault(key, value) # if key exit do nothing
## example
myd = myDict()
name = "fred"
myd.add('apples',6)
print('\n', myd)
myd.add('bananas',3)
print('\n', myd)
myd.add('jack', 7)
print('\n', myd)
myd.add(name, myd)
print('\n', myd)
myd.add('apples', 23)
print('\n', myd)
myd.add(name, 2)
print(myd)
I think it would also be useful to point out Python's collections module that consists of many useful dictionary subclasses and wrappers that simplify the addition and modification of data types in a dictionary, specifically defaultdict:
dict subclass that calls a factory function to supply missing values
This is particularly useful if you are working with dictionaries that always consist of the same data types or structures, for example a dictionary of lists.
>>> from collections import defaultdict
>>> example = defaultdict(int)
>>> example['key'] += 1
>>> example['key']
defaultdict(<class 'int'>, {'key': 1})
If the key does not yet exist, defaultdict assigns the value given (in our case 10) as the initial value to the dictionary (often used inside loops). This operation therefore does two things: it adds a new key to a dictionary (as per question), and assigns the value if the key doesn't yet exist. With the standard dictionary, this would have raised an error as the += operation is trying to access a value that doesn't yet exist:
>>> example = dict()
>>> example['key'] += 1
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
KeyError: 'key'
Without the use of defaultdict, the amount of code to add a new element would be much greater and perhaps looks something like:
# This type of code would often be inside a loop
if 'key' not in example:
example['key'] = 0 # add key and initial value to dict; could also be a list
example['key'] += 1 # this is implementing a counter
defaultdict can also be used with complex data types such as list and set:
>>> example = defaultdict(list)
>>> example['key'].append(1)
>>> example
defaultdict(<class 'list'>, {'key': [1]})
Adding an element automatically initialises the list.
Adding keys to dictionary without using add
# Inserting/Updating single value
# subscript notation method
d['mynewkey'] = 'mynewvalue' # Updates if 'a' exists, else adds 'a'
# OR
d.update({'mynewkey': 'mynewvalue'})
# OR
d.update(dict('mynewkey'='mynewvalue'))
# OR
d.update('mynewkey'='mynewvalue')
print(d) # {'key': 'value', 'mynewkey': 'mynewvalue'}
# To add/update multiple keys simultaneously, use d.update():
x = {3:4, 5:6, 7:8}
d.update(x)
print(d) # {'key': 'value', 'mynewkey': 'mynewvalue', 3: 4, 5: 6, 7: 8}
# update operator |= now works for dictionaries:
d |= {'c':3,'d':4}
# Assigning new key value pair using dictionary unpacking.
data1 = {4:6, 9:10, 17:20}
data2 = {20:30, 32:48, 90:100}
data3 = { 38:"value", 99:"notvalid"}
d = {**data1, **data2, **data3}
# The merge operator | now works for dictionaries:
data = data1 | {'c':3,'d':4}
# Create a dictionary from two lists
data = dict(zip(list_with_keys, list_with_values))
dico["new key"] = "value"
I'm trying to append a value (list) to a dictionary unsuccessfully. setting the value by 'equal' works, but appending doesn't.
DurationDict = dict()
DurationDict[str(curYear)] = duration // This works
DurationDict[str(curYear)].append(duration) //This does't work.
any ideas?
If you want to append to the value of dictionary, then value should be a type of list.
lets consider following example:
>>> k = {"a":1}
>>> k["b"] = 2
>>> k["c"] = [2]
>>> k["c"].append("new value") # here you can append because value of c is type of list.
>>> print(k)
{'a': 1, 'c': [2, 'new value'], 'b': 2}
>>> k["b"].append("new value") # here you can not append because value of b is type of int
you can append to a list but not to a dict. python dicts are documented here.
if you want to have a list for all your dictionary entries, you can use defaultdict:
collections import defaultdict
DurationDict = defaultdict(list)
DurationDict[str(curYear)].append(duration)
defaultdict works like a normal dict except it returns the result of the 'factory' - in this case list() if the key you are looking up does not exist yet. you can then append to this (empty) list.
You can only append lists. If you have an empty dictionary d,
d = {} # initialize an empty dictionary
d['a'].append(1) # will NOT work
But if you already had an empty list defined,
d = {'a': []} # initialize with an empty list
df['a'].append(1) # will work
>>> d
{'a': [1]}
I mean why cant we put key of dict as dict?
that means we can't have dictionary having key as another dictionary...
Short answer: because they are mutable containers.
If a dict was hashed, its hash would change as you changed its contents.
This is easy to deal with. Wrap a dict in a frozenset before you hash it. Then, when you need to use it, convert it back to a dict.
>>> unhashable = {'b': 'a', 'a': 'b'}
>>> hashable = frozenset(unhashable.items())
>>> unhashable = dict(hashable)
>>> unhashable
{'a': 'b', 'b': 'a'}
Note that dictionary key order is undefined anyway, so the change in key order doesn't matter.
As others have said, the hash value of a dict changes as the contents change.
However if you really need to use dicts as keys, you can subclass dict to make a hashable version.
>>> class hashabledict(dict):
... def __hash__(self):
... return id(self)
...
>>> hd = hashabledict()
>>> d = dict()
>>> d[hd] = "foo"
>>> d
{{}: 'foo'}
>>> hd["hello"] = "world"
>>> d
{{'hello': 'world'}: 'foo'}
This replaces the hash value used for the dict with the object's address in memory.
None of the mutable container types in Python are hashable, because they are mutable and thus their hash value can change over their lifetime.
For maybe the wrong reasons I've ran into this problem a bunch of times; where I want to reference a full dict as a key to something. I don't need it to be mutable, but I do want to preserve and easily access the dict's members.
The easiest way I've found to make the dict immutable and quickly usable as a key value is to make it a JSON (or serialize in your favorite alternative).
For example:
>>> import json
>>> d = {'hey':1, 'there':2}
>>> d_key = json.dumps(d)
>>> d_key
'{"there": 2, "hey": 1}'
>>> d2 = {d_key: 'crazytown'}
>>> d2
{'{"there": 2, "hey": 1}': 'crazytown'}
It's easy to manipulate, since it's just a string. And, it can be de-serialized into an object if you want to reference its members.
How do I add a key to an existing dictionary? It doesn't have an .add() method.
You create a new key/value pair on a dictionary by assigning a value to that key
d = {'key': 'value'}
print(d) # {'key': 'value'}
d['mynewkey'] = 'mynewvalue'
print(d) # {'key': 'value', 'mynewkey': 'mynewvalue'}
If the key doesn't exist, it's added and points to that value. If it exists, the current value it points to is overwritten.
I feel like consolidating info about Python dictionaries:
Creating an empty dictionary
data = {}
# OR
data = dict()
Creating a dictionary with initial values
data = {'a': 1, 'b': 2, 'c': 3}
# OR
data = dict(a=1, b=2, c=3)
# OR
data = {k: v for k, v in (('a', 1), ('b',2), ('c',3))}
Inserting/Updating a single value
data['a'] = 1 # Updates if 'a' exists, else adds 'a'
# OR
data.update({'a': 1})
# OR
data.update(dict(a=1))
# OR
data.update(a=1)
Inserting/Updating multiple values
data.update({'c':3,'d':4}) # Updates 'c' and adds 'd'
Python 3.9+:
The update operator |= now works for dictionaries:
data |= {'c':3,'d':4}
Creating a merged dictionary without modifying originals
data3 = {}
data3.update(data) # Modifies data3, not data
data3.update(data2) # Modifies data3, not data2
Python 3.5+:
This uses a new feature called dictionary unpacking.
data = {**data1, **data2, **data3}
Python 3.9+:
The merge operator | now works for dictionaries:
data = data1 | {'c':3,'d':4}
Deleting items in dictionary
del data[key] # Removes specific element in a dictionary
data.pop(key) # Removes the key & returns the value
data.clear() # Clears entire dictionary
Check if a key is already in dictionary
key in data
Iterate through pairs in a dictionary
for key in data: # Iterates just through the keys, ignoring the values
for key, value in d.items(): # Iterates through the pairs
for key in d.keys(): # Iterates just through key, ignoring the values
for value in d.values(): # Iterates just through value, ignoring the keys
Create a dictionary from two lists
data = dict(zip(list_with_keys, list_with_values))
To add multiple keys simultaneously, use dict.update():
>>> x = {1:2}
>>> print(x)
{1: 2}
>>> d = {3:4, 5:6, 7:8}
>>> x.update(d)
>>> print(x)
{1: 2, 3: 4, 5: 6, 7: 8}
For adding a single key, the accepted answer has less computational overhead.
"Is it possible to add a key to a Python dictionary after it has been created? It doesn't seem to have an .add() method."
Yes it is possible, and it does have a method that implements this, but you don't want to use it directly.
To demonstrate how and how not to use it, let's create an empty dict with the dict literal, {}:
my_dict = {}
Best Practice 1: Subscript notation
To update this dict with a single new key and value, you can use the subscript notation (see Mappings here) that provides for item assignment:
my_dict['new key'] = 'new value'
my_dict is now:
{'new key': 'new value'}
Best Practice 2: The update method - 2 ways
We can also update the dict with multiple values efficiently as well using the update method. We may be unnecessarily creating an extra dict here, so we hope our dict has already been created and came from or was used for another purpose:
my_dict.update({'key 2': 'value 2', 'key 3': 'value 3'})
my_dict is now:
{'key 2': 'value 2', 'key 3': 'value 3', 'new key': 'new value'}
Another efficient way of doing this with the update method is with keyword arguments, but since they have to be legitimate python words, you can't have spaces or special symbols or start the name with a number, but many consider this a more readable way to create keys for a dict, and here we certainly avoid creating an extra unnecessary dict:
my_dict.update(foo='bar', foo2='baz')
and my_dict is now:
{'key 2': 'value 2', 'key 3': 'value 3', 'new key': 'new value',
'foo': 'bar', 'foo2': 'baz'}
So now we have covered three Pythonic ways of updating a dict.
Magic method, __setitem__, and why it should be avoided
There's another way of updating a dict that you shouldn't use, which uses the __setitem__ method. Here's an example of how one might use the __setitem__ method to add a key-value pair to a dict, and a demonstration of the poor performance of using it:
>>> d = {}
>>> d.__setitem__('foo', 'bar')
>>> d
{'foo': 'bar'}
>>> def f():
... d = {}
... for i in xrange(100):
... d['foo'] = i
...
>>> def g():
... d = {}
... for i in xrange(100):
... d.__setitem__('foo', i)
...
>>> import timeit
>>> number = 100
>>> min(timeit.repeat(f, number=number))
0.0020880699157714844
>>> min(timeit.repeat(g, number=number))
0.005071878433227539
So we see that using the subscript notation is actually much faster than using __setitem__. Doing the Pythonic thing, that is, using the language in the way it was intended to be used, usually is both more readable and computationally efficient.
dictionary[key] = value
If you want to add a dictionary within a dictionary you can do it this way.
Example: Add a new entry to your dictionary & sub dictionary
dictionary = {}
dictionary["new key"] = "some new entry" # add new dictionary entry
dictionary["dictionary_within_a_dictionary"] = {} # this is required by python
dictionary["dictionary_within_a_dictionary"]["sub_dict"] = {"other" : "dictionary"}
print (dictionary)
Output:
{'new key': 'some new entry', 'dictionary_within_a_dictionary': {'sub_dict': {'other': 'dictionarly'}}}
NOTE: Python requires that you first add a sub
dictionary["dictionary_within_a_dictionary"] = {}
before adding entries.
The conventional syntax is d[key] = value, but if your keyboard is missing the square bracket keys you could also do:
d.__setitem__(key, value)
In fact, defining __getitem__ and __setitem__ methods is how you can make your own class support the square bracket syntax. See Dive Into Python, Classes That Act Like Dictionaries.
You can create one:
class myDict(dict):
def __init__(self):
self = dict()
def add(self, key, value):
self[key] = value
## example
myd = myDict()
myd.add('apples',6)
myd.add('bananas',3)
print(myd)
Gives:
>>>
{'apples': 6, 'bananas': 3}
This popular question addresses functional methods of merging dictionaries a and b.
Here are some of the more straightforward methods (tested in Python 3)...
c = dict( a, **b ) ## see also https://stackoverflow.com/q/2255878
c = dict( list(a.items()) + list(b.items()) )
c = dict( i for d in [a,b] for i in d.items() )
Note: The first method above only works if the keys in b are strings.
To add or modify a single element, the b dictionary would contain only that one element...
c = dict( a, **{'d':'dog'} ) ## returns a dictionary based on 'a'
This is equivalent to...
def functional_dict_add( dictionary, key, value ):
temp = dictionary.copy()
temp[key] = value
return temp
c = functional_dict_add( a, 'd', 'dog' )
Let's pretend you want to live in the immutable world and do not want to modify the original but want to create a new dict that is the result of adding a new key to the original.
In Python 3.5+ you can do:
params = {'a': 1, 'b': 2}
new_params = {**params, **{'c': 3}}
The Python 2 equivalent is:
params = {'a': 1, 'b': 2}
new_params = dict(params, **{'c': 3})
After either of these:
params is still equal to {'a': 1, 'b': 2}
and
new_params is equal to {'a': 1, 'b': 2, 'c': 3}
There will be times when you don't want to modify the original (you only want the result of adding to the original). I find this a refreshing alternative to the following:
params = {'a': 1, 'b': 2}
new_params = params.copy()
new_params['c'] = 3
or
params = {'a': 1, 'b': 2}
new_params = params.copy()
new_params.update({'c': 3})
Reference: What does `**` mean in the expression `dict(d1, **d2)`?
There is also the strangely named, oddly behaved, and yet still handy dict.setdefault().
This
value = my_dict.setdefault(key, default)
basically just does this:
try:
value = my_dict[key]
except KeyError: # key not found
value = my_dict[key] = default
E.g.,
>>> mydict = {'a':1, 'b':2, 'c':3}
>>> mydict.setdefault('d', 4)
4 # returns new value at mydict['d']
>>> print(mydict)
{'a':1, 'b':2, 'c':3, 'd':4} # a new key/value pair was indeed added
# but see what happens when trying it on an existing key...
>>> mydict.setdefault('a', 111)
1 # old value was returned
>>> print(mydict)
{'a':1, 'b':2, 'c':3, 'd':4} # existing key was ignored
This question has already been answered ad nauseam, but since my
comment
gained a lot of traction, here it is as an answer:
Adding new keys without updating the existing dict
If you are here trying to figure out how to add a key and return a new dictionary (without modifying the existing one), you can do this using the techniques below
Python >= 3.5
new_dict = {**mydict, 'new_key': new_val}
Python < 3.5
new_dict = dict(mydict, new_key=new_val)
Note that with this approach, your key will need to follow the rules of valid identifier names in Python.
If you're not joining two dictionaries, but adding new key-value pairs to a dictionary, then using the subscript notation seems like the best way.
import timeit
timeit.timeit('dictionary = {"karga": 1, "darga": 2}; dictionary.update({"aaa": 123123, "asd": 233})')
>> 0.49582505226135254
timeit.timeit('dictionary = {"karga": 1, "darga": 2}; dictionary["aaa"] = 123123; dictionary["asd"] = 233;')
>> 0.20782899856567383
However, if you'd like to add, for example, thousands of new key-value pairs, you should consider using the update() method.
Here's another way that I didn't see here:
>>> foo = dict(a=1,b=2)
>>> foo
{'a': 1, 'b': 2}
>>> goo = dict(c=3,**foo)
>>> goo
{'c': 3, 'a': 1, 'b': 2}
You can use the dictionary constructor and implicit expansion to reconstruct a dictionary. Moreover, interestingly, this method can be used to control the positional order during dictionary construction (post Python 3.6). In fact, insertion order is guaranteed for Python 3.7 and above!
>>> foo = dict(a=1,b=2,c=3,d=4)
>>> new_dict = {k: v for k, v in list(foo.items())[:2]}
>>> new_dict
{'a': 1, 'b': 2}
>>> new_dict.update(newvalue=99)
>>> new_dict
{'a': 1, 'b': 2, 'newvalue': 99}
>>> new_dict.update({k: v for k, v in list(foo.items())[2:]})
>>> new_dict
{'a': 1, 'b': 2, 'newvalue': 99, 'c': 3, 'd': 4}
>>>
The above is using dictionary comprehension.
First to check whether the key already exists:
a={1:2,3:4}
a.get(1)
2
a.get(5)
None
Then you can add the new key and value.
Add a dictionary (key,value) class.
class myDict(dict):
def __init__(self):
self = dict()
def add(self, key, value):
#self[key] = value # add new key and value overwriting any exiting same key
if self.get(key)!=None:
print('key', key, 'already used') # report if key already used
self.setdefault(key, value) # if key exit do nothing
## example
myd = myDict()
name = "fred"
myd.add('apples',6)
print('\n', myd)
myd.add('bananas',3)
print('\n', myd)
myd.add('jack', 7)
print('\n', myd)
myd.add(name, myd)
print('\n', myd)
myd.add('apples', 23)
print('\n', myd)
myd.add(name, 2)
print(myd)
I think it would also be useful to point out Python's collections module that consists of many useful dictionary subclasses and wrappers that simplify the addition and modification of data types in a dictionary, specifically defaultdict:
dict subclass that calls a factory function to supply missing values
This is particularly useful if you are working with dictionaries that always consist of the same data types or structures, for example a dictionary of lists.
>>> from collections import defaultdict
>>> example = defaultdict(int)
>>> example['key'] += 1
>>> example['key']
defaultdict(<class 'int'>, {'key': 1})
If the key does not yet exist, defaultdict assigns the value given (in our case 10) as the initial value to the dictionary (often used inside loops). This operation therefore does two things: it adds a new key to a dictionary (as per question), and assigns the value if the key doesn't yet exist. With the standard dictionary, this would have raised an error as the += operation is trying to access a value that doesn't yet exist:
>>> example = dict()
>>> example['key'] += 1
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
KeyError: 'key'
Without the use of defaultdict, the amount of code to add a new element would be much greater and perhaps looks something like:
# This type of code would often be inside a loop
if 'key' not in example:
example['key'] = 0 # add key and initial value to dict; could also be a list
example['key'] += 1 # this is implementing a counter
defaultdict can also be used with complex data types such as list and set:
>>> example = defaultdict(list)
>>> example['key'].append(1)
>>> example
defaultdict(<class 'list'>, {'key': [1]})
Adding an element automatically initialises the list.
Adding keys to dictionary without using add
# Inserting/Updating single value
# subscript notation method
d['mynewkey'] = 'mynewvalue' # Updates if 'a' exists, else adds 'a'
# OR
d.update({'mynewkey': 'mynewvalue'})
# OR
d.update(dict('mynewkey'='mynewvalue'))
# OR
d.update('mynewkey'='mynewvalue')
print(d) # {'key': 'value', 'mynewkey': 'mynewvalue'}
# To add/update multiple keys simultaneously, use d.update():
x = {3:4, 5:6, 7:8}
d.update(x)
print(d) # {'key': 'value', 'mynewkey': 'mynewvalue', 3: 4, 5: 6, 7: 8}
# update operator |= now works for dictionaries:
d |= {'c':3,'d':4}
# Assigning new key value pair using dictionary unpacking.
data1 = {4:6, 9:10, 17:20}
data2 = {20:30, 32:48, 90:100}
data3 = { 38:"value", 99:"notvalid"}
d = {**data1, **data2, **data3}
# The merge operator | now works for dictionaries:
data = data1 | {'c':3,'d':4}
# Create a dictionary from two lists
data = dict(zip(list_with_keys, list_with_values))
dico["new key"] = "value"