Declaring a multi dimensional dictionary in python

Question

I need to make a two dimensional dictionary in python. e.g. new_dic[1][2] = 5

When I make new_dic = {}, and try to insert values, I get a <

Answer 1

One simple way is to just use tuples as keys to a regular dictionary. So your example becomes this:

new_dic[(1, 2)] = 5

The downside is that all usages have to be with this slightly awkward convention, but if that's OK, this is all you need.

Answer 2

A multi-dimensional dictionary is simply a dictionary where the values are themselves also dictionaries, creating a nested structure:

new_dic = {}
new_dic[1] = {}
new_dic[1][2] = 5

You'd have to detect that you already created new_dic[1] each time, though, to not accidentally wipe that nested object for additional keys under new_dic[1].

You can simplify creating nested dictionaries using various techniques; using dict.setdefault() for example:

new_dic.setdefault(1, {})[2] = 5

dict.setdefault() will only set a key to a default value if the key is still missing, saving you from having to test this each time.

Simpler still is using the collections.defaultdict() type to create nested dictionaries automatically:

from collections import defaultdict

new_dic = defaultdict(dict)
new_dic[1][2] = 5

defaultdict is just a subclass of the standard dict type here; every time you try and access a key that doesn't yet exist in the mapping, a factory function is called to create a new value. Here that's the dict() callable, which produces an empty dictionary when called.

Demo:

>>> new_dic_plain = {}
>>> new_dic_plain[1] = {}
>>> new_dic_plain[1][2] = 5
>>> new_dic_plain
{1: {2: 5}}
>>> new_dic_setdefault = {}
>>> new_dic_setdefault.setdefault(1, {})[2] = 5
>>> new_dic_setdefault
{1: {2: 5}}
>>> from collections import defaultdict
>>> new_dic_defaultdict = defaultdict(dict)
>>> new_dic_defaultdict[1][2] = 5
>>> new_dic_defaultdict
defaultdict(<type 'dict'>, {1: {2: 5}})

Answer 3

Simply, you can use defaultdict

from collections import defaultdict
new_dic = defaultdict(dict)
new_dic[1][2]=5
>>>new_dic
defaultdict(<type 'dict'>, {1: {2: 5}})

Answer 4

Do you mean dict or list?

And if you mean dict do you want the second level to be another dict? or a list?

For a dict to work you need to have declared the keys ahead of time.

So if it's dicts in dicts you need something like this:

new_dic = {}
try:
    new_dic[1][2] = 5
except KeyError:
    new_dic[1] = {2:5}

Answer 5

For project, I needed to have 2D dict of class instances, where indeces are float numbers (coordinates). What I did, was to create 2D dict using default dict, and pass my class name as a type. For ex.:

class myCoordinates:
    def __init__(self, args)
    self.x = args[0]
    self.y = args[1]

and then when I tried to create dictionary:

table = mult_dim_dict(2, myCoordinates, (30, 30))

where function 'mult_dim_dict' defined as:

def mult_dim_dict(dim, dict_type, params):
    if dim == 1:
        if params is not None:
            return defaultdict(lambda: dict_type(params))
        else: 
            return defaultdict(dict_type)
    else:
        return defaultdict(lambda: mult_dim_dict(dim - 1, dict_type, params))

Note: you cannot pass multiple arguments, instead you can pass a tuple, containing all of your arguments. If your class, upon creation, does not need any variables to be passed, 3rd argument of function will be None:

class myCoors:
def __init__(self, tuple=(0, 0)):
    self.x, self.y = tuple

def printCoors(self):
    print("x = %d, y = %d" %(self.x, self.y))


def mult_dim_dict(dim, dict_type, params):
    if dim == 1:
        if params is not None:
            return defaultdict(lambda: dict_type(params))
        else:
            return defaultdict(dict_type)
    else:
        return defaultdict(lambda: mult_dim_dict(dim - 1, dict_type, params))

dict = mult_dim_dict(2, myCoors, None)
dict['3']['2'].x = 3
dict['3']['2'].y = 2

dict['3']['2'].printCoors()  # x = 3, y = 2 will be printed

dict = mult_dim_dict(2, myCoors, (30, 20))
dict['3']['2'].printCoors()  # x = 30, y = 20 will be printed

Answer 6

Here is a dictionary that contains another dictionary as the value for key 1:

>>> new_dic = {}
>>> new_dic[1] = {2:5}
>>> new_dic
{1: {2: 5}}

The problem that you had with

new_dic={}
new_dic[1][2]=5

is that new_dic[1] does not exist, so you can't add a dictionary (or anything for that matter) to it.