How do I join a list of values into a Python dictionary?

Question

I am trying to join a list to a dictionary in Python 3 and return the sum of the key values.

So far, I can't join the two, I've tried using get and set and am not succeeding.

I also tried a for loop with set linking listy and dict2, like this:

dict2 = {
1: "A",
2: "B",
3: "C"
}

listy = ['A', 'M', 'B', 'A']

for k in dict2:
    if set(listy) & set(dict2[value]):
        print(dict2.key)

This is the error I'm getting in IPython:

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-291-5a5e2eb8d7f8> in <module>
     10 
     11 for k in dict2:
---> 12     if set(listy) & set(dict2[value]):
     13         print(dict2.key)
     14 

TypeError: unhashable type: 'list'

Answer 1

You can use a list-comprehension:

[x for x in listy if x in set(dict2.values())]

In code :

dict2 = {
1: "A",
2: "B",
3: "C"
}

listy = ['A', 'M', 'B', 'A']

print([x for x in listy if x in set(dict2.values())])
# ['A', 'B', 'A']

Answer 2

You probably meant to use dict[k] instead of dict2[value]

Also your dictionary's entries contain single values (not lists) so you can use the in operator:

for example:

 # if listy is a dictionary or a small list, you don't need to build a set 

 for key,value in dict2.items():
    if value in listy:
        print(key)

or :

 # If listy is a large list, you should build a set only once 

 listySet = set(listy)
 for key,value in dict2.items():
    if value in listySet:
        print(key)

If you have a lot of code to perform on the "joined" data, you could structure the condition like this:

 for key,value in dict2.items():
    if value not in listy: continue
    print(key)
    ... do more stuff ...

If you're only looking for a sum, you can do it more directly:

# counting sum of dict2 keys matching each value in listy  
# select sum(dict2.key) from listy join dict2 where dict2.value = listy.value
# (note that an inverted dict2 would be better suited for that)

result = sum(key*listy.count(value) for key,value in dict2.items())

# counting sum of keys in dict2 that have a value in listy 
# select sum(dict2.key) from dict2 where exists listy.value = dict2.value

result = sum(key for key,value in dict2.items() if value in listy)

In short, you have to implement the linking logic that the RDBMS query optimizer normally does for you in SQL.

Answer 3

Your task will be easier if you flip the keys and values in your dictionary. I assume that there are no duplicate values.

dict2 = {1: "A", 2: "B", 3: "C"}
lookup = {value: key for key, value in dict2.items()}

Now lookup is {'A': 1, 'B': 2, 'C': 3} . Now you can loop over the list:

listy = ['A', 'M', 'B', 'A']
result = []
for key in listy:
    if key in lookup:
        result.append(key)

Now result is ['A', 'B', 'A'] . The code will be shorter with a list comprehension:

result = [key for key in listy if key in lookup]

As far as I unterstood the question you want to get the sum of the keys in dict2 for every entry in listy that has a corresponding value in dict2 . If you have created the lookup dictionary you can do the following to get the single values.

[lookup.get(key, 0) for key in listy]
# -> [1, 0, 2, 1]

If a key doesn't appear in the dictionary it gets a default value of 0 .

To get the sum is easy now

sum(lookup.get(key, 0) for key in listy)
# -> 4