Mapping input to dictionary value

Question

Part of a script that I am building requires me to map a user input to a list (possible inputs) which is then associated to a key (the result) in a dictionary. I have already manage to obtain the result but I want to know is there a better way that I can go about this?

user_input = "bought"
output = None

input_mapping = {"buy": ["bought", "buys", "purchased"],
                 "sell": ["sold", "sells", "sale"]}

for key, values in input_mapping.items():
    if user_input in values:
        output = key

print(output)

input / output:

user_input = "sale"
>>> sell

user_input = "bought"
>>> buy

Many thanks!

Already asked here. Take a look. https://stackoverflow.com/questions/8023306/get-key-by-value-in-dictionary — Yossi Levi, Sep 23 '20 at 10:20

Amit Yungman · Answer 1 · 2020-09-23T12:04:01.850

2

In this case, keeping the reverse dictionary will yield the best performance. Searching a dictionary is very good, whilst your method requires searching the entire data.

I would build a simple reverse dictionary and use it:

reversed_dictionary = {item[0]: item[1] for sublist in [[(v,k) for v in input_mapping[k]] for k in input_mapping.keys()] for item in sublist}

edited Sep 23 '20 at 12:04

answered Sep 23 '20 at 10:18

Amit Yungman

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@RaymondC. I've added a simple line to reverse a dictionary, for your use. – Amit Yungman Sep 23 '20 at 12:04

score 1 · Accepted Answer · answered Sep 23 '20 at 10:19

1

It might seem more redundant to have multiple keys point to the same value, but from a peformance aspect, it's more efficient to lookup the key on the dictionary O(1) than the value on the list O(N).

input_mapping = {
  "bought": "buy",
  "buys": "buy",
  "purchased": "buy",
  "sold": "sell",
  "sells": "sell",
  "sale": "sell"
}

user_input = "bought"
output = input_mapping[user_input]
print(output)

Code Output:

buy

answered Sep 23 '20 at 10:19

solid.py

2,782
5
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Combined with what Amit mentioned, I decided to go with your answer. – Raymond C. Sep 23 '20 at 10:37

score 1 · Answer 3 · answered Sep 23 '20 at 10:22

You should reverse the mapping, going from output -> [input], to a input -> output mapping:

mapping = {
  "bought": "buy", "buys": "buy", "purchased": "buy", "sold": "sell", "sells": "sell",
  "sale": "sell"}
output = mapping[input]

That would be faster and more intuitive. I would prefer your solution, if the data becomes really large. It is a tradeoff.

Mapping input to dictionary value

3 Answers3