Inverting a Dictionary with List Values

Inverting a dictionary with list values

I've tried around and you want to use val not in inverse but it can't be checked if a "list is in a dict". (val is a list)

For your code a simple change will do what you want:

def invert_dict(d): 
    inverse = dict() 
    for key in d: 
        # Go through the list that is saved in the dict:
        for item in d[key]:
            # Check if in the inverted dict the key exists
            if item not in inverse: 
                # If not create a new list
                inverse[item] = [key] 
            else: 
                inverse[item].append(key) 
    return inverse

pythonic way to reverse a dict where values are lists?

This is well-suited for collections.defaultdict:

>>> from collections import defaultdict
>>> numbers_by_letter = defaultdict(list)
>>> for k, seq in letters_by_number.items():
...     for letter in seq:
...         numbers_by_letter[letter].append(k)
... 
>>> dict(numbers_by_letter)
{'a': [1, 3, 4], 'b': [1, 2, 5], 'c': [1, 3, 5], 'd': [1, 2, 4]}

Note that you don't really need the final dict() call (a defaultdict will already give you the behavior you probably want), but I included it here because the result from your question is type dict.

Inverting a dictionary with sets/lists as values

dict.fromkeys(d, set()) just creates one set and puts that in every dictionary entry. You need to create a new set for each entry:

inverse_dict = {k: set() for k in d}

Reverse / invert a dictionary mapping

Python 3+:

inv_map = {v: k for k, v in my_map.items()}

Python 2:

inv_map = {v: k for k, v in my_map.iteritems()}

How to reverse a dictionary (whose values are lists) in Python?

You can do it very simply like this:

newdict = {}
for key, value in olddict.items():
    for string in value:
        newdict.setdefault(string, []).append(key)

How can I flatten a dictionary with a list for values?

Use a dictionary comprehension with nested iteration:

>>> d = {
...     "USA": ["US", "United States"],
...     "SGP": ["JP", "Japan", "Singapore"]
... }
>>> {i: k for k, v in d.items() for i in v}
{'US': 'USA', 'United States': 'USA', 'JP': 'SGP', 'Japan': 'SGP', 'Singapore': 'SGP'}

• k, v in d.items() -> k = "USA", ..., v = ["US", "United States"], ...
• i in v -> i = "US", ...

hence:

• {i: k ...} -> {"US": "USA", ...}

Python 3: How Do I Invert A Dictionary That Has A List As A Value

You can use a defaultdict to remove the else statement and to check if the key exists. If it doesn't exists an empty list will be created.

from pprint import pprint

summon_locations = {
    "Solaire": ['Gargoyles', 'Gaping Dragon', "Ornstein/Smough"],
    "Gotthard": ['Abyss Watchers', 'Pontiff Sulyvahn', "Grand Archives"],
    "Lucatiel": ['Lost Sinner', 'Smelter Demon', 'Abyss Watchers'],
}

if __name__ == '__main__':
    from collections import defaultdict

    inverted = defaultdict(list)

    for key, values in summon_locations.items():
        for value in values:
            inverted[value].append(key)

    pprint(inverted)

Output

defaultdict(<class 'list'>,
            {'Abyss Watchers': ['Gotthard', 'Lucatiel'],
             'Gaping Dragon': ['Solaire'],
             'Gargoyles': ['Solaire'],
             'Grand Archives': ['Gotthard'],
             'Lost Sinner': ['Lucatiel'],
             'Ornstein/Smough': ['Solaire'],
             'Pontiff Sulyvahn': ['Gotthard'],
             'Smelter Demon': ['Lucatiel']})

Which is similar to the the one-liner

[inverted[value].append(key) for key, values in summon_locations.items() for value in values]

But one liners are not always better, in this case I would stay with the two for loops.

Invert a dictionary with list values in ansible

The playbook

- hosts: localhost
  vars:
    dict1:
      host1: [tag1, tag2]
      host2: [tag1]
  tasks:
    - set_fact:
        dict2: "{{ dict2|default({})|combine({item: list_of_hosts}) }}"
      loop: "{{ dict1.values()|flatten|unique }}"
      vars:
        list_of_hosts: "{{ dict1|
                           dict2items|
                           selectattr('value', 'contains', item)|
                           map(attribute='key')|
                           list }}"
    - debug:
        var: dict2

gives

  dict2:
    tag1: [host1, host2]
    tag2: [host1]

      vars:
        list_of_hosts: "{{ dict1|dict2items|json_query(_query) }}"
        _query: '[?value.contains(@, `{{ item }}`)].key'

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