Python - dict - search by value

data = {
    'a': 'hello',
    'b': 'world',
    'c': 'hello'
}

# ----- example 1 -----

# invert dictionary - it loses element 'hello':'a'

inverted_data = {val: key for key, val in data.items()}

print(inverted_data)

# {'hello': 'c', 'world': 'b'}

print(inverted_data.get("hello"))

# 'c'

print(inverted_data.get("other"))

# None

# ----- example 2 -----

# find all keys using "list comprehension"

result = [key for key, val in data.items() if val == "hello"]

print(result)

# ['a', 'c']

result = [key for key, val in data.items() if val == "other"]

print(result)

# []  # empty list

# --- example 3 ---

# find all keys using "for" loop

result = []

for key, val in data.items():
    if val == "hello":
        result.append(key)

print(result)

# ['a', 'c']

result = []

for key, val in data.items():
    if val == "other":
        result.append(key)

print(result)

# []  # empty list

# ----- example 4 -----

# invert dictionary using list as elements - it doesn't lose element 'hello':'a'

inverted_data = dict()

for key, val in data.items():
    if val not in inverted_data:
        inverted_data[val] = []
    inverted_data[val].append(key)

print(inverted_data)

# {'world': ['b'], 'hello': ['a', 'c']}

print(inverted_data.get('hello'))

# ['a', 'c']

print(inverted_data.get('other'))

# None

# ----- example 5 -----

# invert dictionary using defaultdict - it doesn't lose element 'hello':'a'

from collections import defaultdict

inverted_data = defaultdict(list)

for key, val in data.items():
    inverted_data[val].append(key)

print(inverted_data)

# defaultdict(<class 'list'>, {'world': ['b'], 'hello': ['a', 'c']})

print(inverted_data.get('hello'))

# ['a', 'c']

print(inverted_data.get('other'))

# None