To find every key whose value matches a target, scan the dictionary’s key-value pairs with items():
matches = [key for key, value in data.items() if value == target]For just the first match, use next() with a fallback. Both approaches search the dictionary’s entries; dictionaries are indexed by key, not by value.
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Find every key with a matching value
A list comprehension returns all keys whose values compare equal to target:
data = {"first": "ready", "second": "waiting", "third": "ready"}
target = "ready"
matches = [key for key, value in data.items() if value == target]
print(matches) # ['first', 'third']items() supplies each key and its value together, so the condition can compare the value while collecting the corresponding key. If multiple keys share the target value, they all appear in the result.
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Return only the first matching key
If one result is enough, pass a generator expression to next():
match = next((key for key, value in data.items() if value == target), None)This returns the first matching key in dictionary iteration order. If there is no match, it returns None. If None could itself be a key in your dictionary, use a unique sentinel so you can distinguish “no match” from a matching key:
missing = object()
match = next((key for key, value in data.items() if value == target), missing)
if match is missing:
print("No matching key")
else:
print(match)Use a loop when a match needs custom handling
An explicit loop makes it straightforward to perform an action for each match, such as printing keys or applying additional conditions:
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for key, value in data.items():
if value == target:
print(key)The loop visits entries in dictionary iteration order and does not stop after a match. Add break inside the if block if you want it to stop at the first one.
Choose between scanning and a reverse index
The scan-based examples are useful for occasional searches, values that may not be hashable, or dictionaries that change frequently. Each search checks entries until it finds a match or reaches the end.
If you need repeated lookups in mostly unchanged data and each value can be used as a dictionary key, build a reverse dictionary once:
value_to_key = {value: key for key, value in data.items()}
key = value_to_key.get(target)This is convenient when values are unique. If several original keys share a value, the reverse dictionary retains only the last key encountered for that value; each assignment replaces the previous one.
To preserve all keys for each hashable value, build a multimap instead:
from collections import defaultdict
value_to_keys = defaultdict(list)
for key, value in data.items():
value_to_keys[value].append(key)
matching_keys = value_to_keys.get(target, [])Values used as reverse-dictionary keys must be hashable. Lists and dictionaries, for example, cannot be dictionary keys, so search those values with a scan instead. A reverse index also needs to be rebuilt or updated when the original dictionary changes; otherwise, it may no longer reflect the current entries.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Equality and iteration order
The examples use ==, which tests equality of the complete values. For dictionaries or other structured values, decide whether you want the whole value to match or only a particular field; for a field comparison, change the condition in the comprehension or loop.
Python guarantees dictionary insertion order starting with Python 3.7. Therefore, a first-match scan selects the earliest matching entry in that order. Updating a value for an existing key does not move the key; deleting and then reinserting it puts it at the end.
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For background on dictionary iteration, equality, hashability, and ordering, see the Python tutorial on data structures and the Python language reference on the data model.
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