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dict.keys() / values() / items()

Gets dynamic views of dict keys, values, or (key, value) pairs.

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data.keys()
data.values()
data.items()
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What it does

Overview

Gets dynamic views of dict keys, values, or (key, value) pairs.

In Python 3, dict.keys(), dict.values(), and dict.items() return dynamic view objects (eys, alues, and tems). Unlike Python 2's list-returning methods, these views provide a live reflection of the dictionary's state; any modifications to the dictionary are immediately visible in the view. Performance is highly optimized: membership testing using the 'in' operator on a 'keys' view is O(1) because it leverages the underlying hash table. The 'keys' and 'items' views also function like sets, supporting set-theoretic operations such as intersection (&) and union (|) if the dictionary values are hashable. These objects are extremely memory-efficient as they do not duplicate the dictionary's contents. Furthermore, since Python 3.7, these views are guaranteed to maintain the insertion order of the keys.

Quick reference

Syntax

data.keys()
data.values()
data.items()

See it in practice

Examples

1

Iterating with Key-Value Unpacking

inventory = {'apples': 5, 'oranges': 10}
for fruit, count in inventory.items():
    print(f'{fruit}: {count}')
Output:
apples: 5 oranges: 10

The .items() method returns a view of tuples, allowing for clean sequence unpacking directly in the loop header.

2

Dynamic View Behavior

prices = {'cpu': 300}
view = prices.keys()
prices['gpu'] = 500
print('gpu' in view)
print(list(view))
Output:
True ['cpu', 'gpu']

Changes to the dictionary automatically update existing view objects without requiring a new method call.

3

Set Operations on Dictionary Keys

admin_perms = {'read', 'write', 'delete'}
user_perms = {'id': 1, 'read': True, 'write': True}

common = admin_perms & user_perms.keys()
print(common)
Output:
{'read', 'write'}

Dictionary keys views support set operations, making it trivial to find intersections or differences between keys and other sets or dictionaries.

Debug faster

Common Errors

1

TypeError

Cause: Attempting to access a view object by index (e.g., keys()[0]). View objects are iterable but do not support indexing or slicing.

Fix: Convert the view object to a list using the list() constructor if numerical indexing is required.

data = {'a': 1}
# Bad: data.keys()[0]
# Good:
key_list = list(data.keys())
print(key_list[0])
2

RuntimeError

Cause: Modifying the size of the dictionary (adding or removing items) while iterating over a view object.

Fix: Iterate over a static copy of the keys by wrapping the view in list() if you need to delete items during the loop.

d = {'a': 1, 'b': 2}
# Raises RuntimeError:
# for k in d.keys(): del d[k]

# Correct:
for k in list(d.keys()):
    del d[k]

Runtime support

Compatibility

Python 3.8+

Views exist since Python 3, dynamic and lightweight.

Source: MDN Web Docs

Common questions

Frequently Asked Questions

Gets dynamic views of dict keys, values, or (key, value) pairs.

TypeError: Convert the view object to a list using the list() constructor if numerical indexing is required. RuntimeError: Iterate over a static copy of the keys by wrapping the view in list() if you need to delete items during the loop.