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dict is a built-in type representing an associative mapping or

. A dictionary supports indexing using

and key membership testing using

; both operations take constant time. Unfrozen dictionaries are mutable, and may be updated by assigning to

or by calling certain methods. Dictionaries are iterable; iteration yields the sequence of keys in insertion order. Iteration order is unaffected by updating the value associated with an existing key, but is affected by removing then reinserting a key.

d = {0: 0, 2: 2, 1: 1}
[k for k in d]  # [0, 2, 1]
d.pop(2)
d[0], d[2] = "a", "b"
0 in d, "a" in d  # (True, False)
[(k, v) for k, v in d.items()]  # [(0, "a"), (1, 1), (2, "b")]

There are three ways to construct a dictionary:

1. A dictionary expression {k: v, ...} yields a new dictionary with the specified key/value entries, inserted in the order they appear in the expression. Evaluation fails if any two key expressions yield the same value.
2. A dictionary comprehension {k: v for vars in seq} yields a new dictionary into which each key/value pair is inserted in loop iteration order. Duplicates are permitted: the first insertion of a given key determines its position in the sequence, and the last determines its associated value.

   {k: v for k, v in (("a", 0), ("b", 1), ("a", 2))}  # {"a": 2, "b": 1}
   {i: 2*i for i in range(3)}  # {0: 0, 1: 2, 2: 4}
   

3. A call to the built-in dict function returns a dictionary containing the specified entries, which are inserted in argument order, positional arguments before named. As with comprehensions, duplicate keys are permitted.

None dict.clear()

Remove all items from the dictionary.

unknown dict.get(key, default=None)

Returns the value for

if

is in the dictionary, else

. If

is not given, it defaults to

, so that this method never throws an error.

list dict.items()

Returns the list of key-value tuples:

{2: "a", 4: "b", 1: "c"}.items() == [(2, "a"), (4, "b"), (1, "c")]

list dict.keys()

Returns the list of keys:

{2: "a", 4: "b", 1: "c"}.keys() == [2, 4, 1]

unknown dict.pop(key, default=unbound)

Removes a

from the dict, and returns the associated value. If no entry with that key was found, remove nothing and return the specified

value; if no default value was specified, fail instead.

tuple dict.popitem()

Remove and return the first

pair from the dictionary.

is useful to destructively iterate over a dictionary, as often used in set algorithms. If the dictionary is empty, the

call fails.

unknown dict.setdefault(key, default=None)

If

is in the dictionary, return its value. If not, insert key with a value of

and return

.

defaults to

.

None dict.update(pairs=[], **kwargs)

Updates the dictionary first with the optional positional argument,

, then with the optional keyword arguments If the positional argument is present, it must be a dict, iterable, or None. If it is a dict, then its key/value pairs are inserted into this dict. If it is an iterable, it must provide a sequence of pairs (or other iterables of length 2), each of which is treated as a key/value pair to be inserted. Each keyword argument

causes the name/value pair to be inserted into this dict.

list dict.values()

Returns the list of values:

{2: "a", 4: "b", 1: "c"}.values() == ["a", "b", "c"]

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