dict
dict is a built-in type representing an associative mapping or dictionary. A dictionary supports indexing usingd[k] and key membership testing using k in d; both operations take constant time. Unfrozen dictionaries are mutable, and may be updated by assigning to d[k] 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:
- 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. - 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} - 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.
clear
None dict.clear()Remove all items from the dictionary.
get
unknown dict.get(key, default=None)Returns the value for
key if key is in the dictionary, else default. If default is not given, it defaults to None, so that this method never throws an error.
Parameters
| Parameter | Description |
|---|---|
key
|
The key to look for. |
default
|
The default value to use (instead of None) if the key is not found. |
items
list dict.items()Returns the list of key-value tuples:
{2: "a", 4: "b", 1: "c"}.items() == [(2, "a"), (4, "b"), (1, "c")]
keys
list dict.keys()Returns the list of keys:
{2: "a", 4: "b", 1: "c"}.keys() == [2, 4, 1]
pop
unknown dict.pop(key, default=unbound)Removes a
key from the dict, and returns the associated value. If no entry with that key was found, remove nothing and return the specified default value; if no default value was specified, fail instead.
Parameters
| Parameter | Description |
|---|---|
key
|
The key. |
default
|
a default value if the key is absent. |
popitem
tuple dict.popitem()Remove and return an arbitrary
(key, value) pair from the dictionary. popitem() is useful to destructively iterate over a dictionary, as often used in set algorithms. If the dictionary is empty, calling popitem() fails. It is deterministic which pair is returned.
setdefault
unknown dict.setdefault(key, default=None)If
key is in the dictionary, return its value. If not, insert key with a value of default and return default. default defaults to None.
Parameters
| Parameter | Description |
|---|---|
key
|
The key. |
default
|
a default value if the key is absent. |
update
None dict.update(pairs=[], **kwargs)Updates the dictionary first with the optional positional argument,
pairs, 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 name=value causes the name/value pair to be inserted into this dict.
Parameters
| Parameter | Description |
|---|---|
pairs
|
Either a dictionary or a list of entries. Entries must be tuples or lists with exactly two elements: key, value. |
kwargs
|
Dictionary of additional entries. |
values
list dict.values()Returns the list of values:
{2: "a", 4: "b", 1: "c"}.values() == ["a", "b", "c"]