Python: Dictionary Methods

By Xah Lee. Date: . Last updated: .

Length

len(d)
Return number of items in d. For example, len({"a":8,"b":9})⇒ 2

Get, Set, Delete

d[k]
Return the value of key k if exist, else KeyError. 
xx = {"a": 1, "b": 2}
print(xx["b"] == 2)
d.get(k)
Return the value of key k if exist, else return None. 
xx = {"a": 1, "b": 2}
print(xx.get("b") == 2)
d.get(k, v)
Return the value of key k if exist, else return v. 
xx = {"a": 1, "b": 2}
print(xx.get("c", 8) == 8)
d[k] = v
Set a value v.
del d[k]
Remove the value of key k if exist, else KeyError.

Check Existence

k in d
Return True if d has a key k, else False.
k not in d
The opposite of in

Get All Keys, Get All Keys Values

d.keys()
Return a iterator that's all key's values. (each element is a new copy) 
dd = {"a": 1, "b": 2, "c": 3}

print(dd.keys())
# dict_keys(['a', 'b', 'c'])
d.values()
Return a iterator that's all key's values. (each element is a new copy) 
dd = {"a": 1, "b": 2, "c": 3}

print(dd.values())
# dict_values([1, 2, 3])
d.items()
Return a iterator of 2-tuples, each is (key, value). (each element is a new copy) 
dd = {"a": 1, "b": 2, "c": 3}

print(dd.items())
# dict_items([('a', 1), ('b', 2), ('c', 3)])

Pop, Update

d.pop(k)
Remove and return its value if key k exist, else KeyError.
d.pop(k, v)
Remove and return its value if key k exist, else v.
d.popitem()
Remove and return arbitrary (key, value) pair. If d is empty, KeyError.

Set Value, Update

d.setdefault(k)
If key k exist, return its value, else, add key with a value of None.
d.setdefault(k, v)
If key k exist, return its value, else, insert key with a value of v
d.update(v)
Update the dictionary with the key/value pairs from v, overwriting existing keys.

Return None.

v can be a dictionary or iterable (list, tuple) where each element is iterable of length 2, or can be a key1=val1, key2=val2, … 

# example of xdict.update

xx = {"a": 1}

xx.update([[3, 4], ("a", 2)])
print(xx == {"a": 2, 3: 4})

xx.update([(5, 6), (7, 8)])
print(xx == {"a": 2, 3: 4, 5: 6, 7: 8})

xx.update(aa=8, bb=9)
print(xx == {"a": 2, 3: 4, 5: 6, 7: 8, "aa": 8, "bb": 9})

Clear, Copy

d.clear()
Remove all items. [see Python: dict={} vs dict.clear()]
d.copy()
Return a shallow copy of d. [see How to Copy a Nested List in Python?]
d.fromkeys(seq)
Return a new dictionary with keys from sequence seq (list or tuple). The values are all None.
d.fromkeys(seq, v)
Return a new dictionary with keys from sequence seq. The values are all v. 
# example of xdictionary.fromkeys

xx = {"a": 1, "b": 2, "c": 3}

h2 = xx.fromkeys([8, 9, "a"])
print(h2 == {8: None, 9: None, "a": None})

h3 = xx.fromkeys([8, 9, 10], "x")
print(h3 == {8: "x", 9: "x", 10: "x"})

print(xx == {"a": 1, "b": 2, "c": 3})

Dictionary View

d.viewitems()
A view to a kill 😨
d.viewkeys()
A view to a kill 😨
d.viewvalues()
A view to a kill 😨

Change Lists into Dictionary

Example of using zip(…). It does transposition.

# convert 2 lists into a dictionary, use zip()

aa = [1, 2, 3]
bb = ["a", "b", "c"]

print(dict(zip(aa, bb)))
# {1: 'a', 2: 'b', 3: 'c'}

# example of using zip()

aa = [1, 2, 3, 4]
bb = ["a", "b", "c", "d"]
cc = [10, 20, 30, 40]

print(zip(aa, bb, cc))
# <zip object at 0x106f40b88>

print(list(zip(aa, bb, cc)))
# [(1, 'a', 10), (2, 'b', 20), (3, 'c', 30), (4, 'd', 40)]

Python 2, Loop Thru Key/Value Pairs

These are not in python 3.

d.has_key(k)
Return true if key k exist in d.
d.iterkeys()
Return a iterator. Each element is a key. A short syntax is iter(d)
d.itervalues()
Return a iterator. Each element is a key's value.
d.iteritems()
Return a iterator. Each element is (key, value) pair.

Python Data Structure