Dictionaries

A dictionary is a mutable container that stores key-value pairs. Keys must be immutable (strings, numbers, or tuples), while values can be of any type.

my_dict = {"key": "value"}

Keys are unique within a dictionary; values may repeat.

Dictionary Methods

Method	Description
dict.get(key)	Returns the value for key, or None if not found
key in dict	Returns True if key exists in the dictionary
dict.items()	Returns a view of (key, value) pairs
dict.keys()	Returns a view of all keys
dict.values()	Returns a view of all values
dict.clear()	Removes all items
dict.setdefault(key, value)	Returns value if key exists; otherwise inserts key with value

Note

dict.has_key(key) was removed in Python 3. Use key in dict instead.

Examples

>>> d = {"Name": "Zara", "Age": 7, "Class": "First"}
>>> d["Name"]
'Zara'

>>> # Add a new entry
>>> d["School"] = "DPS School"
>>> d
{'Name': 'Zara', 'Age': 7, 'Class': 'First', 'School': 'DPS School'}

>>> # Update an existing entry
>>> d["Age"] = 8

>>> # List keys and values
>>> list(d.keys())
['Name', 'Age', 'Class', 'School']
>>> list(d.values())
['Zara', 8, 'First', 'DPS School']

>>> # Delete an entry
>>> del d["Name"]

Find a Key by Value

>>> d = {'george': 16, 'amber': 19}

>>> # Using a loop
>>> for name, age in d.items():
...     if age == 16:
...         print(name)
george

>>> # Using list comprehension
>>> [k for k, v in d.items() if v == 16]
['george']

Append to a List Inside a Dictionary

# Using setdefault (concise)
d.setdefault(key, []).append(value)

# Equivalent long form
if key in d:
    d[key].append(value)
else:
    d[key] = [value]

Iteration

>>> mydict = {"Hour": 13, "Day": "Monday"}
>>> for key, value in mydict.items():
...     print(key, value)
Hour 13
Day Monday

>>> knights = {"gallahad": "the pure", "robin": "the brave"}
>>> for k, v in knights.items():
...     print(k, v)
gallahad the pure
robin the brave

Warning

Python 2 vs Python 3 Use dict.items() instead of dict.iteritems() — the latter was removed in Python 3.