Dictionary Syntax#

A dictionary maps keys to values. Given a key, you can look up its associated value in constant time — like looking up a word in a reference book.

Creating Dictionaries#

Use braces with key: value pairs. Run this and edit the entries:

>>> e2sp = {"one": "uno", "two": "dos", "three": "tres"}
>>> word_count = {}       # empty dict
>>> ages = {"Alice": 30, "Bob": 25}
>>> e2sp
{'one': 'uno', 'two': 'dos', 'three': 'tres'}
>>> word_count
{}
>>> ages
{'Alice': 30, 'Bob': 25}

You can also use dict() with keyword arguments. Try it live:

>>> e2sp = dict(one="uno", two="dos", three="tres")
>>> e2sp
{'one': 'uno', 'two': 'dos', 'three': 'tres'}

Accessing and Modifying Entries#

Use square brackets to get or set a value by key. Run this and edit the keys and values:

>>> e2sp = {"one": "uno", "two": "dos", "three": "tres"}
>>> e2sp["one"]
'uno'
>>> e2sp["four"] = "cuatro"   # add new entry
>>> e2sp["two"] = "DOS"       # update existing entry
>>> e2sp
{'one': 'uno', 'two': 'DOS', 'three': 'tres', 'four': 'cuatro'}

Accessing a key that does not exist raises a KeyError. Use dict.get(key, default) to avoid the exception. Try both forms live:

>>> e2sp = {"one": "uno", "two": "dos", "three": "tres"}
>>> e2sp.get("five", "not found")
'not found'
>>> e2sp.get("one", "not found")
'uno'
>>> e2sp["five"]
Traceback (most recent call last):
    ...
KeyError: 'five'

Membership Test#

Use in to test whether a key is present. Try it live:

>>> e2sp = {"one": "uno", "two": "dos", "three": "tres"}
>>> "three" in e2sp
True
>>> "seven" in e2sp
False

Removing Entries#

del d[key] removes the entry (raises KeyError if absent).
d.pop(key) removes and returns the value; accepts an optional default.
d.clear() removes all entries.

Run this and watch the dictionary shrink:

>>> e2sp = {"one": "uno", "two": "dos", "three": "tres"}
>>> del e2sp["two"]
>>> removed = e2sp.pop("three", None)
>>> removed
'tres'
>>> len(e2sp)         # number of remaining entries
1
>>> e2sp
{'one': 'uno'}

Iterating Over a Dictionary#

A dictionary can be iterated several ways. Run this and edit the loops:

>>> e2sp = {"one": "uno", "two": "dos", "three": "tres"}
>>> for key in e2sp:               # iterate over keys (insertion order)
...     print(key, "->", e2sp[key])
...
one -> uno
two -> dos
three -> tres
>>> for key in e2sp.keys():        # explicit keys view
...     print(key)
...
one
two
three
>>> for val in e2sp.values():      # values view
...     print(val)
...
uno
dos
tres
>>> for key, val in e2sp.items():  # key-value pairs
...     print(f"{key}: {val}")
...
one: uno
two: dos
three: tres

Build and modify a dictionary below. Try adding keys, updating values, and calling .get() with a missing key to see how it differs from [] access:

>>> e2sp = {"one": "uno", "two": "dos", "three": "tres"}
>>> e2sp["four"] = "cuatro"             # add a new entry
>>> e2sp["one"]
'uno'
>>> e2sp.get("five", "not found")       # safe lookup with default
'not found'
>>> "three" in e2sp
True
>>> for key, val in e2sp.items():
...     print(f"{key}: {val}")
...
one: uno
two: dos
three: tres
four: cuatro

Key Type Restriction#

Dictionary keys must be immutable — strings, numbers, and tuples are valid keys; lists are not. This restriction exists because of how hash tables work (see Dictionary Efficiency).

Dictionary Syntax

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