Dictionary Syntax

Dictionary Syntax#

Note

Source: Adapted from the C# edition (dictionaries/dictionarysyntax.rst). Python’s dict replaces C#’s Dictionary<K,V>. No type annotation is needed in the declaration. Python dicts preserve insertion order (guaranteed since Python 3.7) and use key in d in place of ContainsKey.

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:

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

You can also use dict() with keyword arguments:

e2sp = dict(one="uno", two="dos", three="tres")

Accessing and Modifying Entries#

Use square brackets to get or set a value by key:

print(e2sp["one"])       # "uno"
e2sp["four"] = "cuatro"  # add new entry
e2sp["two"] = "DOS"      # update existing entry

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

print(e2sp.get("five", "not found"))   # "not found"
print(e2sp.get("one", "not found"))    # "uno"

Membership Test#

Use in to test whether a key is present (replaces C#’s ContainsKey):

print("three" in e2sp)    # True
print("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.

del e2sp["two"]
removed = e2sp.pop("three", None)
print(len(e2sp))    # number of remaining entries

Iterating Over a Dictionary#

e2sp = {"one": "uno", "two": "dos", "three": "tres"}

for key in e2sp:             # iterate over keys (insertion order)
    print(key, "->", e2sp[key])

for key in e2sp.keys():      # explicit keys view
    print(key)

for val in e2sp.values():    # values view
    print(val)

for key, val in e2sp.items():  # key-value pairs
    print(f"{key}: {val}")

Output of the last loop:

one: uno
two: dos
three: tres

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).