Dictionaries in Python: Key-Value Pairs and Methods

What is a dictionary? Key-value pairs

A dictionary (dict) stores data as key: value pairs in { }.

marks = {"Ravi": 85, "Asha": 92}

• Each key must be unique. It must be immutable: str, int, float or tuple (not a list).
• A value can be any type and can repeat.
• A dict is mutable: pairs can be added, changed and removed.
• Since Python 3.7, the pairs keep the order in which they were added, but we reach them by key, not by index.

Creating a dictionary

d = {}                          # empty
d = dict()                      # empty
d = {"roll": 5, "name": "Ira"}
d = dict(roll=5, name="Ira")
d = dict([("roll", 5), ("name", "Ira")])

d = {}
for i in range(3):              # from input
    n = input("Name: ")
    d[n] = int(input("Marks: "))

Accessing and traversing

d[key] gives the value; a missing key gives KeyError. d.get(key, default) returns the default (or None) instead of an error.

key in d checks if a key exists (it checks keys, not values).

for k in d:                  # keys
    print(k, d[k])
for k, v in d.items():       # pairs
    print(k, v)

Updating and deleting

• d[key] = value: if the key exists, its value is changed; if not, a new pair is added.
• d.update(d2): adds or changes many pairs at once.
• del d[key]: deletes a pair (KeyError if missing).
• d.pop(key): deletes and returns the value.
• d.popitem(): deletes and returns the last added pair.
• d.clear(): empties the dictionary.

Dictionary methods and functions

• len(d): number of pairs
• d.keys(), d.values(), d.items(): views of keys, values, pairs
• d.get(k, default)
• d.update(d2), d.pop(k), d.popitem(), d.clear()
• d.copy(): a separate copy
• dict.fromkeys(keys, value): new dict with the given keys, all set to value
• d.setdefault(k, v): returns d[k]; if k is missing, adds it with v
• sorted(d): sorted list of keys; max(d), min(d) work on keys

Try it: count the letters

Type this program and run it with your own name:

word = input("Word: ")
d = {}
for ch in word:
    d[ch] = d.get(ch, 0) + 1
print(d)

Predict the output for "banana" first. Then open step 6 of the 3D, type the same word, and watch each locker fill.

Key formulas and definitions

• d = {key: value, …}; keys unique and immutable
• d[k] = v → add if new, change if old
• d.get(k, default) avoids KeyError
• keys(), values(), items(), update(), pop(), popitem(), clear(), copy(), fromkeys(), setdefault()

Worked examples

Common mistakes

• Using d[0] to get the "first" item; a dict is reached by key, not index.
• Using a list as a key; keys must be immutable.
• Expecting "value in d" to search values; in checks keys only.
• Thinking a repeated key keeps both values; the later one replaces the earlier.