Python Dictionaries and Dictionary Methods

A dictionary maps keys to values: a student id to a name, a product code to a price, a word to its count. Lookups by key are fast (O(1) on average), dictionaries keep insertion order (since Python 3.7), and JSON data from APIs maps directly onto them.

This lesson covers creating dictionaries, reading and updating values safely, every dictionary method — get, keys, values, items, update, pop, popitem, setdefault, fromkeys, clear, copy — merging, dict comprehensions and nested dictionaries.

Creating and Accessing

Create with braces {"name": "Asha", "age": 24}, dict(name="Asha"), dict(zip(keys, values)) or a comprehension. Keys must be hashable and unique; values can be anything. d[key] raises KeyError for missing keys, while d.get(key, default) returns a default. Assigning d[key] = value adds or updates.

The Methods

Every dict method:

• get(k, default) — safe read.
• keys(), values(), items() — live views for iteration.
• update(other) — merge in another dict or key/value pairs; d1 | d2 and d1 |= d2 also merge (3.9+).
• pop(k, default) — remove and return a value; popitem() removes the last inserted pair.
• setdefault(k, default) — return the value, inserting the default if the key is missing.
• fromkeys(keys, value) — class method creating a dict with the same value for each key.
• clear() — remove everything; copy() — shallow copy.

Iterating and Transforming

Iterating a dict yields keys; use .items() for key/value pairs. Dict comprehensions build or filter dictionaries in one expression. Sort by value with sorted(d.items(), key=lambda kv: kv[1]). Do not add or remove keys while iterating over a dict.

Nested Dictionaries

Real data is often nested — an API response with a user containing an address containing a city. Access with chained keys (data["user"]["address"]["city"]) or safely with chained get calls. For counting and grouping, collections.Counter and defaultdict (see the collections lesson) are even more convenient.

Examples

student = {"name": "Asha", "age": 24, "courses": ["Python"]}
print(student["name"], student.get("city"), student.get("city", "Unknown"))
student["city"] = "Pune"
student["age"] += 1
student["courses"].append("SQL")
print(student)
print(dict(zip(["a", "b"], [1, 2])), dict(x=1, y=2))
try:
    student["email"]
except KeyError as e:
    print("KeyError:", e)

Asha None Unknown
{'name': 'Asha', 'age': 25, 'courses': ['Python', 'SQL'], 'city': 'Pune'}
{'a': 1, 'b': 2} {'x': 1, 'y': 2}
KeyError: 'email'

prices = {"pen": 10, "book": 250}
print(list(prices.keys()), list(prices.values()), list(prices.items()))

prices.update({"bag": 899, "pen": 12})
print("update    :", prices)
removed = prices.pop("book")
print("pop       :", removed, prices, prices.pop("lamp", "not found"))
print("popitem   :", prices.popitem(), prices)
print("setdefault:", prices.setdefault("ink", 40), prices.setdefault("pen", 99), prices)
print("fromkeys  :", dict.fromkeys(["a", "b", "c"], 0))
copy_ = prices.copy()
prices.clear()
print("clear     :", prices, "copy:", copy_)
print("merge     :", {"a": 1, "b": 2} | {"b": 3, "c": 4})

['pen', 'book'] [10, 250] [('pen', 10), ('book', 250)]
update    : {'pen': 12, 'book': 250, 'bag': 899}
pop       : 250 {'pen': 12, 'bag': 899} not found
popitem   : ('bag', 899) {'pen': 12}
setdefault: 40 12 {'pen': 12, 'ink': 40}
fromkeys  : {'a': 0, 'b': 0, 'c': 0}
clear     : {} copy: {'pen': 12, 'ink': 40}
merge     : {'a': 1, 'b': 3, 'c': 4}

scores = {"Asha": 91, "Ravi": 72, "Meera": 88, "Kiran": 65}
for name, score in scores.items():
    print(f"{name}: {score}")

top = sorted(scores.items(), key=lambda kv: kv[1], reverse=True)[:2]
print("Top 2:", top)

passed = {n: s for n, s in scores.items() if s >= 70}
print(passed)

words = ["apple", "avocado", "banana", "blueberry", "cherry"]
groups = {}
for w in words:
    groups.setdefault(w[0], []).append(w)
print(groups)

Asha: 91
Ravi: 72
Meera: 88
Kiran: 65
Top 2: [('Asha', 91), ('Meera', 88)]
{'Asha': 91, 'Ravi': 72, 'Meera': 88}
{'a': ['apple', 'avocado'], 'b': ['banana', 'blueberry'], 'c': ['cherry']}

response = {
    "user": {"id": 7, "name": "Asha", "address": {"city": "Pune", "pin": "411001"}},
    "orders": [{"id": 101, "total": 2999}, {"id": 102, "total": 499}],
}
print(response["user"]["address"]["city"])
print(response.get("user", {}).get("phone", {}).get("mobile", "no phone"))
print(sum(o["total"] for o in response["orders"]))

Pune
no phone
3498