🐍 Python📅 Day 13

Dictionaries in Python

Store data as key–value pairs, look values up by key, add, update and delete entries, and loop over a dictionary.

🎯

Learning objectives

→Create dictionaries and read values by key
→Add, update and delete key–value pairs
→Loop over keys, values and items
→Use get(), update(), pop(), setdefault() and fromkeys(), and count things with a dictionary

💡 Key points

A dictionary (dict) stores key: value pairs inside { }. You look up a value by its key, not by a position.
Keys must be unique and immutable (str, int, tuple). Values can be anything, even lists or other dicts.
d[key] raises KeyError if the key is missing; d.get(key, default) returns the default instead.
d[key] = value adds the pair if the key is new, or replaces the old value if the key already exists.
keys(), values() and items() let you loop over a dict; items() gives (key, value) tuples.
del d[key] and d.pop(key) remove one entry; d.clear() empties the dictionary.
Since Python 3.7, dictionaries remember the order in which keys were added.

💻 Code examples(6)

#1Create and access

python

student = {"name": "Amit", "class": 12,
           "marks": 87}
print(student["name"])
print(student["marks"])
print(len(student))          # number of pairs

# print(student["age"])      # KeyError: 'age'
print(student.get("age"))
print(student.get("age", "not given"))

Output

Amit
87
3
None
not given

Square brackets need the key to exist. get() is the safe way to read a key that may be missing.

#2Add, update and delete

python

prices = {"pen": 10, "pencil": 5}
prices["eraser"] = 3       # new key: added
prices["pen"] = 12         # existing key: updated
print(prices)

del prices["pencil"]
print(prices)
removed = prices.pop("eraser")
print(removed, prices)

prices.update({"pen": 15, "ruler": 20})
print(prices)

Output

{'pen': 12, 'pencil': 5, 'eraser': 3}
{'pen': 12, 'eraser': 3}
3 {'pen': 12}
{'pen': 15, 'ruler': 20}

Dictionaries are mutable. update() merges another dict in: existing keys get new values and new keys are added.

#3Looping over a dictionary

python

marks = {"Amit": 87, "Priya": 92, "Ravi": 76}
print(list(marks.keys()))
print(list(marks.values()))
print(list(marks.items()))

for name, m in marks.items():
    print(f"{name} scored {m}")

avg = sum(marks.values()) / len(marks)
print("Class average:", avg)

Output

['Amit', 'Priya', 'Ravi']
[87, 92, 76]
[('Amit', 87), ('Priya', 92), ('Ravi', 76)]
Amit scored 87
Priya scored 92
Ravi scored 76
Class average: 85.0

A plain for name in marks: loop gives the keys. Use items() when you need both the key and the value.

#4Counting the frequency of characters

python

text = "programming"
freq = {}
for ch in text:
    if ch in freq:
        freq[ch] += 1
    else:
        freq[ch] = 1
print(freq)

# the same thing, shorter, with get()
freq2 = {}
for ch in text:
    freq2[ch] = freq2.get(ch, 0) + 1
print(freq2 == freq)

Output

{'p': 1, 'r': 2, 'o': 1, 'g': 2, 'a': 1, 'm': 2, 'i': 1, 'n': 1}
True

This counting pattern works for letters, words, votes or anything else. in on a dict checks the keys.

#5fromkeys, setdefault, copy and popitem

python

subjects = ["Maths", "Physics", "CS"]
marks = dict.fromkeys(subjects, 0)
print(marks)

marks["CS"] = 95
marks.setdefault("English", 80)  # missing: added
marks.setdefault("CS", 10)       # exists: no change
print(marks)

backup = marks.copy()
print(marks.popitem())      # removes last added
print(sorted(backup))       # keys, A to Z
print(max(backup, key=backup.get))

Output

{'Maths': 0, 'Physics': 0, 'CS': 0}
{'Maths': 0, 'Physics': 0, 'CS': 95, 'English': 80}
('English', 80)
['CS', 'English', 'Maths', 'Physics']
CS

copy() makes an independent dict, so popitem() on marks does not touch backup. max(d, key=d.get) finds the key with the biggest value.

#6Student records: a dict of dicts

python

students = {
    101: {"name": "Amit", "marks": 87},
    102: {"name": "Priya", "marks": 92},
    103: {"name": "Ravi", "marks": 76},
}
roll = 102
if roll in students:
    s = students[roll]
    print(f"{s['name']} scored {s['marks']}")

toppers = [s["name"] for s in students.values()
           if s["marks"] >= 85]
print("85 and above:", toppers)

Output

Priya scored 92
85 and above: ['Amit', 'Priya']

Roll numbers make good keys because they are unique. Each value is another dict, reached with a second pair of brackets.

🎯 Practice

Q1. What does d.get('x', 0) return if 'x' is not a key of d?+

0, the default you passed. d['x'] would raise KeyError instead.

Q2. Can a list be used as a dictionary key?+

No. Keys must be immutable, so a list raises TypeError: unhashable type: 'list'. Use a tuple instead.

Q3. Write a program to count how many times each word appears in 'the cat and the hat'.+

count = {} for w in 'the cat and the hat'.split(): count[w] = count.get(w, 0) + 1 print(count) # {'the': 2, 'cat': 1, 'and': 1, 'hat': 1}

Q4. What is printed? d = {'a': 1, 'b': 2}; d['a'] = 5; print(len(d))+

2. Assigning to an existing key replaces its value; it does not add a new pair.

Q5. What is the difference between pop() and popitem()?+

pop(key) removes the given key and returns its value. popitem() removes and returns the last added (key, value) pair.

📝 Notes

List or dictionary?

A list is best when items are in a sequence and you reach them by position: marks[0].
A dictionary is best when each value has a label and you reach it by name: marks["Amit"].

Looking up a key is fast even when the dictionary holds thousands of entries, because Python does not search through the items one by one.

Dictionary comprehension

Like the list comprehensions you saw with loops, you can build a dict in one line:

squares = {n: n * n for n in range(1, 6)}
print(squares)    # {1: 1, 2: 4, 3: 9, 4: 16, 5: 25}

passed = {k: v for k, v in marks.items() if v >= 33}

Building a dict with dict()

d1 = dict(name="Neha", age=16)
d2 = dict([("a", 1), ("b", 2)])
d3 = dict(zip(["x", "y"], [10, 20]))

All three are handy when the data comes from somewhere else, such as two lists or user input.

Common mistakes

KeyError: reading d["Rahul"] when there is no such key. Check with if "Rahul" in d: or use get().
Using positions: d[0] does not mean "first item" — it looks for the key 0.
Changing size while looping: deleting keys inside for k in d: raises RuntimeError: dictionary changed size during iteration. Loop over list(d) instead.
Duplicate keys: {"a": 1, "a": 2} keeps only the last value, {'a': 2}.
Thinking in checks values: 87 in marks looks at keys. Use 87 in marks.values().

Next: Day 14 — Sets, for collections of unique items.