Dictionaries — key/value storage

dicts map keys to values. O(1) lookup. Insertion-ordered since Python 3.7.

script.py v3.1

# Create
person = {
    'name': 'Ada',
    'age': 36,
    'city': 'London',
    'skills': ['Python', 'ML'],
}

# Access
print('name:', person['name'])
print('age:', person.get('age'))
print('phone (default):', person.get('phone', 'N/A'))  # no KeyError

# Add / update
person['email'] = 'ada@example.com'
person['age'] = 37
print('after add/update:', person)

# Iterate
for key in person:
    print(f'  {key}: {person[key]}')

for key, value in person.items():
    print(f'  {key} -> {value}')

# Delete
del person['email']
age = person.pop('age')
print(f'popped age: {age}, remaining keys: {list(person.keys())}')

# Dict comprehension
squares = {n: n*n for n in range(5)}
print('squares:', squares)

Python output click Run to execute

Click "Run" to execute the code on the left.
The output will appear here.

🧠 How it works

dicts are hash tables — keys must be hashable (str, int, tuple, frozenset; NOT list or dict). person['key'] raises KeyError if missing. person.get('key', default) returns default if missing. items() yields (key, value) pairs. keys() yields keys. values() yields values. del d[k] removes key k. d.pop(k) removes and returns the value. Dict comprehension: {k_expr: v_expr for item in iter if cond}. Useful: dict(zip(keys, values)) builds a dict from two lists. Counter, defaultdict, OrderedDict in collections module for special cases.

✏️ Try these changes

Change person['name'] to person['phone']
→ KeyError
Change .get('phone', 'N/A') to .get('phone')
→ returns None instead of 'N/A'
Add 'skills' to the iteration filter — skip non-string values
→ more complex iteration
Change squares to {n: n*n for n in range(10) if n % 2 == 0}
→ only even squares

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Lists — append, slice, sort, comprehensions

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if / elif / else + ternary