Learning

Dictionaries Mastery

Creating Dictionaries

python

# Literal syntax
person = {
    'name': 'Alice',
    'age': 30,
    'city': 'New York'
}

# Constructor
empty = dict()
from_pairs = dict([('a', 1), ('b', 2)])  # {'a': 1, 'b': 2}
from_kwargs = dict(name='Bob', age=25)   # {'name': 'Bob', 'age': 25}

# fromkeys (all keys get same value)
keys = ['a', 'b', 'c']
d = dict.fromkeys(keys, 0)  # {'a': 0, 'b': 0, 'c': 0}

Accessing Values

python

d = {'x': 10, 'y': 20}

# Square brackets (KeyError if missing!)
print(d['x'])  # 10
# print(d['z'])  # KeyError!

# get() method (Returns None or default if missing)
print(d.get('y'))      # 20
print(d.get('z'))      # None
print(d.get('z', 99))  # 99

# setdefault (Get value, or INSERT if missing)
val = d.setdefault('z', 99)
# d is now {'x': 10, 'y': 20, 'z': 99}

Modifying Dictionaries

python

d = {'a': 1, 'b': 2}

# Add / Update
d['c'] = 3       # Add new
d['a'] = 100     # Update existing

d.update({'d': 4, 'a': 200})  # Update multiple

# Delete
del d['b']       # Deletes key 'b' (KeyError if missing)
val = d.pop('c')  # Removes 'c' and returns 3
val = d.pop('z', None) # Safe pop with default
d.clear()        # Empty the dict

Iteration Methods

python

d = {'name': 'Alice', 'age': 30, 'city': 'NYC'}

# Keys (default iteration)
for key in d:
    print(key)
for key in d.keys():
    print(key)

# Values
for val in d.values():
    print(val)

# Key-Value Pairs (most common!)
for key, val in d.items():
    print(f'{key}: {val}')

Dictionary Comprehension

python

# Basic
squares = {x: x**2 for x in range(5)}
# {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}

# With condition
evens = {x: x**2 for x in range(10) if x % 2 == 0}

# Transform existing dict
prices = {'apple': 1.2, 'banana': 0.5}
taxed = {k: round(v * 1.1, 2) for k, v in prices.items()}

# Invert dict (values must be unique!)
original = {'a': 1, 'b': 2}
inverted = {v: k for k, v in original.items()}

Nested Dictionaries

python

users = {
    'u1': {'name': 'Alice', 'role': 'admin'},
    'u2': {'name': 'Bob', 'role': 'user'}
}
print(users['u1']['name'])  # Alice

Key Rules

•Use d.get(key, default) instead of d[key] to avoid KeyError on missing keys
•setdefault() is like get() but it ALSO inserts the key with the default value if it's missing
•Always use d.items() to iterate over both keys and values simultaneously
•dict.update() can take another dict or keyword arguments to update multiple keys at once
•Dictionary comprehension syntax: {key_expr: val_expr for item in iterable if condition}

Your Task

Create a function `invert_dict(d)` that inverts a dictionary. If the original dict has duplicate values, group the keys into a list (e.g., {'a': 1, 'b': 1} → {1: ['a', 'b']}). Create a function `deep_get(d, keys, default=None)` that safely accesses nested dictionaries, e.g., deep_get(d, ['a', 'b', 'c']) without throwing KeyError.

Learning

Dictionaries Mastery

Creating Dictionaries

python

# Literal syntax
person = {
    'name': 'Alice',
    'age': 30,
    'city': 'New York'
}

# Constructor
empty = dict()
from_pairs = dict([('a', 1), ('b', 2)])  # {'a': 1, 'b': 2}
from_kwargs = dict(name='Bob', age=25)   # {'name': 'Bob', 'age': 25}

# fromkeys (all keys get same value)
keys = ['a', 'b', 'c']
d = dict.fromkeys(keys, 0)  # {'a': 0, 'b': 0, 'c': 0}

Accessing Values

python

d = {'x': 10, 'y': 20}

# Square brackets (KeyError if missing!)
print(d['x'])  # 10
# print(d['z'])  # KeyError!

# get() method (Returns None or default if missing)
print(d.get('y'))      # 20
print(d.get('z'))      # None
print(d.get('z', 99))  # 99

# setdefault (Get value, or INSERT if missing)
val = d.setdefault('z', 99)
# d is now {'x': 10, 'y': 20, 'z': 99}

Modifying Dictionaries

python

d = {'a': 1, 'b': 2}

# Add / Update
d['c'] = 3       # Add new
d['a'] = 100     # Update existing

d.update({'d': 4, 'a': 200})  # Update multiple

# Delete
del d['b']       # Deletes key 'b' (KeyError if missing)
val = d.pop('c')  # Removes 'c' and returns 3
val = d.pop('z', None) # Safe pop with default
d.clear()        # Empty the dict

Iteration Methods

python

d = {'name': 'Alice', 'age': 30, 'city': 'NYC'}

# Keys (default iteration)
for key in d:
    print(key)
for key in d.keys():
    print(key)

# Values
for val in d.values():
    print(val)

# Key-Value Pairs (most common!)
for key, val in d.items():
    print(f'{key}: {val}')

Dictionary Comprehension

python

# Basic
squares = {x: x**2 for x in range(5)}
# {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}

# With condition
evens = {x: x**2 for x in range(10) if x % 2 == 0}

# Transform existing dict
prices = {'apple': 1.2, 'banana': 0.5}
taxed = {k: round(v * 1.1, 2) for k, v in prices.items()}

# Invert dict (values must be unique!)
original = {'a': 1, 'b': 2}
inverted = {v: k for k, v in original.items()}

Nested Dictionaries

python

users = {
    'u1': {'name': 'Alice', 'role': 'admin'},
    'u2': {'name': 'Bob', 'role': 'user'}
}
print(users['u1']['name'])  # Alice

Key Rules

•Use d.get(key, default) instead of d[key] to avoid KeyError on missing keys
•setdefault() is like get() but it ALSO inserts the key with the default value if it's missing
•Always use d.items() to iterate over both keys and values simultaneously
•dict.update() can take another dict or keyword arguments to update multiple keys at once
•Dictionary comprehension syntax: {key_expr: val_expr for item in iterable if condition}

Your Task