Advanced Dictionaries

Nested dicts, defaultdict, Counter, OrderedDict, and dictionary patterns.

Overview

Master advanced dictionary operations.

Nested Dictionaries

# Access nested values
config = {
    "database": {
        "host": "localhost",
        "port": 5432,
        "credentials": {
            "user": "admin",
            "pass": "secret"
        }
    }
}

# Safe access
def safe_get(d, keys, default=None):
    for key in keys:
        if isinstance(d, dict) and key in d:
            d = d[key]
        else:
            return default
    return d

print(safe_get(config, ["database", "host"]))  # localhost
print(safe_get(config, ["database", "missing"]))  # None

defaultdict

from collections import defaultdict

# Count occurrences
words = ["apple", "banana", "apple", "cherry", "banana", "apple"]
word_count = defaultdict(int)

for word in words:
    word_count[word] += 1

print(dict(word_count))  # {'apple': 3, 'banana': 2, 'cherry': 1}

# Group by
students = [
    ("Alice", "Math"),
    ("Bob", "Science"),
    ("Charlie", "Math"),
    ("Diana", "Science"),
    ("Eve", "Math")
]

groups = defaultdict(list)
for name, subject in students:
    groups[subject].append(name)

print(dict(groups))
# {'Math': ['Alice', 'Charlie', 'Eve'], 'Science': ['Bob', 'Diana']}

Counter

from collections import Counter

# Count elements
text = "hello world"
char_count = Counter(text)
print(char_count.most_common(3))  # [('l', 3), ('o', 2), ('h', 1)]

# Count list elements
fruits = ["apple", "banana", "apple", "cherry", "banana", "apple"]
fruit_count = Counter(fruits)
print(fruit_count["apple"])  # 3

# Update counters
counter1 = Counter(a=3, b=1)
counter2 = Counter(a=1, b=2)
print(counter1 + counter2)  # Counter({'a': 4, 'b': 3})
print(counter1 - counter2)  # Counter({'a': 2})

Dictionary Patterns

# Invert dictionary
original = {"a": 1, "b": 2, "c": 3}
inverted = {v: k for k, v in original.items()}
print(inverted)  # {1: 'a', 2: 'b', 3: 'c'}

# Merge dictionaries
dict1 = {"a": 1, "b": 2}
dict2 = {"b": 3, "c": 4}

# Python 3.9+
merged = dict1 | dict2

# Earlier versions
merged = {**dict1, **dict2}
print(merged)  # {'a': 1, 'b': 3, 'c': 4}

# Filter dictionary
data = {"a": 1, "b": 2, "c": 3, "d": 4}
filtered = {k: v for k, v in data.items() if v > 2}
print(filtered)  # {'c': 3, 'd': 4}

# Sort by value
sorted_dict = dict(sorted(data.items(), key=lambda x: x[1]))
print(sorted_dict)  # {'a': 1, 'b': 2, 'c': 3, 'd': 4}

Practice

Create a word frequency analyzer using Counter.