Question 1

How do I check if a key exists in a dictionary?

Accepted Answer

Use the 'in' operator to check if a key exists. This is the most Pythonic and efficient way.

Example code:
my_dict = {'name': 'Alice', 'age': 30}

# Check if key exists
if 'name' in my_dict:
    print("Key exists!")

# Check if key doesn't exist
if 'email' not in my_dict:
    print("Key not found")

# Alternative: use .get() which returns None if key missing
value = my_dict.get('email')  # Returns None
value = my_dict.get('email', 'default@example.com')  # Returns default

Question 2

How do I iterate over a dictionary?

Accepted Answer

You can iterate over keys, values, or both using .keys(), .values(), or .items() methods.

Example code:
person = {'name': 'Alice', 'age': 30, 'city': 'NYC'}

# Iterate over keys (default)
for key in person:
    print(key)

# Iterate over values
for value in person.values():
    print(value)

# Iterate over key-value pairs (most common)
for key, value in person.items():
    print(f"{key}: {value}")

Question 3

How do I sort a dictionary by value?

Accepted Answer

Use the sorted() function with a key parameter to sort by values. Python 3.7+ preserves insertion order.

Example code:
scores = {'Alice': 85, 'Bob': 92, 'Charlie': 78}

# Sort by value (ascending)
sorted_asc = dict(sorted(scores.items(), key=lambda x: x[1]))
# {'Charlie': 78, 'Alice': 85, 'Bob': 92}

# Sort by value (descending)
sorted_desc = dict(sorted(scores.items(), key=lambda x: x[1], reverse=True))
# {'Bob': 92, 'Alice': 85, 'Charlie': 78}

# Sort by key
sorted_by_key = dict(sorted(scores.items()))
# {'Alice': 85, 'Bob': 92, 'Charlie': 78}

Question 4

How do I merge two dictionaries?

Accepted Answer

Python 3.9+ supports the | operator. For older versions, use .update() or ** unpacking.

Example code:
dict1 = {'a': 1, 'b': 2}
dict2 = {'b': 3, 'c': 4}

# Python 3.9+ merge operator (creates new dict)
merged = dict1 | dict2  # {'a': 1, 'b': 3, 'c': 4}

# Python 3.5+ unpacking
merged = {**dict1, **dict2}

# Update in place (modifies dict1)
dict1.update(dict2)

# Note: Later values override earlier ones for duplicate keys

Question 5

What's the difference between dict[key] and dict.get(key)?

Accepted Answer

dict[key] raises KeyError if key doesn't exist, while dict.get(key) returns None (or a default value).

Example code:
person = {'name': 'Alice'}

# Direct access - raises KeyError if missing
try:
    age = person['age']  # KeyError!
except KeyError:
    age = None

# .get() - safe access with optional default
age = person.get('age')  # Returns None
age = person.get('age', 0)  # Returns 0

# Use direct access when you're certain the key exists
# Use .get() when the key might not exist

Question 6

How do I remove a key from a dictionary?

Accepted Answer

Use del, .pop(), or .popitem() depending on your needs.

Example code:
person = {'name': 'Alice', 'age': 30, 'city': 'NYC'}

# del - removes key, raises KeyError if missing
del person['city']

# .pop() - removes and returns value, can provide default
age = person.pop('age')  # Returns 30
email = person.pop('email', None)  # Returns None, no error

# .popitem() - removes and returns last inserted item
last = person.popitem()  # Returns ('name', 'Alice')

# .clear() - removes all items
person.clear()  # person is now {}

Question 7

How do I create a dictionary from two lists?

Accepted Answer

Use the zip() function combined with dict() to create a dictionary from parallel lists.

Example code:
keys = ['name', 'age', 'city']
values = ['Alice', 30, 'NYC']

# Create dict from two lists
person = dict(zip(keys, values))
# {'name': 'Alice', 'age': 30, 'city': 'NYC'}

# Using dictionary comprehension
person = {k: v for k, v in zip(keys, values)}

# With enumerate for indexed dict
items = ['apple', 'banana', 'cherry']
indexed = dict(enumerate(items))
# {0: 'apple', 1: 'banana', 2: 'cherry'}

Question 8

What is a dictionary comprehension?

Accepted Answer

Dictionary comprehension is a concise way to create dictionaries, similar to list comprehensions.

Example code:
# Basic syntax: {key: value for item in iterable}

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

# With condition
even_squares = {x: x**2 for x in range(10) if x % 2 == 0}
# {0: 0, 2: 4, 4: 16, 6: 36, 8: 64}

# Transform existing dict
prices = {'apple': 1.0, 'banana': 0.5}
discounted = {k: v * 0.9 for k, v in prices.items()}
# {'apple': 0.9, 'banana': 0.45}

# Swap keys and values
inverted = {v: k for k, v in prices.items()}

Question 9

How do I handle nested dictionaries?

Accepted Answer

Use chained .get() calls or write a helper function for safe nested access.

Example code:
data = {
    'user': {
        'profile': {
            'name': 'Alice',
            'settings': {'theme': 'dark'}
        }
    }
}

# Chained .get() - safe but verbose
theme = data.get('user', {}).get('profile', {}).get('settings', {}).get('theme')

# Helper function for dot-notation access
def get_nested(d, path, default=None):
    keys = path.split('.')
    for key in keys:
        if isinstance(d, dict):
            d = d.get(key, default)
        else:
            return default
    return d

theme = get_nested(data, 'user.profile.settings.theme')  # 'dark'
missing = get_nested(data, 'user.profile.email', 'N/A')  # 'N/A'

Question 10

What is defaultdict and when should I use it?

Accepted Answer

defaultdict automatically creates default values for missing keys. Use it when building collections like lists or counters.

Example code:
from collections import defaultdict

# defaultdict(list) - perfect for grouping
groups = defaultdict(list)
for item in [('a', 1), ('b', 2), ('a', 3)]:
    groups[item[0]].append(item[1])
# {'a': [1, 3], 'b': [2]}

# defaultdict(int) - perfect for counting
counts = defaultdict(int)
for char in 'hello':
    counts[char] += 1
# {'h': 1, 'e': 1, 'l': 2, 'o': 1}

# defaultdict(set) - unique values per key
tags = defaultdict(set)
tags['python'].add('programming')
tags['python'].add('programming')  # No duplicates
# {'python': {'programming'}}

Question 11

How do I copy a dictionary?

Accepted Answer

Use .copy() for shallow copy or copy.deepcopy() for nested dictionaries.

Example code:
import copy

original = {'name': 'Alice', 'scores': [90, 85, 88]}

# Shallow copy - nested objects are still shared!
shallow = original.copy()
shallow['scores'].append(95)  # Modifies original too!

# Deep copy - completely independent
deep = copy.deepcopy(original)
deep['scores'].append(100)  # Only affects deep copy

# Dict constructor also creates shallow copy
another_shallow = dict(original)

Question 12

Are dictionaries ordered in Python?

Accepted Answer

Yes! Since Python 3.7, dictionaries maintain insertion order. This is guaranteed in the language specification.

Example code:
# Insertion order is preserved
person = {}
person['name'] = 'Alice'
person['age'] = 30
person['city'] = 'NYC'

for key in person:
    print(key)  # Prints: name, age, city (in order)

# OrderedDict from collections is still useful for:
# - move_to_end() method
# - Comparing order between dicts
from collections import OrderedDict
od = OrderedDict()
od['a'] = 1
od['b'] = 2
od.move_to_end('a')  # Move 'a' to end

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