Python Basics — Accessing & Modifying Dictionaries

Accessing & Modifying Dictionaries

Learn how to read, add, update, and delete dictionary entries.

Bracket Notation

user = {"name": "Alice", "age": 30, "city": "New York"}

print(user["name"])  # Alice
print(user["age"])   # 30

If the key doesn’t exist, you get a KeyError:

# print(user["email"])  # KeyError: 'email'

.get() — Safe Access

user = {"name": "Alice", "age": 30}

print(user.get("name"))       # Alice
print(user.get("email"))      # None (no error!)
print(user.get("email", "N/A"))  # N/A (custom default)

Rule of thumb: Use [] when you’re sure the key exists. Use .get() when a key might be missing.

Adding Key-Value Pairs

user = {"name": "Alice"}

user["age"] = 30
user["email"] = "alice@example.com"

print(user)
# {'name': 'Alice', 'age': 30, 'email': 'alice@example.com'}

Updating Existing Values

user = {"name": "Alice", "age": 30}

user["age"] = 31  # update existing key
print(user)  # {'name': 'Alice', 'age': 31}

Deleting Entries

del Statement

user = {"name": "Alice", "age": 30, "city": "New York"}

del user["city"]
print(user)  # {'name': 'Alice', 'age': 30}

# KeyError if key doesn't exist
# del user["phone"]  # KeyError: 'phone'

.pop() — Delete and Return Value

user = {"name": "Alice", "age": 30, "city": "New York"}

city = user.pop("city")
print(city)  # New York
print(user)  # {'name': 'Alice', 'age': 30}

# With default (no error if missing)
phone = user.pop("phone", "N/A")
print(phone)  # N/A

.popitem() — Remove Last Inserted

user = {"name": "Alice", "age": 30, "city": "New York"}

last = user.popitem()
print(last)  # ('city', 'New York')
print(user)  # {'name': 'Alice', 'age': 30}

.setdefault() — Get or Set

Returns the value if the key exists. If not, inserts the key with the given default:

user = {"name": "Alice"}

# Key exists — returns existing value
name = user.setdefault("name", "Unknown")
print(name)  # Alice

# Key doesn't exist — inserts and returns default
age = user.setdefault("age", 0)
print(age)   # 0
print(user)  # {'name': 'Alice', 'age': 0}

Practical Use — Grouping

words = ["apple", "banana", "avocado", "blueberry", "cherry"]
grouped = {}

for word in words:
    first_letter = word[0]
    grouped.setdefault(first_letter, []).append(word)

print(grouped)
# {'a': ['apple', 'avocado'], 'b': ['banana', 'blueberry'], 'c': ['cherry']}

KeyError Handling

user = {"name": "Alice"}

# Option 1: Check first
if "email" in user:
    print(user["email"])
else:
    print("No email")

# Option 2: get() with default
print(user.get("email", "No email"))

# Option 3: try/except
try:
    print(user["email"])
except KeyError:
    print("No email")

Checking Key Existence

user = {"name": "Alice", "age": 30}

print("name" in user)     # True
print("email" in user)    # False
print("email" not in user)  # True

in checks keys, not values.

Summary

• d["key"] — access (raises KeyError if missing)
• d.get("key", default) — safe access
• d["key"] = value — add or update
• del d["key"] — delete (raises KeyError if missing)
• d.pop("key", default) — delete and return
• d.popitem() — delete and return last pair
• d.setdefault("key", default) — get or insert
• "key" in d — check if key exists

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