Dictionaries in Python

A dictionary stores data as key-value pairs. Instead of looking things up by position (like a list), you look them up by a meaningful key. You write a dictionary with curly braces:

student = {
    "name": "Asha",
    "age": 21,
    "city": "Jalgaon",
}

# Look up a value by its key
print(student["name"])    # Asha
print(student["age"])     # 21

Think of it like a real dictionary: you look up a word (the key) to find its meaning (the value). Let's dig in.

Creating and accessing

Each entry is key: value, separated by commas. Keys are usually strings, and they must be unique:

prices = {
    "pen": 20,
    "notebook": 60,
    "eraser": 10,
}

print(prices["notebook"])   # 60

Accessing a missing key with square brackets raises a KeyError. The safer way is .get(), which returns None (or a default you choose) instead of crashing:

print(prices.get("pencil"))         # None — no crash
print(prices.get("pencil", 0))      # 0 — your chosen default

Adding and updating

Assigning to a key adds it if new, or updates it if it already exists:

student = {"name": "Asha", "age": 21}

student["city"] = "Jalgaon"     # add a new key
student["age"] = 22             # update an existing key

print(student)
# {'name': 'Asha', 'age': 22, 'city': 'Jalgaon'}

Removing items

student = {"name": "Asha", "age": 22, "city": "Jalgaon"}

del student["city"]              # remove a key
removed = student.pop("age")     # remove and return its value
print(removed)                   # 22
print(student)                   # {'name': 'Asha'}

Looping through a dictionary

You'll often loop over the keys, the values, or both:

prices = {"pen": 20, "notebook": 60, "eraser": 10}

Loop over keys (the default)

for item in prices: print(item)

Loop over keys and values together using .items()

for item, price in prices.items(): print(f"{item} costs {price}")

Just the values

total = 0 for price in prices.values(): total += price print(f"Total: {total}") # 90

## Checking if a key exists

Use the `in` keyword — it checks keys, not values:

```python
prices = {"pen": 20, "notebook": 60}

if "pen" in prices:
    print("We sell pens")

if "stapler" not in prices:
    print("No stapler here")

A practical example

# Count how many times each word appears in a sentence

sentence = "the cat sat on the mat the cat slept"
words = sentence.split()       # split into a list of words

counts = {}                    # start with an empty dictionary
for word in words:
    # If we've seen the word, add 1; otherwise start at 1
    counts[word] = counts.get(word, 0) + 1

print(counts)
# {'the': 3, 'cat': 2, 'sat': 1, 'on': 1, 'mat': 1, 'slept': 1}

Common mistakes

• Accessing a missing key with []: d["missing"] raises a KeyError. Use d.get("missing") when a key might not exist.
• Using a list as a key: Keys must be immutable (strings, numbers, tuples). A list as a key raises a TypeError.
• Duplicate keys: If you write the same key twice, the last value wins — the earlier one is silently overwritten.
• Confusing keys and values with in: "pen" in prices checks the keys. To check values, use 20 in prices.values().
• Forgetting .items() when looping for pairs: for k, v in prices: fails. You need for k, v in prices.items():.

FAQ

Are dictionaries ordered? Yes. As of Python 3.7+, dictionaries keep items in the order you inserted them.

Can values be any type? Yes — values can be numbers, strings, lists, even other dictionaries. Only the keys have the immutability restriction.

How is a dictionary different from a list? A list uses numeric positions (0, 1, 2...). A dictionary uses meaningful keys you choose. Use a dictionary when each item has a natural label.

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Dictionaries are one of Python's most-used tools. Pair them with Lists in Python and learn about unique collections in Sets in Python. See all topics on the Python learning hub.

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