How to Make a Dictionary in Python

What you’ll build or solve

You’ll create dictionaries in Python, add and update key-value pairs, and read values safely without KeyError.

When this approach works best

This approach works best when you need to:

• Store related data by name, like {"name": "Anna", "city": "Boston"}.
• Look up values fast by a key, like prices["apple"].
• Count or group items, like counting words or grouping orders by status.

Avoid this approach when:

• You only need items in order and access by position. A list is simpler.
• You need multiple values for the same key. Use a list as the value, like {"tag": ["a", "b"]}.

Prerequisites

• Python 3 installed
• You know what variables are
• You know what a list is
• You can run a short Python script

Step-by-step instructions

1) Create a dictionary

Use curly braces with key: value pairs.

person = {"name": "Anna", "age": 29, "city": "Boston"}
print(person)

Keys are usually strings, but they can also be numbers or tuples.

2) Start empty and add items

This works well when you build data step by step.

person = {}
person["name"] = "Ana"
person["age"] = 29
print(person)

What to look for:

Assigning person["age"] = 29 creates the key if it is missing, or updates it if it already exists.

3) Read values safely

Use square brackets when you are sure the key exists. Use get() when it might not.

person = {"name": "Ana", "age": 29}

print(person["name"])
print(person.get("city"))

get() returns None if the key is missing. You can provide a default value:

print(person.get("city", "Unknown"))

4) Update or overwrite a value

Assign to an existing key to change its value.

person = {"name": "Ana", "age": 29}
person["age"] = 30
print(person)

If the key does not exist, this assignment creates it.

5) Loop through keys and values

Looping helps you print, transform, or validate dictionaries.

person = {"name": "Anna", "age": 29, "city": "Boston"}

for key, value in person.items():
    print(key, value)

If you only need keys, use for key in person:.

If you only need values, use for value in person.values():.

Examples you can copy

1) Count words in a sentence

text = "learn python learn dictionaries"
counts = {}

for word in text.split():
    counts[word] = counts.get(word, 0) + 1

print(counts)

This pattern avoids KeyError by using get() with a default.

2) Build a simple price lookup

prices = {"apple": 1.2, "banana": 0.8, "orange": 1.5}

item = "banana"
print(prices.get(item, "missing"))

3) Group tasks by status

tasks = [
    ("Write outline", "todo"),
    ("Draft post", "doing"),
    ("Publish", "done"),
]

by_status = {}

for title, status in tasks:
    if status not in by_status:
        by_status[status] = []
    by_status[status].append(title)

print(by_status)

Each key stores a list of related items.

Common mistakes and how to fix them

Mistake 1: Forgetting the : between key and value

You might write:

person = {"name" "Ana"}

Why it breaks:

Dictionaries require key: value pairs, so this causes a syntax error.

Correct approach:

person = {"name": "Ana"}

Mistake 2: Using an unhashable type as a key

You might write:

bad_key = ["a", "b"]
data = {bad_key: 1}

Why it breaks:

Lists can change, so Python does not allow them as dictionary keys.

Correct approach:

good_key = ("a", "b")
data = {good_key: 1}
print(data)

Tuples are immutable, so Python allows them as keys.

Mistake 3: Getting a KeyError when reading a missing key

You might write:

person = {"name": "Ana"}
print(person["city"])

Why it breaks:

Square bracket access fails if the key does not exist.

Correct approach:

person = {"name": "Ana"}
print(person.get("city", "Unknown"))

Troubleshooting

If you see KeyError, switch to dict.get(key) or check if key in dict_name: before accessing.

If you see SyntaxError near a dictionary literal, check for missing : or missing commas between pairs.

If you see TypeError: unhashable type, you used a mutable type like a list as a key. Use a string, number, or tuple instead.

If your loop prints only keys, use .items() to get both keys and values.

Quick recap

• Create a dictionary with {} and key: value pairs.
• Add or update items with d[key] = value.
• Read safely with d.get(key, default) when keys may be missing.
• Update values by assigning to an existing key.
• Loop with for key, value in d.items(): to access both.