Overview, Historical Timeline, Problems & Solutions
An Overview of Python Dictionaries
What is a Python dictionary?
You use a dictionary when you want to look up a value using a key. In real life, you might use a dictionary to look up the meaning of a word. In Python, you use a dictionary to look up a value using a word, number, or other key.
A Python dictionary is a built-in type that holds a set of key–value pairs. You use it when you want to store and retrieve data using keys instead of numbers. Keys must be unique and must not change.
Python lets you create dictionaries using braces and colons.
person = {"name": "Ada", "age": 36}
print(person["name"]) # prints: Ada
The braces {} hold the dictionary. Each entry inside has a key and a value, separated by a colon :.
How do keys and values work in a Python dictionary?
Each key in a Python dictionary points to one value. You cannot have the same key twice. A key can be a number, string, or tuple. A value can be anything.
When you write d[k], you get the value for key k in dictionary d. You can also use this form to add or change entries. If the key is not found, Python will raise an error unless you use a safe method.
Python lets you access and update dictionary values using square brackets.
d = {"a": 1}
print(d["a"]) # prints: 1
d["b"] = 2
print(d["b"]) # prints: 2
You can also use the .get() method to get a value safely. If the key is missing, .get() returns None or a value you choose.
Are Python dictionaries mutable?
Yes. Python dictionaries are mutable. This means you can add, remove, or update key–value pairs after the dictionary is created.
Python lets you change a dictionary after you create it.
data = {"a": 1}
data["b"] = 2
del data["a"]
print(data) # prints: {'b': 2}
You can use del to remove a key. You can use .clear() to remove all keys.
What types can be used as keys in Python dictionaries?
A Python dictionary key must not change. This means you cannot use lists or other dictionaries as keys. You can use strings, numbers, or tuples that contain only strings or numbers.
Python lets you use fixed types like strings and numbers as keys.
lookup = {1: "one", "two": 2, (3, 4): "pair"}
print(lookup["two"]) # prints: 2
print(lookup[(3, 4)]) # prints: pair
If you use a key that changes, Python will raise an error when you try to use it.
A Historical Timeline of Python Dictionaries
Where do Python’s dictionary rules come from?
Python dictionaries come from the idea of using a key to find a value. This idea started in early programming and grew as languages added better tools for storing structured data. Python shaped dictionaries to be fast, safe, and easy to use.
People invented key–value storage
1956 — Hashing in Symbol Tables IBM used hashing to store and find values by name in early compilers.
1973 — Associative arrays in AWK AWK added dynamic arrays with named keys for text processing.
People designed Python dictionaries
1991 — Built-in dictionaries with {} syntax Python 0.9.0 added dictionaries with literal notation and subscript access.
2000 — Dictionary views and .get() method Python 2.0 introduced .get() and views for keys, values, and items.
People expanded dictionary features
2010 — Dictionary comprehensions Python 3.0 added {k: v for k, v in iterable} syntax for fast creation.
2016 — Dictionary insertion order preserved Python 3.6 preserved the order of insertion as an implementation detail.
2018 — Ordered dictionaries by design Python 3.7 made insertion order an official part of the language.
People protected dictionary behavior
2023 — Key rules unchanged Python continued to forbid mutable types as keys to protect consistency.
2025 — Core dictionary type stable Python did not add new built-in mapping types or change the dictionary model.
Problems & Solutions with Python Dictionaries
How do you use Python dictionaries the right way?
Dictionaries help you match keys to values. You can look up names, counts, or any data by using a clear and simple key. The examples below show problems you face in daily tasks and how Python solves them with dictionaries.
Problem: How do you group facts about one thing in Python?
You are writing a program to store facts about a person. You could use a list, but then you must remember the order of each item. You want a way to label each fact with a name.
Problem: A list of values does not let you label each part clearly.
Solution: Use a dictionary so each value is stored under a key that tells what it is.
Python lets you store labeled data using dictionaries.
person = {"name": "Ada", "age": 36}
print(person["name"]) # prints: Ada
The key "name" tells what the value means. You do not need to guess the order of items.
Problem: How do you check if a key exists before using it in Python?
You are using data from a user, and you are not sure if all the expected keys are present. If you try to use a missing key, Python raises an error.
Problem: Using a missing key with [] causes a crash.
Solution: Use .get() to check for the key safely and return a default if it is missing.
Python lets you safely check for missing keys using .get().
data = {"name": "Ada"}
print(data.get("age")) # prints: None
print(data.get("age", 0)) # prints: 0
You can also check if a key exists using the in keyword.
Problem: How do you count things efficiently in Python?
You are counting how many times each word appears in a list. You do not want to write a long list of if-statements.
Problem: Manually tracking counts is slow and error-prone.
Solution: Use a dictionary to store each word and update the count each time it appears.
Python lets you count values fast using keys.
words = ["apple", "banana", "apple"]
counts = {}
for word in words:
counts[word] = counts.get(word, 0) + 1
print(counts) # prints: {'apple': 2, 'banana': 1}
Each key holds the count for that word. You use .get() to start at 0 the first time.
Problem: How do you update part of a record in Python?
You have a dictionary with several values, and you want to change just one. You do not want to create a new object each time.
Problem: Updating many fields without structure takes time.
Solution: Use dictionary assignment to change only the key you need.
Python lets you update one value by using its key.
profile = {"name": "Ada", "age": 36}
profile["age"] = 37
print(profile) # prints: {'name': 'Ada', 'age': 37}
This is fast and clear. You only touch what you need.
Problem: How do you store data with non-string keys in Python?
You are mapping points on a grid to values. Using strings as keys is slow and hard to manage.
Problem: Strings as keys do not help when your data is numeric or spatial.
Solution: Use tuples as keys to store positions or combinations.
Python lets you use tuples as keys to store structured data.
grid = {}
grid[(0, 0)] = "start"
grid[(1, 2)] = "checkpoint"
print(grid[(1, 2)]) # prints: checkpoint
The tuple key holds two numbers. You can now look up a location using (x, y).
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Mike Vincent is an American software engineer and app developer from Los Angeles, California. More about Mike Vincent
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