CodeBlanks Logo Icon CodeBlanks
Login Register

Dict and set comprehension

Learn {k: v for ...} and {x for ...}.

Dict comprehension

A dict comprehension builds a dictionary in one line. It uses curly braces and a colon:

squares = {n: n * n for n in [1, 2, 3]}
# {1: 1, 2: 4, 3: 9}

The shape is: {key: value for variable in list}. The key and value are computed for each item. If a key appears twice, the later one overwrites the earlier.

Set comprehension

A set comprehension also uses curly braces but no colon:

unique = {x % 3 for x in [1, 2, 4, 7]}
# {0, 1, 2}

The result is a set: each value appears once. The only difference from a list comprehension is the curly brackets.

A set is unordered and unique; if order matters, use a list.

Practice Write a squares dict

Assign to squares a dict comprehension that holds n: n * n pairs for each n in [1, 2, 3, 4]. Then print print(squares).

Starter code

main.py
# Build a squares dict with a comprehension

Sample solution

main.py
squares = {n: n * n for n in [1, 2, 3, 4]}
print(squares)

Expected output

{1: 1, 2: 4, 3: 9, 4: 16}

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Which is a dict comprehension?

  • [k: v for k, v in pairs]
  • (k: v for k, v in pairs)
  • {k: v for k, v in pairs} Correct answer
  • <k, v for k, v in pairs>

Explanation: Curly braces and a colon mark a dict comprehension.

2 Match the comprehension types

Term Meaning
[...] List
{k: v} Dict
{x} Set
(...) Generator

3 Complete the set comprehension

Question 
unique = {x % 3 for x in [____, 2, 4, 7]}
Answer 
unique = {x % 3 for x in [1, 2, 4, 7]}

Explanation: [1, 2, 4, 7] is the list, x % 3 gives the remainders.

All lessons in Beginner

1 Python basics

  1. What is Python?
  2. REPL and print
  3. Comments
  4. Indentation blocks
  5. python -c and scripts
  6. Summary - Python basics

2 Variables

  1. Variable assignment
  2. Multiple assignment
  3. Naming variables
  4. id and type
  5. Swapping variables
  6. Summary - Variables

3 Data types

  1. int and float
  2. bool
  3. str basics
  4. None
  5. Type conversion
  6. Summary - Data types

4 Project: Calculator

  1. Project design
  2. Add/subtract
  3. Multiply/divide
  4. Error handling
  5. Putting it together

5 Operators

  1. Arithmetic
  2. Comparison
  3. Logical
  4. Bitwise
  5. Membership and identity
  6. Summary - Operators

6 String basics

  1. f-string
  2. Concatenation
  3. str.format
  4. Raw strings
  5. Triple-quote
  6. Summary - String basics

7 Conditionals

  1. The if statement
  2. elif and else
  3. Nested if
  4. Truthiness
  5. Conditional expression
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. range
  3. The while loop
  4. break and continue
  5. Loop else
  6. Summary - Loops

9 Project: Shopping cart

  1. Cart design
  2. Adding items
  3. Calculating totals
  4. Updating quantity
  5. Putting it together

10 Lists

  1. Declaring lists
  2. Index and slice
  3. List methods
  4. in and count
  5. Sorting lists
  6. Summary - Lists

11 List comprehension

  1. Comprehension basics
  2. Filtered comprehension
  3. Dict and set comprehension
  4. Nested comprehension
  5. Generator expression
  6. Summary - List comprehension

12 Tuple, set and dict

  1. Tuple
  2. Set
  3. Dict basics
  4. Dict methods
  5. Dict comprehension
  6. Summary - Tuple, set and dict

13 Functions

  1. def
  2. return
  3. Docstring
  4. Default parameters
  5. *args and **kwargs
  6. Summary - Functions

14 Function parameters

  1. Positional parameters
  2. Keyword parameters
  3. Keyword-only
  4. Positional-only
  5. Type hints basics
  6. Summary - Function parameters

15 Project: To-do CLI

  1. Task model
  2. Adding tasks
  3. Listing tasks
  4. Completing tasks
  5. Putting it together

16 Advanced string methods

  1. split and join
  2. strip, lstrip, rstrip
  3. replace
  4. find and index
  5. startswith, endswith
  6. Summary - Advanced strings

17 Input and output

  1. input()
  2. int/float conversion
  3. print parameters
  4. f-string formatting
  5. sys.argv
  6. Summary - Input and output

18 Error handling

  1. try/except
  2. else and finally
  3. raise
  4. Exception hierarchy
  5. Custom exceptions
  6. Summary - Error handling

19 Modules

  1. import
  2. from import
  3. as alias
  4. __name__ == '__main__'
  5. Standard library
  6. Summary - Modules

20 Beginner final

  1. Project design
  2. Data model
  3. Business logic
  4. Error handling
  5. Putting it together