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Positional-only

Learn positional-only parameters with /.

Positional-only parameters

Parameters before a / become positional-only; they cannot be passed as keyword:

def compute(a, b, /):
    return a + b

compute(1, 2)       # valid
compute(a=1, b=2)   # error

This is used when parameter names are an implementation detail; so renaming them later does not break calls.

This feature was added in Python 3.8.

Why positional-only?

Positional-only parameters are ideal when the parameter name is not important for the user:

def add(x, y, /):
    return x + y

Here x and y can be renamed; calls still work. The call is add(3, 4).

Some functions in the standard library (len, pow) are defined this way.

Practice Write a positional-only function

Define a function called compute with a and b positional-only parameters (then /). The body does return a + b. Assign compute(3, 4) to result. Then print print(result).

Starter code

main.py
# Define the compute function

Sample solution

main.py
def compute(a, b, /):
    return a + b
result = compute(3, 4)
print(result)

Expected output

7

Questions and answers

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

1 Match positional-only features

Term Meaning
/ Positional separator
positional-only Cannot be keyword
compute(1, 2) Valid call
Python 3.8 Version

2 Complete the positional-only call

Question 
compute(____, 2)
Answer 
compute(1, 2)

Explanation: compute(1, 2) is a positional call.

3 In def f(a, /, b): ... which call is valid?

  • f(a=1, 2)
  • f(1, b=2) Correct answer
  • f(1, 2)
  • f(a=1, b=2)

Explanation: a is positional-only (passed as 1), b can be keyword.

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