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.

Exercise 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).

Show example solution

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

Test yourself

Match positional-only features

/

Positional separator
positional-only

Cannot be keyword
compute(1, 2)

Valid call
Python 3.8

Version
Complete the positional-only call
```
compute(____, 2)
```
Show answer
```
compute(1, 2)
```
In def f(a, /, b): ... which call is valid?
- f(a=1, 2)
- f(1, b=2)
- f(1, 2)
- f(a=1, b=2)
Show answer

f(1, b=2) — a is positional-only (passed as 1), b can be keyword.

Next lesson: Type hints basics

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