Structured bindings in Python

Question

C++17 introduced the new structured bindings syntax:

std::pair<int, int> p = {1, 2};
auto [a, b] = p;

Is there something similar in python3? I was thinking of using the "splat" operator to bind class variables to a list, which can be unpacked and assigned to multiple variables like such:

class pair:
    def __init__(self, first, second):
        self.first = first
        self.second = second
...

p = pair(1, 2)
a, b = *p

Is this possible? And if so, how would I go by implementing this to work for my own classes?

A tuple in Python works as a simple solution to this problem. However, built in types don't give much flexibility in implementing other class methods.

Are you looking for a tuple? p = (1, 2) a, b = p

Alan Birtles
– Alan Birtles

2022-03-14 23:00:08 +00:00
Commented Mar 14, 2022 at 23:00 — Alan Birtles
– Alan Birtles, Commented Mar 14, 2022 at 23:00

Weston · Accepted Answer · 2022-12-14 10:24:40Z

6

Yes, you can use __iter__ method since iterators can be unpacked too:

class pair:
    def __init__(self, first, second):
        self.first = first
        self.second = second
    def __iter__(self):
        # Use tuple's iterator since it is the closest to our use case.
        return iter((self.first, self.second))

p = pair(1, 2)
a, b = p
print(a, b) # Prints 1 2

edited Dec 14, 2022 at 10:24

Weston

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answered Mar 14, 2022 at 23:01

Quimby

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2 Comments

Kelly Bundy Over a year ago

a, b = *p still doesn't work with that.

Kelly Bundy Over a year ago

yield self.first yield self.second might be a good alternative.

BrokenBenchmark · Accepted Answer · 2022-03-14 23:01:57Z

2

You can use tuples and tuple unpacking (see the bottom of this section in the Python documentation):

p = (1, 2)
a, b = p
print(a, b) # Prints 1 2

answered Mar 14, 2022 at 23:01

BrokenBenchmark

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1 Comment

Blackgaurd Over a year ago

The problem with using built in types is that I can't implement other class methods, but thanks for the help.

Kelly Bundy · Accepted Answer · 2022-03-15 15:30:03Z

1

An alternative with dataclass:

from dataclasses import dataclass, astuple

@dataclass
class pair:
    first: int
    second: int

p = pair(1, 2)
a, b = astuple(p)
print(a, b)

Output (Try it online!):

1 2

answered Mar 15, 2022 at 15:30

Kelly Bundy

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1 Comment

Weston Over a year ago

I would still be inclined to add def __iter__(self): return iter(astuple(self)) to remove the need for astuple call in user code

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