I am confused about where the r in the below code

Question

I am confused about where the r in the below code comes from in the combinations function. A bit new to Python.

from itertools import chain, combinations

def powerset(iterable):
    "powerset([1,2,3]) --> () (1,) (2,) (3,) (1,2) (1,3) (2,3) (1,2,3)"
    s = list(iterable)
    return chain.from_iterable(combinations(s, r) for r in range(len(s)+1))

[https://docs.python.org/3/reference/expressions.html#displays-for-lists-sets-and-dictionaries](https://docs.python.org/3/reference/expressions.html#displays-for-lists-sets-and-dictionaries) — wwii, Jul 17 '22 at 03:10

score 5 · Accepted Answer · answered Jul 17 '22 at 03:04

The variable r is the iterable of the generator comprehension statement passed to the function chain.from_iterable(). If you unpack the comprehension, it would be analogous to the below code.

from itertools import chain, combinations

def powerset(iterable):
    s = list(iterable)
    res = []
    for r in range(len(s) + 1):
        res.append(combinations(s, r))
    return chain.from_iterable(res)

Both the above for loop and the provided generator comprehension use the variable r as the iterator.

You can read more about generator comprehension from this Stack Overflow answer. While not identical, you can read more about list comprehension in the Python Documentation here.

I am confused about where the r in the below code

1 Answers1