The ,= operator

Question

While looking at some Python code I noticed usage of what looked like ,= operator:

a ,= b

After experimentation and very close ins

Answer 1

The statement;

a, = b

• assumes b is iterable
• take first value from b and assigns it to a (near to a = b[0])
• asserts that b has no more values (i.e. len(b) == 1)

In that sense it is not the same as a = b[0]. The latter would work with b = ['a', 'b'], while the first will raise a ValueError: too many values to unpack. On the other side:

l = [x]
it = iter(l)
a, = it    #fine when a=it[0] raises TypeError: 'listiterator' object has no attribute '__getitem__'

TL/DR: if b is a list or a tuple with one single element, both a, = b and a = b[0] behave the same, but in other cases the may behave differently