How does python "throw away" variable work?

Question

Lets say I have:

[obj for (_, obj) in stack]

This code assumes that the first object in stack is a tuple, and throws away the first part of the tuple.

What happens if the code is not a tuple, but a single object?

Does it just ignore the thrown-away part and take the whole object?

Answer 1

_ it's just a convention, any other name will behave same way.

Name _ simply points to first element of unpacked tuple. When that name goes out of scope reference, counter is decreased, there are no other named referencing to "first element of unpacked tuple", and that object may be safely garbage-collected.

Since _ is only convention, attempt to unpack tuple with _ will behave same as with any other name - it'll raise an exception, one of following:

a, b = 1  # TypeError: 'int' object is not iterable
a, b = () # ValueError: need more than 0 values to unpack
a, b = (1, 2, 3) # ValueError: too many values to unpack