Convert list to set based on duplicate of only certain values of a tuple

Question

In general, converting a list to set is simple, as below:

a = [1,2,3,1]
set_a = set(a)    # set([1, 2, 3])

Now, I want to convert a list of tuples to set, only considering the first value of tuple.

a = [(1,"a1"), (2,"b2"), (3, "c3"), (1, "d4")]
set_a = some_magic(a)

# 1) set_a = set([(1,"a1"), (2,"b2"), (3, "c3")]) or
# 2) set_a = set([(1, "d4"), (2,"b2"), (3, "c3")])
# Both (1) or (2) are acceptible outputs.

Is there a "one-line" trick I could use instead of some_magic function mentioned above?

I want to avoid making a separate list for book-keeping which of the first index of tuples are already used [which would have been the obvious answer otherwise]

Answer 1

try this:

set(dict(a).items())

dict(a) will turn the list into a dictionary where the keys get overwritten by each occurrence

the .items() extracts each key-value pair into a dict_items list of tuples

the set() turns it into the set you want

Note: this will give you the second example, keeping the very last unique tuple key. If you want the first one, a different approach will be needed