Sets

Unordered, no duplicates

A set holds unique values with no particular order:

tags = {"python", "beginner", "python"}   # {"python", "beginner"}
empty = set()    # {} would make an empty dictionary

Two things sets are good at. First, removing duplicates:

unique = list(set(items))

Second, answering "is this in here?" — for a large collection, x in some_set is far faster than x in some_list, because a set does not have to look through everything.

They also do the operations you remember from school maths:

a | b    # union — everything in either
a & b    # intersection — only what is in both
a - b    # difference — in a but not b

The trade-off: no order and no indexing. my_set[0] is an error.

Your turn: find the tags common to both lists, and everything either post used.

De-duplicating while keeping order

set() loses the original order, which is often fine and sometimes not. When order matters, walk the list and use a set only to remember what you have already seen:

seen = set()
result = []
for item in items:
    if item not in seen:
        seen.add(item)
        result.append(item)

Your turn: write unique(items) returning the items with duplicates removed, in the order they first appeared.