迭代地建立元组集

Question

I am trying to iteratively build a set of tuples using set.add(tuple) . The problem is that first tuple does not remain encapsulated as a tuple. Its members enter the set as unique elements.

# What I'd like to do
s = set((1,2))
s.add((3,4))
s.add((5,6))

>>>s
>>>{1, 2, (3, 4), (5, 6)}

Of course I can build the whole thing up as a list of tuples and then convert it to a set, but I'd like to know if it is possible to avoid casting from list to set.

# Works with some extra work
l = [(1,2)]
l.append((3,4))
l.append((5,6))
s = set(l)

>>>s
>>>> {(1, 2), (3, 4), (5, 6)}

Thanks.

Answer 1

You should create your set like this:

s = {(1, 2)}

If you do not, the set() "constructor" while unpack all elements from your tuple and add them to a new empty set.

Answer 2

On Python 2.7 and later, you can use a set literal:

s = {(1, 2)}

If you want to use set to make a set whose one element is thing , you do

s = set([thing])

where the argument to set is a 1-element list. If thing is (1, 2)`, that means you do

s = set([(1, 2)])

Answer 3

The one-argument set constructor takes an iterable of things to put into the set. In particular, it does not accept an object and create the singleton set containing that object.

You didn't get an error, because tuples are iterable; the set constructor dutifully added each element of the tuple to the set.

Better is to use the constructor {(1,2)} rather than set(((1,2),)) or set([(1,2)]) ; the {...} syntax is overloaded to allow creating set instances as well as dict instancess. (but you can't make an empty set this way; in face of ambiguity, python assumes dict )