Adding the number 1 to a set has no effect

>>> 1 == True
True

I believe your problem is that 1 and True are the same value, so 1 is "already in the set".

>>> st
{'a', True, 'Vanilla'}
>>> 1 in st
True

In mathematical operations True is itself treated as 1:

>>> 5 + True
6
>>> True * 2
2
>>> 3. / (True + True)
1.5

Though True is a bool and 1 is an int:

>>> type(True)
<class 'bool'>
>>> type(1)
<class 'int'>

Because 1 in st returns True, I think you shouldn't have any problems with it. It is a very strange result though. If you're interested in further reading, @Lattyware points to PEP 285 which explains this issue in depth.

1 is equivalent to True as 1 == True returns true. As a result the insertion of 1 is rejected as a set cannot have duplicates.

I believe, though I'm not certain, that because hash(1) == hash(True) and also 1 == True that they are considered the same elements by the set. I don't believe that should be the case, as 1 is True is False, but I believe it explains why you can't add it.