Python set constructor syntax

Question

Does anyone know the justification for this confusing set construction syntax? I spent a day unable to find this bug because I missed a comma in constructing a set.

> {1 2} 
SyntaxError: invalid syntax  # This makes sense.
> {'a' 'b'} = set(['ab'])    # This does not.

Answer 1

That's not a set construction syntax thing. You're running into implicit string literal concatenation, a confusing and surprising corner of the language:

>>> 'a' 'b'
'ab'

If you write two string literals next to each other, they're implicitly combined into one string. (This only works with literals; str(3) str([]) is a syntax error, not '3[]' .)

Answer 2

This has nothing to do with sets.

Two string literals separated by whitespace are considered one string literal.

rationale = ('This is quite useful when you need to construct '
             'a long literal without useless "+" and without '
             'the indentation and newlines which triple-quotes bring.')

Answer 3

Do you mean

>>> {'a' 'b'} == set(['ab']) 
True

?

That's just because 2 strings are concatenated to 1 string:

>>> type('a' 'b')
<class 'str'>
>>> len('a' 'b')
2
>>> print('a' 'b')
ab