list comprehension vs general ex

As an expression(in the grammar):

• [i for i in [1,2,3]] is a list comprehension;
• (i for i in [1,2,3]) is a general expression;
• But i for i in [1,2,3] by itself is a syntax error, and that's just the way it is. There must be something surrounding it. Unless you have ( or [ around it, it's not a valid expression, because the for keyword is not valid at that point.