The parens are required. What you've written raises a SyntaxError. You can omit them in the generator expression if it's being passed directly as the only para

The parens are required. What you've written raises a SyntaxError. You can omit them in the generator expression if it's being passed directly as the only parameter to a function: halve_evens_only = list(i/2 for i in nums if i % 2 != 0)