Should a return statement have parentheses?

Question

Suppose we have in Python 3.x (and I guess in Python 2.6 and in Python 2.7 too) the following functions:

>>> def dbl_a(p): return p*2
>>> d         


        

Answer 1

There are generally 4 uses for the parentheses () in Python.

1. It acts the same way as most of the other mainstream languages - it's a construct to force an evaluation precedence, like in a math formula. Which also means it's only used when it is necessary, like when you need to make sure additions and subtractions happen first before multiplications and divisions.
2. It is a construct to group immutable values together in the same spirit as a similar set notation in math. We call this a tuple in Python. Tuple is also a basic type. It is a construct to make an empty tuple and force operator precedence elevation.
3. It is used to group imported names together in import statements so you don't have to use the multi-line delimiter \. This is mostly stylistic.
4. In long statements like

    decision = (is_female and under_30 and single
                or
                is_male and above_35 and single)

the parenthesis is an alternative syntax to avoid hitting the 80 column limit and having to use \ for statement continuation.

In any other cases, such as inside the if, while, for predicates and the return statement I'd strongly recommend not using () unless necessary or aid readability (defined by the 4 points above). One way to get this point across is that in math, (1) and just 1 means exactly the same thing. The same holds true in Python.

People coming from the C-family of languages will take a little bit getting used to this because the () are required in control-flow predicates in those languages for historical reasons.

Last word for return statements, if you are only returning 1 value, omit the (). But if you are returning multiple values, it's OK to use () because now you are returning a grouping, and the () enforces that visually. This last point is however stylistic and subject to preference. Remember that the return keywords returns the result of a statement. So if you only use , in your multiple assignment statements and tuple constructions, omit the (), but if you use () for value unpacking and tuple constructions, use () when you are returning multiple values in return. Keep it consistent.