[Python-3000] PEP-3125 -- remove backslash continuation (original) (raw)

Guido van Rossum guido at python.org
Wed May 2 20:15:04 CEST 2007

On 5/2/07, Andrew Koenig <ark at acm.org> wrote:

Looking at PEP-3125, I see that one of the rejected alternatives is to allow any unfinished expression to indicate a line continuation.

I would like to suggest a modification to that alternative that has worked successfully in another programming language, namely Stu Feldman's EFL. EFL is a language intended for numerical programming; it compiles into Fortran with the interesting property that the resulting Fortran code is intended to be human-readable and maintainable by people who do not happen to have access to the EFL compiler. Anyway, the (only) continuation rule in EFL is that if the last token in a line is one that lexically cannot be the last token in a statement, then the next line is considered a continuation of the current line. Python currently has a rule that if parentheses are unbalanced, a newline does not end the statement. If we were to translate the EFL rule to Python, it would be something like this: The whitespace that follows an operator or open bracket or parenthesis can include newline characters. Note that if this suggestion were implemented, it would presumably be at a very low lexical level--even before the decision is made to turn a newline followed by spaces into an INDENT or DEDENT token. I think that this property solves the difficulty-of-parsing problem. Indeed, I think that this suggestion would be easier to implement than the current unbalanced-parentheses rule. Note also that like the current backslash rule, the space after the newline would be just space, with no special significance. So to rewrite the examples from the PEP: "abc" + # Plus is an operator, so it continues "def" # The extra spaces before "def" do not constitute an INDENT "abc" # Line does not end with an operator, so statement ends + "def" # The newline and spaces constitute an INDENT -- this is a syntax error ("abc" # I have no opinion about keeping the unbalanced-parentheses rule -- + "def") # but I do think that it is harder to parse (and also harder to read) # than what I am proposing.

I am worried that (as no indent is required on the next line) it will accidentally introduce legal interpretations for certain common (?) typos, e.g.

x = y+ # Used to be y+1, the 1 got dropped f(x)

Still, if someone wants to give implementing this a try we could add this to the PEP.

-- --Guido van Rossum (home page: http://www.python.org/~guido/)

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