[Python-ideas] Line continuations with comments

[Mike Graham]

Backslash line continuations are mostly to be avoided and making a change
like this would seem to

a. make them slightly less obvious when they are used,
b. increase their use in cases where there aren't even long lines of code
involved, and
c. seem to encourage their use in general.

It seems to me that using parentheses is an already-existing,
somewhat-better way to do what you're doing in your examples.

Mike


On Wed, May 22, 2013 at 12:13 AM, Ron Adam <ron3200 at gmail.com> wrote:

>
> There were a few people who liked the idea of having comments after a line
> continuation.
>
> I was able to make a small patch that removed the some of the restrictions
> on the '\' for testing some ideas which does the following.
>
>     * Allow a line to continue on the same line.
>
>     * Skips comments before checking for the new line
>       after a back slash.
>
>
> Here are some examples...
>
>
> These are technically the same.
>
> >>> 'aaa' \
> ... 'bbb' \
> ... 'ccc'
> 'aaabbbccc'
>
> >>> 'aaa' \ 'bbb' \ 'ccc'
> 'aaabbbccc'
>
> Yes there is't much need for this, but I wanted to see if it would work
> and if the test suit passes.  It does. ;-)
>
>
> You can put a comment after a line continuation.
>
> >>> 'aaa'    \# one
> ... 'bbb'    \# two
> ... 'ccc'     # three
> 'aaabbbccc'
>
>
> Works with expressions too.
>
> >>> result = \
> ...        + 111  \# A
> ...        + 222  \# B
> ...        + 333  \# C
> ...        + 444   # D
> >>> result
> 1110
>
>
> But if it has a space between the \ and the #, the line is continued on
> the same line instead of the following line.
>
> >>> 'aaa' \ #comment
> 'aaa'
>
> The reason \# works, but not \ #, is when the comment comes directly after
> the back slash, it's removed and leaves a (backslash + new-line) pair.
>
> Removing the white space before the new line check caused some errors in
> the test suite.  I haven't figured out why yet.  So this doesn't do that
> for now.
>
>
>
> Currently you get this if you try any of these examples.
>
> >>> 'abc' \#comment
>   File "<stdin>", line 1
>     'abc' \#comment
>                   ^
> SyntaxError: unexpected character after line continuation character
>
>
>
>
> Only one of pythons tests fail, and I don't think it's related.
>
>     test test_urllib2_localnet failed
>
>
>
> See the diff below if you want to play with it.  It's not big.
>
> Cheers,
>     Ron
>
>
>
>
> diff -r 155e6fb309f5 Parser/tokenizer.c
> --- a/Parser/tokenizer.c        Tue May 21 21:02:04 2013 +0200
> +++ b/Parser/tokenizer.c        Tue May 21 22:10:31 2013 -0500
> @@ -1391,18 +1391,31 @@
>
>   again:
>      tok->start = NULL;
> +
> +    c = tok_nextc(tok);
> +
> +    /* Check if continuing line */
> +    if (tok->cont_line == 1 && c == '\n') {
> +        tok->cont_line = 0;
> +        c = tok_nextc(tok);
> +    }
> +
>      /* Skip spaces */
> -    do {
> +    while (c == ' ' || c == '\t' || c == '\014') {
>          c = tok_nextc(tok);
> -    } while (c == ' ' || c == '\t' || c == '\014');
> +        tok->cont_line = 0;
> +    }
>
>      /* Set start of current token */
>      tok->start = tok->cur - 1;
>
>      /* Skip comment */
> -    if (c == '#')
> +    if (c == '#') {
>          while (c != EOF && c != '\n')
>              c = tok_nextc(tok);
> +        tok_backup(tok, c);
> +        goto again;
> +    }
>
>      /* Check for EOF and errors now */
>      if (c == EOF) {
> @@ -1641,12 +1654,6 @@
>
>      /* Line continuation */
>      if (c == '\\') {
> -        c = tok_nextc(tok);
> -        if (c != '\n') {
> -            tok->done = E_LINECONT;
> -            tok->cur = tok->inp;
> -            return ERRORTOKEN;
> -        }
>          tok->cont_line = 1;
>          goto again; /* Read next line */
>      }
>
>
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