>> The "two steps" method is not promising: parsing the format string >> twice is slower than other methods. > > The "1.5 steps" method is more promising -- first parse the format string in > an efficient internal representation, and then allocate the output string > and then write characters (or enlarge and widen the buffer, but with more > information in any case). The internal representation can be cached (as for > struct module) that for a repeated formatting will reduce the cost of > parsing to zero.
I implemented something like that, and it was not efficient and very complex. See for example the (incomplete) patch for str%args attached to the issue #14687: http://bugs.python.org/file25413/pyunicode_format-2.patch IMO this approach is less efficient than the "Unicode writer" approach because: - you have to create many substrings or temporary strings in the first step, or (worse) compute each argument twice: the writer approach is more efficient here because it avoids computing substrings and temporary strings - you have to parse the format string twice, or you have to write two versions of the code: first create a list of items, then concatenate items. The PyAccu method concatenates substrings at the end, it is less efficient than the writer method (e.g. it has to create a string of N fill characters to pad to WIDTH characters). - the code is more complex than the writer method (which is very similar to what is used in Python 2.7 and 3.2) I wrote a much more complex patch for str%args to remember variables of the first step to avoid most of the parsing work in the second step. The patch was very complex and hard to maintain. I chose to not publish it and try another approach (the Unicode writer). Note: I'm talking about str%args and str.format(args), the Unicode writer is not the most efficient method for *any* function creating strings! Victor _______________________________________________ Python-Dev mailing list Python-Dev@python.org http://mail.python.org/mailman/listinfo/python-dev Unsubscribe: http://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
