Steven D'Aprano <steve+pyt...@pearwood.info> added the comment:
Eric is correct that this is a CPython optimization, it is not a language feature. Furthermore, it is an optimization that can be affected by rather subtle factors such as the operating system memory management. Here is a thread demonstrating that code that relied on this optimization ran fast on Linux and slow on Windows, literally hundreds of times slower than other tools: https://mail.python.org/archives/list/python-...@python.org/thread/EQO6HDZXHFCP4DBEE5Q7MYLHPJMGJ7DQ/ If you prefer the old mailman archives, here are a few relevant posts: https://mail.python.org/pipermail/python-dev/2009-August/091125.html https://mail.python.org/pipermail/python-dev/2009-September/091582.html https://mail.python.org/pipermail/python-dev/2009-September/091592.html Best practice is to *not* rely on this optimization, but to use str.join. That ensures that: - your code remains fast if run on alternate implementations; - your code remains fast if the OS memory management changes; - your code remains fast if you change your code slightly. For example, I expect both of the following to show the documented quadratic behaviour: - Prepending instead of appending - keeping two or more references to the string being appended to partials = [] current = '' for s in list_of_strings: partials.append(current) current += s There may be others. Perhaps we ought to document this optimization specifically so we can suggest that it is intended for improving performance of casual scripts, and that *best practice* for professional quality code remains str.join. ---------- nosy: +steven.daprano _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue37512> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
