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[debbugs-tracker] bug#15295: closed (which-func-mode slow in long Python

From: GNU bug Tracking System
Subject: [debbugs-tracker] bug#15295: closed (which-func-mode slow in long Python tuple)
Date: Tue, 24 Dec 2013 20:09:02 +0000

Your message dated Tue, 24 Dec 2013 17:08:09 -0300
with message-id <address@hidden>
and subject line 
has caused the debbugs.gnu.org bug report #15295,
regarding which-func-mode slow in long Python tuple
to be marked as done.

(If you believe you have received this mail in error, please contact

15295: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=15295
GNU Bug Tracking System
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--- Begin Message --- Subject: which-func-mode slow in long Python tuple Date: Fri, 06 Sep 2013 19:47:19 -0500 User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.8; rv:17.0) Gecko/20130801 Thunderbird/17.0.8 I happen to have a Python source file that has a relatively long tuple at the module top level, i.e. a Python source file containing:

foo = (
    "item 1",
    "item 2",
    # ...and so on for ~500 lines

I also use which-function-mode. If I go to the end of that tuple and move the cursor in to it, Emacs becomes unusably slow. It will appear to lock up and eat 100% CPU for 10-20 seconds each time I move the cursor within the end of that tuple. Emacs remains responsive at the top of the tuple.

I think this is happening because python-info-current-defun is slow when dealing with long tuples. (Maybe lists, dicts, and other things too; I only tested tuples.) Here's some elisp to produce a test case and benchmark python-info-current-defun:

  (set-buffer (generate-new-buffer "*test*"))
  (insert "foo = (\n")
  (dotimes (_ 500) (insert "    \"bar\",\n"))
  (insert ")\n")
  (forward-line -2)
  (message "%S" (benchmark-run (python-info-current-defun))))

This makes a python-mode buffer named "*test*" containing only a 500-item Python tuple, as in my above example. On my hardware, the above benchmark-run yields a result such as "(7.364507 131 0.9572049999999979)", i.e. 7.3 seconds to run.

Once that *test* buffer is created, feel free to turn on which-function-mode in there and see that Emacs locks up every time you move the cursor around in the end of that tuple. (which-function-mode seems to be taking about twice the time reported by benchmark-run. Perhaps it's calling python-info-current-defun twice?)

I have reproduced this behavior with "emacs -Q" using an Emacs I just built from trunk, looks like revision 114162. (I get Emacs from Git, where the master branch is 0f1532f2fe2.) I have also reproduced this with python.el from the emacs-24 branch, looks like revision 111403.

        Thanks to everyone who develops Emacs, an indispensable tool for me!


--- End Message ---
--- Begin Message --- Subject: Date: Tue, 24 Dec 2013 17:08:09 -0300 User-agent: mu4e; emacs 24.3.1
Fixed in revno 115736.

Thanks Dale for such detailed recipe.

This patch banishes initial thoughts of `python-syntax-context' being a
bad idea.  `python-syntax-context' is nothing than a thin semantic
wrapper over `syntax-ppss'. It makes code easier to grasp for newcomers
to Elisp and has almost no impact on itself, it's optional argument is a
`syntax-ppss' list which can be used instead to lower the amount of
calls to it (as it is happening in this new patch I've just committed).

The problem here was that `python-nav-beginning-of-statement' was coded
awfully (looking for the statement beginning line by line). Now it
should be extremely fast compared to that.

Using OP's suggested recipe, here are the elp results for when
which-func is triggered inside the big tuple:

    python-info-current-defun                      2           0.003719249   
    python-nav-beginning-of-defun                  2           0.0036946010  
    python-nav--beginning-of-defun                 2           0.003685751   
    python-nav-backward-block                      2           0.001836524   
    python-nav-forward-block                       2           0.0018315750  
    python-info-looking-at-beginning-of-defun      6           0.000889166   
    python-nav-beginning-of-statement              4           0.000437251   
    python-syntax-context-type                     6           5.009e-06     

And this is the benchmark-run result: (0.020715153 0 0.0)


--- End Message ---

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