Why does stack.inspect take so long?

[Tim Peters]

[63q2o4i02 at sneakemail.com]
> Hi, I've written a top-down recursive decent parser for SPICE circuit
> descriptions.  For debugging purposes, I wanted each production
> rule/function to know what its own name was, so at the beginning of
> each rule/function, I make a call to inspect.stack()[0][3] (I think...)
> and that fetches the name from someplace.  However, with a bunch of
> test text input, this takes ~10 seconds to run.  When I comment out the
> inspect.stack() function from each production/function, the program
> speeds up to 0.04s, or 250x !!  I loaded up the profiler to see what
> was there, and although I don't understand the output entirely, it
> looks like the stack frame functions are used in disproportionately
> large amounts compared to other often-used functions.  This is a
> *recursive* parser, so it's being called a lot even for simple things.
> I'm wondering still why there is a 250x speed up when I comment it out.
>  Any clues?

Look at the source code (inspect.py, function `stack()`).  It
obviously takes time proportional to the total depth of the call
stack, and for a recursive-descent parser that's likely to be highly
non-trivial most of the time.

It should go much faster to use a function that doesn't crawl the
entire call stack.  For example,

>>> import sys, inspect
>>> def name_of_caller():
...     return inspect.getframeinfo(sys._getframe(1), context=0)[2]
>>> def f():
...     print "my name is", name_of_caller()
>>> f()
my name is f

name_of_caller() takes time independent of the call-stack depth.

The "context=0" is to avoid wasting time sucking up and packaging
source-code lines you don't want anyway.