[TIP] branch coverage

Andrew Dalke dalke at dalkescientific.com
Thu Jan 24 05:41:26 PST 2008

Hi all,

m h sesquile at gmail.com:
>  I really don't want to write a python compiler, but
> am assuming tracing at that level won't be supported in cpython, but
> perhaps might in pypy....

On Jan 24, 2008, at 12:32 PM, Laura Creighton wrote:
> I think that Andrew Dalke, who is cc'd to this note, has already done
> a bunch of work which would be very useful to you.

I was talking with Laura a few days ago about a lightning talk I  
wanted to present at PyCon - branch coverage.  There's no easy way to  
do it with Python.  The closest is to use the compiler module to  
generate the AST then instrument the AST.  The problem is, the  
compiler module is a bear to work with and doesn't record everything  
I want.  For example, in the coverage report I want to see which  
branches weren't covered, pinpointed to the character range of the  
expression.  Python's AST doesn't have byte positions.

What I've been doing over the last couple of days is getting a PLY  
grammar for Python.  As of last night it parses (and builds the  
trivial concrete syntax tree) of the entire standard library.  After  
the AST works I plan to convert code like this (from line 548 of  

     if close_fds and (stdin is not None or stdout is not None or
                       stderr is not None):
         raise ValueError("close_fds is not supported on Windows "
                          "platforms if you redirect stdin/stdout/ 

into something equivalent to this mess

   __reached_statement(100)  # assuming this is statement number 100

   if close_fds:
     __branch_is_true(1)  # each branch also gets a unique id, with
                          # some table mapping that to source file  
and byte range

     if stdin is not None:
       __result_bool = True
       __result_obj = stdin

       if stdout is not None:
         __result_bool = True
       __result_obj = stdout

         if stderr is not None:
           __result_bool = True
           __result_obj = stderr
           __result_bool = False
           __result_obj = stderr
     __result_bool = False
     __result_obj = close_fds

  if __result_bool:
         raise ValueError("close_fds is not supported on Windows "
                          "platforms if you redirect stdin/stdout/ 

where __branch_is_true and __branch_is_false and __reached_statement  
keep track of which branch points and lines were executed.  (Along  
with a filename, module name, and md5 checksum to prevent version skew.)

This horrible if statement expansion mess is needed because it's the  
only way to keep Python guarantees:

   -- short circuiting

   -- the bool check is only done once per term

Otherwise something like

    if _bool_check(1, close_fds) and (_bool_check(2, stdin is not  
None) ... )

would work, where

   def _bool_check(branch_number, obj):
     if obj:
     return obj

This is simple, but it calls bool(obj) twice.

   -- the correct object is returned

I had another hack which looked like

     if (_about_to_call(1) and close_fds or is_false(1)) and  (...:

along with some state tracking.  This handles the booleanness  
correctly, but does not return the correct object during assignment

   x = a and (b or c or d)

Once I have the modified AST, what should I do with it?

I could generate raw Python code, which would be ugly, have the  
comments stripped out, and line numbers changed.  Or I could generate  
byte code.

If the latter, I was thinking to write a .py -> .pyc compiler, but do  
I use it like compileall?  Or do I generate the .pyc files in another  
directory, which is used for the coverage testing.  Where do I keep  
the coverage results?  Probably all in a single directly, named after  
the Python module name.

Do people only care about if the branch was true/false or are the  
number of tests also important?  What about the number of times a  
line was executed, vs. a flag saying that it was covered?

				dalke at dalkescientific.com

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