testing hg using subprocess.Popen()

[Chris Jerdonek]

I have a need to write some tests that can check more than what
"unified tests" currently allow.  For example, I would like to use
Python to compare the stdout of `hg` against some
dynamically-evaluated Python expressions.  Also, some of the
substrings I would like to check vary across multiple lines which
isn't currently supported by the per-line "filtering" functionality
(glob, re, etc).

So as not to add new test scripts, my idea was to convert an existing
unified test into a "pure unit test" (i.e. the kind of test mentioned
at the end of this section [2]), and then use subprocess.Popen() to
invoke `hg`.

Does anyone have any thoughts on this?  I didn't find any examples
like this in the test suite.  It seems like a helper function to call
`hg` using subprocess.Popen() would be useful to have in the tests
folder.  However, I didn't come across any examples of helper test
functions being imported in any of the Python unit tests.

For DRY purposes, etc, it seems like it would be useful to be able to
share code across Python unit tests.  Starting to do this might be
another way to chip away at the following problem mentioned in the
wiki: "The test suite is slow. And the test suite is slow because it
is highly redundant.." [1]

I'm asking now because I wanted to hear people's thoughts before I
prepare my next patch.  Thanks.

--Chris

[1] http://mercurial.selenic.com/wiki/WritingTests#No_more_new_test_scripts.21
[2] http://mercurial.selenic.com/wiki/WritingTests#Writing_a_Python_unit_test