[TIP] Going From Untested Perl to Tested Python

Noah Gift noah.gift at gmail.com
Sun Feb 1 19:27:20 PST 2009

On Mon, Feb 2, 2009 at 4:05 PM, Paul Hildebrandt
<Paul.Hildebrandt at disneyanimation.com> wrote:
> Kumar McMillan wrote:
>> On Sun, Feb 1, 2009 at 5:16 PM, Noah Gift <noah.gift at gmail.com> wrote:
>>> Hi,
>>> I am starting to convert a rather large code base from untested Perl
>>> to tested Python.  One idea I had was to write tests in Python, that
>>> tests the Perl code.  Once I have the core functionality worked out,
>>> the next step is to rewrite it in Python, and finish off unit testing.
>>>  Has anyone else done something like this, and if so do you have
>>> recommendations?
>> Hi Noah.
>> I did exactly this once for a set of command line scripts (migrated
>> untested perl scripts to python scripts, implementing all new scripts
>> in python-land).  I attempted to create tests for all the legacy
>> scripts so I could simply swap them out with a python one and have the
>> tests still pass.  I spent a lot of time doing crazy DB setup and file
>> layout setup so that the tests could execute the perl scripts in a
>> subprocess.  I would not recommend this approach for a few reasons.
>> First, it is a huge pain to go through all the setup needed for such a
>> high level "black box" testing approach.  Secondly, the legacy script
>> tests were hard to maintain since they relied so much on a database.
>> Yes, they did need to be maintained because the perl was never fully
>> ported to Python (there was just too much).
>> I would suggest an approach like this.  Assume the legacy perl code
>> just works and don't touch it.  Make a plan for how much you can
>> realistically migrate to Python and begin all new work in Python.
>> Start the new Python project with tests and don't worry about testing
>> the legacy Perl code.  If you find yourself in a situation where you
>> need to fix a bug in the legacy perl code consider rewriting it in
>> Python first and if that's not feasible then maybe that's a good
>> enough excuse to create tests for perl code before you fix the bug.
>> Otherwise, in my experience, it wasn't worth the effort to retrofit
>> the perl code with tests.
>> YMMV of course :)
>> Kumar
> +1 that is how we've done the same thing.

Good to hear to different takes on it.  Of course Paul, working in
film like I do, has a particularly good grasp of the problem I am
facing :)  I suppose there is something to be said for not wasting
resources on Perl code, when you can just rewrite it in Python.  As
Ben mentioned, it is probably going to be frustrating because the code
is often not testable anyway.

>>> For the Python code that is untested I am using Pythoscope, which is
>>> quite nice so far.
>>> --
>>> Cheers,
>>> Noah
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