Refactor/Rewrite Perl code in Python

Steven D'Aprano steve+comp.lang.python at
Sun Jul 24 13:39:54 CEST 2011

On Sun, Jul 24, 2011 at 7:29 PM, Shashwat Anand <anand.shashwat at>

> How do I start ?
> The idea is to rewrite module by module.
> But how to make sure code doesn't break ?

By testing it.

Read up on "test driven development".

At this point, you have this:

Perl modules: A, B, C, D
Python modules: none
Python tests: none

Now, before you rewrite each Perl module in Python, first write a good,
comprehension test suite in Python for that module. You need to have tests
for each function and method. Test that they do the right thing for both
good data and bad data. If you have functional requirements for the Perl
modules, use that as your reference, otherwise use the Perl code as the

For example, this might be a basic test suite for the len() built-in

for empty in ([], {}, (), set([]), ""):
    if len(empty) != 0:
        raise AssertionError('empty object gives non-zero len')

for n in range(1, 5):
    if len("x"*n) != n:
        raise AssertionError('failure for string')
    for kind in (list, tuple, set):
        obj = kind([None]*n)
        if len(obj) != n:
            raise AssertionError('failure for %s' % obj)

if len({'a': 1, 'b': None, 42: 'spam'}) != 3:
    raise AssertionError('failure for dict')

for bad_obj in (23, None, object(), 165.0, True):
    except TypeError:
        # Test passes!
        # No exception means len() fails!
        raise AssertionError('failed test')

Multiply that by *every* function and method in the module, and you have a
moderately good test suite for module A.

(You may want to learn about the unittest module, which will help.)

Now you have:

Perl modules: A, B, C, D
Python modules: none
Python tests: test_A

Now re-write the Python module, and test it against the test suite. If it
fails, fix the failures. Repeat until it passes, and you have:

Perl modules: A, B, C, D
Python modules: A
Python tests: test_A

Now you can be confident that Python A does everything that Perl A does.
Possibly *better* than the Perl module, since if you don't have a test
suite for it, it probably has many hidden bugs.

Continue in this way with the rest of the modules. At the end, you will have
a full test suite against the entire collection of modules.


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