One of the problems with writing code that interacts with a database (for example) is that it’s hard to test; do you have your test suite create, populate and destroy whole databases to test your code against? That’s an awful lot of work. Then there’s the question about testing those tricky edge cases, like certain error conditions: if you’re creating new databases, how do you simulate, say, your database server suffering a catastrophic failure. I can think of one solution, but it would get pretty expensive pretty quickly.

Fortunately, since this is a common enough problem (testing database interaction), smart people have already solved it. In the perl community, for example, DBD::Mock (full disclosure: I work on DBD::Mock some) is one fairly well-known way of testing interaction with database systems. It allows you to say, “Make sure the following SQL statements are executed in the following order, and return these results”. It can also be used to simulate what happens when the connection to the database dies mid-query, for example.

Once you move away from databases, though, to other, though still complex systems, the range of testing options becomes more limited. In my case, I wanted to write several applications and support libraries on top of the Python bindings for Subversion, a popular revision control system. When it came time to write tests for the Subversion-facing code, I faced a dilemma: was I back to creating full-blown SVN repositories for each test case?

My solution was svnmock. The svnmock package does for Subversion’s Python API what DBD::Mock does for perl’s DBI: it makes testing easy. svnmock allows the test writer to say, like with DBD::Mock, “I want the following series of function calls, with this set of parameters and this return value”. While this may seem fairly low-level, it is trivial to write macro-like constructs on top of the current set of primitives. One of my favourite features of svnmock is that it allows you to specify that the return value from api_func_1() must be used as a parameter to api_func_2().

svnmock’s project website may be found here.