Here are the principles of mahotas, in decreasing order of importance:
The first principle is that things should just work. This means two things: (1) there should be no bugs, and (2) interfaces should be flexible or fail well.
To avoid bugs, tests are extensively used. Every reported bug leads to a new test case, so that it never happens again. New features should at least have a smoke test (test that runs the feature and verifies some basic properties of the output).
Interfaces are designed to be as flexible as possible. No specific types are required unless it is really needed or in performance-enhancing features (such as using out parameters).
The user should never be able to crash the Python interpreter with mahotas.
No public function is without a complete docstring. In addition to that hard documentation, there is also soft documentation with examples.
The code should be as fast as possible without sacrificing generality (see just work above). This is why C++ templates are used for type independent code.
The code should be simple.
Try to avoid dependencies. Right now, building mahotas depends on a C++ compiler, numpy. These are unlikely to ever change. To run mahotas, we need numpy and either imread or freeimage.
The imread/freeimage dependency is a soft dependency: everything, except for imread works without it. The code is written to ensure that import-ing mahotas without freeimage will not trigger an error unless the imread() function is used.