  Unit tests are widely used to check source code quality, but they can be too
coarse-grained or ill-suited for testing individual program statements. We
introduce inline tests to make it easier to check for faults in statements. We
motivate inline tests through several language features and a common testing
scenario in which inline tests could be beneficial. For example, inline tests
can allow a developer to test a regular expression in place. We also define
language-agnostic requirements for inline testing frameworks. Lastly, we
implement I-Test, the first inline testing framework. I-Test works for Python
and Java, and it satisfies most of the requirements. We evaluate I-Test on
open-source projects by using it to test 144 statements in 31 Python programs
and 37 Java programs. We also perform a user study. All nine user study
participants say that inline tests are easy to write and that inline testing is
beneficial. The cost of running inline tests is negligible, at 0.007x--0.014x,
and our inline tests helped find two faults that have been fixed by the
developers.
