• Testing scientific software is highly important,
  as it has direct impact on scientific insight.
• Most scientific software is not thoroughly tested
  or even not tested at all.
• Tests can only prove the existence of bugs, not their absence.
• Testing should be done as early as possible, as often as possible,
  and as automatic as possible.
• Test automatisation has dramatic impact on the design
  of the code to be tested (e.g., modularity).
• Test-driven development turns the usual sequence upside down
  and results in high test coverage.
• High test coverage by automatic tests increases the trust in software.
• Test-driven development leads to tests
  that document the code and its specifications.

Python Unittest Module
Unit testing framework originally inspired by JUnit