Defensive Programming

Reporting (un)expected state

• Assertions check for user defined conditions and raise an exception if the condition is False. The syntax is:

  assert expression0
  
  assert expression1, expression2
  
  # Which is equivalent to:
  
  if __debug__:
      if not expression0: raise AssertionError
  
  if __debug__:
      if not expression1: raise AssertionError(expression2)
  

• Raising Exceptions

  In a more complex case you could raise a custom exception that carries the exact meaning you wish to transfer:

  class mySpecialError(Exception):
     """ This error is raised in a special situation.  """
     pass
  
  if myValue < myLimit:
     raise mySpecialError()
  

• Use loggers to easily turn on and off your debug writes.

  import logging
  logging.warning('Warn!')        # Printed by default
  logging.info('Info!')           # Need to ask for more verbose
  logging.debug('details...')     # Need to ask for much more verbose
  

  • The basic idea of loggers is to provide a framework that makes it convenient to choose the current verbosity of the program depending on simple centralized run-time setting.

  • The logger framework can do formatting of the messages, such as including the time.

Checks while writing

Using On-line linting and IDEs can help you fix bugs as you type!

• Pylint is an off-line tool that can check your code for formatting and for certain logical errors that could hint at hidden problems.

• IDEs like spyder, pycharm, eclipse and vscode can warn you of undefined and unused variables, bad syntax and perform further diagnostics of your code while you write.

Divide and conquer

• Separate objects and implement clear interfaces

• Unit testing

  • There are a number of python frameworks for unit testing, e.g. the built in unittest and pytest.

  • Preferrably run automatically in a continuous integration setup

Document well

• You can give type hints about what types are expected, but it is only documentation, not enforced by the runtime.

• Docstrings should include i.a. information about the parameters and return values of functions.