Best practice for Python assert

Question

1. Is there a performance or code maintenance issue with using assert as part of the standard code instead of using it just for debugging purposes?

   Is

   assert x >= 0, 'x is less than zero'
   

   better or worse than

   if x < 0:
       raise Exception, 'x is less than zero'
   

2. Also, is there any way to set a business rule like if x < 0 raise error that is always checked without the try/except/finally so, if at anytime throughout the code x is less than 0 an error is raised, like if you set assert x < 0 at the start of a function, anywhere within the function where x becomes less then 0 an exception is raised?

Answer 1

Asserts should be used to test conditions that should never happen. The purpose is to crash early in the case of a corrupt program state.

Exceptions should be used for errors that can conceivably happen, and you should almost always create your own Exception classes.

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For example, if you're writing a function to read from a configuration file into a dict, improper formatting in the file should raise a ConfigurationSyntaxError, while you can assert that you're not about to return None.

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In your example, if x is a value set via a user interface or from an external source, an exception is best.

If x is only set by your own code in the same program, go with an assertion.