Python assertion style

Question

I'm wondering if what I'm doing is an appropriate method of assertions. I'm trying to making something both concise and very correct for Python's style guides.

try:
    assert self.port_number == 0
    assert self.handle == None
    assert isinstance(port_number, int) or isinstance(port_number, float)
    assert port_number > 0
except AssertionError:
    return -1

*body of code*

return 0

Above is an excerpt of my code that shows how I handle argument assertions. You can assume that I've covered all of the necessary assertions and the input is port_number. Is this considered good style? Is there a better way?

Answer 1

The assert statement should only be used to check the internal logic of a program, never to check user input or the environment. Quoting from the last two paragraphs at http://wiki.python.org/moin/UsingAssertionsEffectively ...

Assertions should not be used to test for failure cases that can occur because of bad user input or operating system/environment failures, such as a file not being found. Instead, you should raise an exception, or print an error message, or whatever is appropriate. One important reason why assertions should only be used for self-tests of the program is that assertions can be disabled at compile time.

If Python is started with the -O option, then assertions will be stripped out and not evaluated. So if code uses assertions heavily, but is performance-critical, then there is a system for turning them off in release builds. (But don't do this unless it's really necessary. It's been scientifically proven that some bugs only show up when a customer uses the machine and we want assertions to help there too. )

With this in mind, there is virtually never a reason to catch an assertion in user code, ever, as the entire point of the assertion failing is to notify the programmer as soon as possible that there is a logic error in the program.