Raise warning in Python without interrupting program

I am trying to raise a Warning in Python without making the program crash / stop / interrupt.

I use the following simple function to check if the user passed a non-zero number to it. If so, the program should warn them, but continue as per normal. It should work like the code below, but should use class Warning(), Error() or Exception() instead of printing the warning out manually.

def is_zero(i):
   if i != 0:
     print "OK"
   else:
     print "WARNING: the input is 0!"
   return i

If I use the code below and pass 0 to the function, the program crashes and the value is never returned. Instead, I want the program to continue normally and just inform the user that he passed 0 to the function.

def is_zero(i):
   if i != 0:
     print "OK"
   else:
     raise Warning("the input is 0!")
   return i

I want to be able to test that a warning has been thrown testing it by unittest. If I simply print the message out, I am not able to test it with assertRaises in unittest.

import warnings
warnings.warn("Warning...........Message")

See the python documentation: here

You shouldn't raise the warning, you should be using warnings module. By raising it you're generating error, rather than warning.

By default, unlike an exception, a warning doesn't interrupt.

After import warnings, it is possible to specify a Warnings class when generating a warning. If one is not specified, it is literally UserWarning by default.

>>> warnings.warn('This is a default warning.')
<string>:1: UserWarning: This is a default warning.

To simply use a preexisting class instead, e.g. DeprecationWarning:

>>> warnings.warn('This is a particular warning.', DeprecationWarning)
<string>:1: DeprecationWarning: This is a particular warning.

Creating a custom warning class is similar to creating a custom exception class:

>>> class MyCustomWarning(UserWarning):
...     pass
... 
... warnings.warn('This is my custom warning.', MyCustomWarning)

<string>:1: MyCustomWarning: This is my custom warning.

For testing, consider assertWarns or assertWarnsRegex.

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As an alternative, especially for standalone applications, consider the logging module. It can log messages having a level of debug, info, warning, error, etc. Log messages having a level of warning or higher are by default printed to stderr.