Raise an exception at import if conditions not met

Question

I have a module that depends on some system settings. For example, to work properly it needs to have an environment variable FOO set. I would like the module to raise an Exception if this condition is not met at import time.

# mymodule.py
if 'FOO' not in sys.environ:
    raise SomeException('ensure that FOO is provided')

I would like to know:

1. Is it the best practice to check those conditions at import time, or maybe it would be better to do it later (when)
2. What type of exception should I raise? Should it be my own class MyModuleImportError(Exception) or maybe some built-in exceptions are more suitable and commonly used? One candidate would be ImportError , but it seems to be reserved for situation when python "fails to find the module definition"

Answer 1

Is this what you are looking for?

>>> import os
>>> if not os.getenv('FOO', False):
    raise OSError('FOO not in envs')

---

1. Yes, you should check these kind of things at the start of your code. Else there is no reason for it to continue running.
2. an OSError (as above)