I put a dict as the default value for an optional argument to a Python function, and pylint (using Sublime package) told me it was dangerous. Can someone explain why this is the case? And is a better alternative to use None instead?
It's dangerous only if your function will modify the argument. If you modify a default argument, it will persist until the next call, so your "empty" dict will start to contain values on calls other than the first one.
Yes, using None is both safe and conventional in such cases.