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Seems there is a bug in the implementation of the IMPORT_NAME opcode in ceval.c.
If the level argument is -1, it is not passed to __import__ (should never happen, but looks correct). If it is an integer != -1 in C long range, it is passed to __import__ (this is correct). But if it is not integer (e.g. None) or can't be converted to C long, an exception is set and __import__ is called with level and not cleared error (this is wrong).
In correct bytecode the level argument can be either integer or None. Default __import__ accepts only integers as the level argument and checks the range. Proposed patch makes the code always passing the level argument to __import__ unless it is None.