What is the underscore prefix for python file name?
In cherryPy for example, there are files like:
__init__.py_cptools.py
How are they different? What does this mean?
Answer
__...__ means reserved Python name (both in filenames and in other names). You shouldn't invent your own names using the double-underscore notation; and if you use existing, they have special functionality.
In this particular example, __init__.py defines the 'main' unit for a package; it also causes Python to treat the specific directory as a package. It is the unit that will be used when you call import cherryPy (and cherryPy is a directory). This is briefly explained in the Modules tutorial.
Another example is the __eq__ method which provides equality comparison for a class. You are allowed to call those methods directly (and you use them implicitly when you use the == operator, for example); however, newer Python versions may define more such methods and thus you shouldn't invent your own __-names because they might then collide. You can find quite a detailed list of such methods in Data model docs.
_... is often used as 'internal' name. For example, modules starting with _ shouldn't be used directly; similarly, methods with _ are supposedly-private and so on. It's just a convention but you should respect it.