From :

https://docs.python.org/2/tutorial/datastructures.html and
https://docs.python.org/3.4/tutorial/datastructures.html (and all other versions) :

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    list.index(x)

        Return the index in the list of the first item whose value is x. It is an error if there is no such item.
----------------------------

From `help(list)` :

----------------------------
 |  index(...)
 |      L.index(value, [start, [stop]]) -> integer -- return first index of value.
 |      Raises ValueError if the value is not present.
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Thus, second and third parameter are undocumented.