What does this mean: key=lambda x: x[1] ?

by

lambda effectively creates an inline function. For example, you can rewrite this example:

max(gs_clf.grid_scores_, key=lambda x: x[1])

Using a named function:

def element_1(x):
    return x[1]

max(gs_clf.grid_scores_, key=element_1)

In this case, max() will return the element in that array whose second element (x[1]) is larger than all of the other elements’ second elements. Another way of phrasing it is as the function call implies: return the max element, using x[1] as the key.

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