Q:

How does slicing work with double square brackets?

I created a string variable stro.
How will the slicing stro[7:][-6] work on stro?
stro = "Python is fun"

print(stro[7:][-6])

# output: i

A:

You are slicing, then indexing:
stro = "Python is fun"
x = stro[7:]  # 'is fun'
y = x[-6]     # 'i'

Since strings are immutable, both x and y are new strings rather than a "view" of an object. Thus stro[7:] returns 'is fun' and indexing the 6th last character returns 'i'.
The syntax is similar to lists: see Understanding Python's slice notation.

