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Python / Core Python Fundamentals Interview Questions

How does slicing work in Python for lists, strings, and tuples?

Slicing extracts a sub-sequence from any sequence type using the notation sequence[start:stop:step]. All three parts are optional and default to the beginning, end, and a step of 1 respectively. Slicing always returns a new object of the same type — it does not modify the original.

data = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

data[2:5]     # [2, 3, 4]        — stop is exclusive
data[:4]      # [0, 1, 2, 3]     — from beginning
data[6:]      # [6, 7, 8, 9]     — to end
data[::2]     # [0, 2, 4, 6, 8]  — every other element
data[::-1]    # [9, 8, 7, 6, 5, 4, 3, 2, 1, 0]  — reverse
data[-3:]     # [7, 8, 9]        — last three elements
data[1:8:3]   # [1, 4, 7]        — start=1, stop=8, step=3

Strings and tuples behave identically. text = 'Hello, World!'; text[7:] gives 'World!'. The [::-1] idiom is the classic one-liner to reverse a string in Python.

Slicing on lists returns a shallow copy. Assigning to a slice mutates the list in-place, which is a powerful but occasionally surprising feature:

data[2:5] = [20, 30]  # replace three elements with two
print(data)  # [0, 1, 20, 30, 5, 6, 7, 8, 9]  — length changed

For more reusable slice objects, slice(start, stop, step) creates a first-class slice that can be stored and reused: s = slice(1, 8, 2); data[s].

What does data[1:7:2] return for data = list(range(10))?
Which slice idiom reverses a sequence in Python?

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