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PCEP: Data Collections: Lists, Tuples, Sets, Dictionaries, and Strings

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Data Collections: Lists, Tuples, Sets, Dictionaries, and Strings

What this domain covers

This domain tests the four built-in collections and strings: indexing, slicing, mutability, core methods, and which operations raise errors. Expect "what is the output" questions involving aliasing and in-place methods.

Core rules

  • Lists are ordered and mutable: a[i], a[i:j], negative indices, len(), in. Methods include append, extend, insert, pop, remove, sort, reverse. sort() modifies in place and returns None.
  • Tuples are ordered and immutable. A one-element tuple needs a comma: (5,). Unpacking: a, b = 1, 2.
  • Sets hold unique, hashable elements and are unordered. {} is an empty dict; set() is an empty set. Operators: | union, & intersection, - difference, ^ symmetric difference; add, discard (no error if absent), remove (raises KeyError).
  • Dictionaries map hashable keys to values, preserve insertion order, and support d[k], get(k, default), keys(), values(), items(), and in (checks keys). d[k] on a missing key raises KeyError.
  • Strings are immutable sequences: s[0], slicing, in, +, *, and methods such as upper, strip, split, join, replace, find. Every method returns a new string.

Worked example

a = [3, 1, 2]; b = a; b.sort(); print(a) prints [1, 2, 3]. b = a copies the reference, not the list, so both names point to the same object. For an independent list use b = a[:], a.copy() or list(a).

Traps

  • a = b aliases; slicing copies.
  • x = a.sort() makes x equal to None.
  • Negative indices count from the end; slices never raise IndexError.
  • s[0] = "H" fails because strings are immutable.
  • Adding a list to a set raises TypeError because lists are unhashable.