Different meanings of brackets in Python

Question

I am curious, what do the 3 different brackets mean in Python programming? Not sure if I'm correct about this, but please correct me if I'm wrong:

• [] - Normally used for dictionaries, list items
• () - Used to identify params
• {} - I have no idea what this does...

Or if these brackets can be used for other purposes, any advice is welcomed! Thanks!

Answer 1

[]: Lists and indexing/lookup/slicing

• Lists: [], [1, 2, 3], [i**2 for i in range(5)]
• Indexing: 'abc'[0] → 'a'
• Lookup: {0: 10}[0] → 10
• Slicing: 'abc'[:2] → 'ab'

(): Tuples, order of operations, generator expressions, function calls and other syntax.

• Tuples: (), (1, 2, 3)
  • Although tuples can be created without parentheses: t = 1, 2 → (1, 2)
• Order of operations: (n-1)**2
• Generator expression: (i**2 for i in range(5))
• Function or method calls: print(), int(), range(5), '1 2'.split(' ')
  • with a generator expression: sum(i**2 for i in range(5))

{}: Dictionaries and sets, as well as string formatting

• Dicts: {}, {0: 10}, {i: i**2 for i in range(5)}
• Sets: {0}, {i**2 for i in range(5)}
• Inside f-strings and format strings, to indicate replacement fields: f'{foobar}' and '{}'.format(foobar)

All of these brackets are also used in regex. Basically, [] are used for character classes, () for grouping, and {} for repetition. For details, see The Regular Expressions FAQ.

Answer 2

() parentheses are used for order of operations, or order of evaluation, and are referred to as tuples. [] brackets are used for lists. List contents can be changed, unlike tuple content. {} are used to define a dictionary in a "list" called a literal.