Different meanings of brackets in Python

Different meanings of brackets in Python

[]: 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.

() 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.

Different meanings of brackets in Python

In addition to Maltysens answer and for future readers: you can define the () and [] operators in a class, by defining the methods:

• __call__(self[, args...]) for ()
• __getitem__(self, key) for []

An example is numpy.mgrid[...]. In this way you can define it on your custom-made objects for any purpose you like.