What do the 3 different brackets mean in Python programming?
[]- Normally used for dictionaries, list items()- Used to identify parameters{}- I don't know what this does
Can these brackets can be used for other purposes?
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4 Answers
Square brackets: []
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'
Parentheses: () (AKA "round brackets")
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)
- Although tuples can be created without parentheses:
- Order of operations:
(n-1)**2 - Generator expressions:
(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))
- with a generator expression:
Curly braces: {}
Dictionaries and sets, as well as in string formatting
- Dicts:
{},{0: 10},{i: i**2 for i in range(5)} - Sets:
{0},{i**2 for i in range(5)}- Except the empty set:
set()
- Except the empty set:
- In string formatting to indicate replacement fields:
- F-strings:
f'{foobar}' - Format strings:
'{}'.format(foobar)
- F-strings:
Regular expressions
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.
Angle brackets: <>
Used when representing certain objects like functions, classes, and class instances if the class doesn't override __repr__(), for example:
>>> print <built-in function print> >>> zip <class 'zip'> >>> zip() <zip object at 0x7f95df5a7340> (Note that these aren't proper Unicode angle brackets, like ⟨⟩, but repurposed less-than and greater-than signs.)
Comments
In addition to Maltysen's 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.
Comments
() 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.
2 Comments
wjandrea
"parentheses ... are referred to as tuples" -- Totally incorrect. Parentheses have a much broader purpose than just for tuples (e.g. calls like
print()), and tuples can be defined without parentheses (e.g. t = 1, 2).Tuple is immutable(order inside it can't be changed once created),and are enclosed in parenthesis,separated by ("," or ','). Tuple is used to store multiple items in a single variable.
Example:
thistupple("apple","banana","mango") print(thistupple) Output:
('apple', 'banana', 'cherry') 
