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!
Answers:
() 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.
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)
- 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))
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()
- In string formatting to indicate replacement fields:
- F-strings:
f'{foobar}'
- Format strings:
'{}'.format(foobar)
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.)
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.
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')
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!
() 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.
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.)
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.
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')