今天來介紹比較運算元。

Identity and Membership Operators

is 和 is not operators 適用於任何 data type，因為比較的是物件的記憶體位置：

0.1 is (3+4j)

<>:1: SyntaxWarning: "is" with a literal. Did you mean "=="?
<>:1: SyntaxWarning: "is" with a literal. Did you mean "=="?
/var/folders/zq/_kqzvld5709fpk2pk5wrblkw0000gn/T/ipykernel_21125/2697711678.py:1: SyntaxWarning: "is" with a literal. Did you mean "=="?
  0.1 is (3+4j)





False

'a' is [1, 2, 3]

<>:1: SyntaxWarning: "is" with a literal. Did you mean "=="?
<>:1: SyntaxWarning: "is" with a literal. Did you mean "=="?
/var/folders/zq/_kqzvld5709fpk2pk5wrblkw0000gn/T/ipykernel_21125/3744826144.py:1: SyntaxWarning: "is" with a literal. Did you mean "=="?
  'a' is [1, 2, 3]





False

in 和 not in 適用於可迭代的物件，測試是否為成員之一：

1 in [1, 2, 3]

True

[1, 2] in [1, 2, 3]

False

[1, 2] in [[1,2], [2,3], 'abc']

True

'key1' in {'key1': 1, 'key2': 2}

True

1 in {'key1': 1, 'key2': 2}

False

Equality Operators

== 和 != 比較雙方的值是否相等。

不同的 data type 仍可能可以用這兩個運算元，條件是它們有某種共通性。

比如，你可以比較 Fraction 和 Decimals 是否相等，但字串和整數則永不相等。

1 == '1'

False

from decimal import Decimal
from fractions import Fraction

Decimal('0.1') == Fraction(1, 10)

True

1 == 1 + 0j

True

True == Fraction(2, 2)

True

False == 0j

True

Ordering Comparisons

很多 data type 可以比較大小。但有些沒辦法，比如複數：

1 + 1j < 2 + 2j

---------------------------------------------------------------------------

TypeError                                 Traceback (most recent call last)

/Users/maotingyang/Downloads/Comparison Operators.ipynb Cell 22 in <cell line: 1>()
----> <a href='vscode-notebook-cell:/Users/maotingyang/Downloads/Comparison%20Operators.ipynb#X31sZmlsZQ%3D%3D?line=0'>1</a> 1 + 1j < 2 + 2j


TypeError: '<' not supported between instances of 'complex' and 'complex'

混合 data type 比大小是可行的，但必須有邏輯：

1 < 'a'

---------------------------------------------------------------------------

TypeError                                 Traceback (most recent call last)

/Users/maotingyang/Downloads/Comparison Operators.ipynb Cell 24 in <cell line: 1>()
----> <a href='vscode-notebook-cell:/Users/maotingyang/Downloads/Comparison%20Operators.ipynb#X33sZmlsZQ%3D%3D?line=0'>1</a> 1 < 'a'


TypeError: '<' not supported between instances of 'int' and 'str'

Decimal('0.1') < Fraction(1, 2)

True

Chained Comparisons

比較可以進行多個連鎖。

比如 a < b < c, Python 其實是在每對比較之間加上了 and： a < b and b < c

1 < 2 < 3

True

特別的是連鎖中的運算元不一定要一樣，可以混用：

a < b > c 等同於 a < b and b > c

不過這樣寫之前最好確認同事看得懂，否則也許後者可以更清楚的表達你想做什麼。

1 < 2 > -5 < 50 > 4

True

1 < 2 == Decimal('2.0')

True

import string
'A' < 'a' < 'z' > 'Z' in string.ascii_letters 

True

好啦，今天就到這邊，我們明天見～

參考：Python 3: Deep Dive (Part 1 - Functional)