“=="和“=="之间有区别吗?和“是"? [英] Is there a difference between "==" and "is"?

问题描述

我的 Google-fu 让我失望了.

在 Python 中，以下两个相等性测试是否等效?

n = 5# 测试一.如果 n == 5:打印耶！"# 测试二.如果 n 是 5:打印耶！"

这是否适用于您将比较实例的对象(例如 list)?

好的，这样回答了我的问题:

L = []L.append(1)如果 L == [1]:打印耶！"# 成立，但是...如果 L 是 [1]:打印耶！"# 没有.

所以 == 测试值 where is 测试它们是否是同一个对象?

解决方案

is 将返回 True 如果两个变量指向同一个对象，== 如果变量引用的对象相等.

<预><代码>>>>a = [1, 2, 3]>>>乙 = 一>>>b 是一个真的>>>b==a真的# 通过切片操作符制作列表`a`的新副本，# 并将其分配给变量`b`>>>b = a[:]>>>b 是一个错误的>>>b==a真的

在您的情况下，第二个测试仅适用于 Python 缓存小整数对象，这是一个实现细节.对于较大的整数，这不起作用:

<预><代码>>>>1000 是 10**3错误的>>>1000 == 10**3真的

同样适用于字符串文字:

<预><代码>>>>一个"是一个"真的>>>"aa" 是 "a" * 2真的>>>x = "a">>>aa"是 x * 2错误的>>>aa"是实习生(x*2)真的

请参阅这个问题 也是.

My Google-fu has failed me.

In Python, are the following two tests for equality equivalent?

n = 5
# Test one.
if n == 5:
    print 'Yay!'

# Test two.
if n is 5:
    print 'Yay!'

Does this hold true for objects where you would be comparing instances (a list say)?

Okay, so this kind of answers my question:

L = []
L.append(1)
if L == [1]:
    print 'Yay!'
# Holds true, but...

if L is [1]:
    print 'Yay!'
# Doesn't.

So == tests value where is tests to see if they are the same object?

解决方案

is will return True if two variables point to the same object, == if the objects referred to by the variables are equal.

>>> a = [1, 2, 3]
>>> b = a
>>> b is a 
True
>>> b == a
True

# Make a new copy of list `a` via the slice operator, 
# and assign it to variable `b`
>>> b = a[:] 
>>> b is a
False
>>> b == a
True

In your case, the second test only works because Python caches small integer objects, which is an implementation detail. For larger integers, this does not work:

>>> 1000 is 10**3
False
>>> 1000 == 10**3
True

The same holds true for string literals:

>>> "a" is "a"
True
>>> "aa" is "a" * 2
True
>>> x = "a"
>>> "aa" is x * 2
False
>>> "aa" is intern(x*2)
True

Please see this question as well.

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