“ ==”与“和“是”？ [英] Is there a difference between "==" and "is"?

问题描述

我的 Google-fu 使我失败了。

在Python中，以下两个相等性测试是否相等？

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!'

是否保留对于要在其中比较实例的对象（例如列表说）是否正确？

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

好吧，所以这种答案我的问题：

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.

所以 == 测试值，其中是进行测试以查看它们是否是同一对象？

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

推荐答案

<$ c如果两个变量指向同一个对象，则$ c> is 将返回 True ， ==

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

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，这将不起作用：

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

对于字符串文字也是如此： / p>

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

请参见这个问题。

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