问题描述
我有以下问题:
x=Symbol('x')
book={1:2*x,2:3*x}
x=2
print(book) >>> {1:2*x,2:3*x}
我曾希望它会打印 {1:4,2:6}
但是如果我设置 book={1:2*x,2:3*x}就在打印语句之前，我得到了想要的结果.
But if I set book={1:2*x,2:3*x}just before the print statement, I get the wanted result.
令人沮丧的是，如果我改为编写 book=book，这应该是相同的(对吗?)，就在打印语句之前，我得到 {1:2*x,2:3*x} - 为什么会这样?
The frustrating thing is, if I instead write book=book, which should be the same (right?), just before the print statement, I get {1:2*x,2:3*x} - why is that?
推荐答案
您正在处理 x 的两个不同值，因为您正在讨论将 x 设置为在一种情况下不是 Symbol('x') 而不是另一种情况.一种情况:
You're dealing with two different values of x because you're talking about setting x to something other than Symbol('x') in one case but not the other. In one case:
x=Symbol('x')
book={1:2*x,2:3*x}
x=2
print(book)
结果:
{1: 2*x, 2: 3*x}
x 在您创建 book 时是 Symbol('x').以后将 x 设置为 2 并不重要.
x was Symbol('x') when you created book. It doesn't matter that you later set x to 2.
在您的其他情况下，如果我理解正确:
In your other case, if I understand you right:
x=Symbol('x')
x=2
book={1:2*x,2:3*x}
print(book) >>> {1:2*x,2:3*x}
结果:
{1: 4, 2: 6}
当您构建 book 时，
x 现在是 2，因此您的结果现在与 sympy 无关.它只是常规的 Python 算术，因此您可以在字典中获得计算值.
x is now 2 when you build book, so your result now has nothing to do with sympy. It's just regular Python arithmetic, and so you get computed values in your dict.
如果你想要的是{1: 4, 2: 6}，你为什么要使用sympy?
If what you want is {1: 4, 2: 6}, why are you using sympy at all?
要回答您的最后一个问题，您要问为什么添加行 book = book 不会改变结果.为什么会呢?那条线没有任何作用.book 在该行运行后与之前的值相同.
To answer your final question, you're asking why adding the line book = book doesn't change the result. Why would it? That line doesn't do anything. book is the same value after that line is run that it was before.
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