如何摆脱sympy替换错误信息？

I have an annoying error message that I cannot figure it out. The following code (in Python 3) gives a correct results:

我有一个恼人的错误消息，我无法弄明白。以下代码（在Python 3中）给出了正确的结果：

from sympy import *
x, y, z, t = symbols('x y z t') 
expr = cos(x) + 1
print(expr.subs(x, y))

The results are:

结果是：

cos(y) + 1

However, when expr is modified or different say,

但是，当expr被修改或者说不同时，

from sympy import *
x, y, z, t = symbols('x y z t') 
expr = 1
print(expr.subs(x, y))

The output is the error message saying,

输出是错误消息，

AttributeError: 'int' object has no attribute 'subs'

How can I fix this? I need subs to handle symbolic expression and numerical when necessary (say symbolic expression was modified in such a way that result is numerical). Regards.

我怎样才能解决这个问题？我需要sub来处理符号表达式和必要时的数值（比如说符号表达式被修改为结果是数字的）。问候。

3 个解决方案

#1

The 1 in your second example is not a SymPy number, just a regular Python integer.

第二个示例中的1不是SymPy编号，只是常规Python整数。

You can use the S() (or sympify()) function to turn it into a SymPy number.

您可以使用S（）（或sympify（））函数将其转换为SymPy数字。

The official docs have a Gotchas entry that describes your problem and gives a few more examples, and a description of what sympify can do.

官方文档有一个描述您的问题的Gotchas条目，并提供了一些示例，以及对可以做什么的描述。

#2

1 is not a Sympy expression. 1 is just a regular Python int. You will need to wrap it in a Sympy Integer if you want to perform substitutions on it. (Also, stop using import *.):

1不是Sympy表达式。 1只是一个普通的Python int。如果要对其执行替换，则需要将其包装在Sympy Integer中。（另外，停止使用import *。）：

import sympy

expr = sympy.Integer(1)

Note that Sympy Integers display identically to regular ints, so if some 1s seem to have a subs method and others don't, those 1s might not all be the same type. You can check with type:

请注意，Sympy Integers与常规整数的显示方式相同，因此如果某些1s似乎有一个subs方法而另一些则没有，那么这些1可能并非都是同一类型。你可以检查类型：

In [14]: x = 1

In [15]: y = sympy.Integer(1)

In [16]: x
Out[16]: 1

In [17]: y
Out[17]: 1

In [18]: type(x)
Out[18]: int

In [19]: type(y)
Out[19]: sympy.core.numbers.One

#3

As in the other answers, 1 is not a SymPy integer, it's a Python integer. Therefore it doesn't have SymPy's methods.

与其他答案一样，1不是SymPy整数，而是Python整数。因此它没有SymPy的方法。

You could declare expr as one of the following to make it work:

您可以将expr声明为以下之一以使其工作：

expr = S.One
expr = S.One
expr = S(1)
expr = S（1）
expr = Integer(1)
expr =整数（1）

SymPy integers are instances of the class Integer, whereas Python integers are int objects.

SymPy整数是Integer类的实例，而Python整数是int对象。

There is an additional trick to avoid this step: execute isympy -i. The -i parameter adds an additional input layer that replaces all digits with Integer(digits) objects, as an example:

还有一个技巧可以避免这一步骤：执行isympy -i。 -i参数添加一个额外的输入层，用Integer（数字）对象替换所有数字，例如：

In [1]: 1
Out[1]: 1

In [2]: In[1]
Out[2]: u'Integer (1 )'

That is, inspecting with IPython what the input 1 is, gives u'Integer (1 )'.

也就是说，用IPython检查输入1是什么，给出u'Integer（1）'。

#1