问题

I apologize for any inconvenience. Here's the translation of the code part you provided:

我想要一个自定义的 SymPy 函数，在转换为字符串时具有特殊行为，行为应该像 _latex 一样。

我尝试了以下代码：

```python
from sympy import Function, Symbol, latex
class TestClass(Function):
    def _latex(self, printer):
        return 'This is a latex test!'
    def __str__(self):
        return 'This is a str test!'

a = TestClass(Symbol('a'))
ab = TestClass(Symbol('a')) + TestClass(Symbol('b'))

对于以下输出：

print(latex(ab))
print(str(a))

预期/期望的结果如下：

This is a latex test! + This is a latex test!
This is a str test!

然而，对于以下输出：

print(str(ab))

输出是：

TestClass(a) + TestClass(b)

如何实现所期望的行为呢？

Please note that the translation is provided only for the code part, as per your request.

<details>
<summary>英文:</summary>

I would like to have a custom SymPy Function with special behavior when it is converted to a string. The behavior should be like for _latex.

I tried the following code:

from sympy import Function, Symbol, latex
class TestClass(Function):
def _latex(self, printer):
return 'This is a latex test!'
def str(self):
return 'This is a str test!'

a = TestClass(Symbol('a'))
ab = TestClass(Symbol('a')) + TestClass(Symbol('b'))

The results for

print(latex(ab))
print(str(a))

are as expected/desired

This is a latex test! + This is a latex test!
This is a str test!

However, for

print(str(ab))

the output is

TestClass(a) + TestClass(b)

How can I archive the desired behavior?

</details>


# 答案1
**得分**: 1

```py
class TestClass(Function):
    def _sympystr(self, printer):
        return printer.doprint('This is a test!')

a = TestClass(Symbol('a'))
ab = TestClass(Symbol('a')) + TestClass(Symbol('b'))

print(a)
print(ab)

输出:

This is a test!
This is a test! + This is a test!

请注意我如何调用打印机：printer.doprint('This is a test!')。在这个简单的情况下，你可以直接返回字符串。但是，如果你的类包含其他sympy对象（符号、函数、数字等），使用这种方法可以创建标准的sympy输出。

class TestClass(Function):
    def _sympystr(self, printer):
        return printer.doprint('This is a test!')

a = TestClass(Symbol('a'))
ab = TestClass(Symbol('a')) + TestClass(Symbol('b'))

print(a)
print(ab)

This is a test!
This is a test! + This is a test!