python异常链

习惯使用java开发，在java开发里有异常链概念和重新抛出异常，在python是怎么实现的呢？

1.异常链

1.1.java实现

    public static void test1() throws Exception{
        throw new Exception("test1 ex");
    }

    public static void test2() throws Exception{
        try {
            test1();
        } catch (Exception e) {
            throw new Exception("test2 ex",e);
        }
    }

    public static void main(String[] args) throws Exception {
        test2();
    }

异常输出：

Exception in thread "main" java.lang.Exception: test2 ex
    at com.epgis.gisserver.EText.test2(EText.java:)
    at com.epgis.gisserver.EText.main(EText.java:)
Caused by: java.lang.Exception: test1 ex
    at com.epgis.gisserver.EText.test1(EText.java:)
    at com.epgis.gisserver.EText.test2(EText.java:)
    ...  more

1.2.python实现

import traceback
def test1():
    raise Exception("ex test1")
def test2():
    try:
        test1()
    except Exception as e:
        raise Exception("ex test2") from e
if __name__ == '__main__':
    test2()

异常输出：

Traceback (most recent call last):
  File "F:/MyProgram/pythonProgram/Nike/ex_test.py", line , in test2
    test1()
  File "F:/MyProgram/pythonProgram/Nike/ex_test.py", line , in test1
    raise Exception("ex test1")
Exception: ex test1

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "F:/MyProgram/pythonProgram/Nike/ex_test.py", line , in <module>
    test2()
  File "F:/MyProgram/pythonProgram/Nike/ex_test.py", line , in test2
    raise Exception("ex test2") from e
Exception: ex test2

2.重新抛出异常

2.1.java实现

	public static void test1() throws Exception{
		throw new Exception("test1 ex");
	}

	public static void test2() throws Exception{
		try {
			test1();
		} catch (Exception e) {
			throw new Exception("test2 ex");
		}
	}

	public static void main(String[] args) throws Exception {
		test2();
	}

异常输出：

Exception in thread "main" java.lang.Exception: test2 ex
    at com.epgis.gisserver.EText.test2(EText.java:)
    at com.epgis.gisserver.EText.main(EText.java:)

可以看出已经将test1的异常给抛弃

2.2.python实现

import traceback
def test1():
    raise Exception("ex test1")
def test2():
    try:
        test1()
    except Exception as e:
        raise Exception("ex test2") from None
if __name__ == '__main__':
    test2()

异常输出：

Traceback (most recent call last):
  File "F:/MyProgram/pythonProgram/Nike/ex_test.py", line , in <module>
    test2()
  File "F:/MyProgram/pythonProgram/Nike/ex_test.py", line , in test2
    raise Exception("ex test2") from None
Exception: ex test2

与java不同的地方一定要from None

如果不使用from会怎么样？

import traceback
def test1():
    raise Exception("ex test1")
def test2():
    try:
        test1()
    except Exception as e:
        raise Exception("ex test2")
if __name__ == '__main__':
    test2()

异常输出：

Traceback (most recent call last):
  File "F:/MyProgram/pythonProgram/Nike/ex_test.py", line , in test2
    test1()
  File "F:/MyProgram/pythonProgram/Nike/ex_test.py", line , in test1
    raise Exception("ex test1")
Exception: ex test1

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "F:/MyProgram/pythonProgram/Nike/ex_test.py", line , in <module>
    test2()
  File "F:/MyProgram/pythonProgram/Nike/ex_test.py", line , in test2
    raise Exception("ex test2")
Exception: ex test2

test1的异常还是输出了