raise
raise 语句用来引发异常。语法如下：
raise expression from expression
如果不带表达式，raise 会重新引发当前作用域内最后一个激活的异常。如果当前作用域内没有激活的异常，将会引发 RuntimeError 来提示错误。
raise
---------------------------------------------------------------------------
RuntimeError Traceback (most recent call last)
<ipython-input-1-9c9a2cba73bf> in <module>
----> 1 raise
RuntimeError: No active exception to reraise
raise 会将第一个表达式求值为异常对象。它必须为 BaseException 的子类或实例。如果它是一个类，当需要时会通过不带参数地实例化该类来获得异常的实例。
type(ZeroDivisionError)
type
raise ZeroDivisionError # 无提示信息
---------------------------------------------------------------------------
ZeroDivisionError Traceback (most recent call last)
<ipython-input-2-798b08d1683c> in <module>
----> 1 raise ZeroDivisionError # 无提示信息
ZeroDivisionError:
raise ZeroDivisionError('分母不能为 0') # 自定义提示信息
---------------------------------------------------------------------------
ZeroDivisionError Traceback (most recent call last)
<ipython-input-17-950b4accf1f2> in <module>
----> 1 raise ZeroDivisionError('分母不能为 0') # 自定义提示信息
ZeroDivisionError: 分母不能为 0
from 子句用于异常串连：如果有该子句，则第二个表达式必须为另一个异常类或实例，它将被关联到所引发的异常:
raise IndexError("索引错误") from NameError('名称错误')
---------------------------------------------------------------------------
NameError Traceback (most recent call last)
NameError: 名称错误
The above exception was the direct cause of the following exception:
IndexError Traceback (most recent call last)
<ipython-input-18-124f83b49e6f> in <module>
----> 1 raise IndexError("索引错误") from NameError('名称错误')
IndexError: 索引错误
try:
print(1 / 0)
except Exception as e:
raise RuntimeError("Something bad happened") from e
---------------------------------------------------------------------------
ZeroDivisionError Traceback (most recent call last)
<ipython-input-13-83aaca0b7e7f> in <module>
1 try:
----> 2 print(1 / 0)
3 except Exception as e:
ZeroDivisionError: division by zero
The above exception was the direct cause of the following exception:
RuntimeError Traceback (most recent call last)
<ipython-input-13-83aaca0b7e7f> in <module>
2 print(1 / 0)
3 except Exception as e:
----> 4 raise RuntimeError("Something bad happened") from e
RuntimeError: Something bad happened
如果一个异常在异常处理器或 finally 中被引发，类似的机制会隐式地发挥作用：
try:
print(1 / 0)
except:
raise RuntimeError("Something bad happened")
---------------------------------------------------------------------------
ZeroDivisionError Traceback (most recent call last)
<ipython-input-16-5576c5c08e42> in <module>
1 try:
----> 2 print(1 / 0)
3 except:
ZeroDivisionError: division by zero
During handling of the above exception, another exception occurred:
RuntimeError Traceback (most recent call last)
<ipython-input-16-5576c5c08e42> in <module>
2 print(1 / 0)
3 except:
----> 4 raise RuntimeError("Something bad happened")
RuntimeError: Something bad happened
try:
print(1 / 0)
finally:
raise RuntimeError("Something bad happened")
---------------------------------------------------------------------------
ZeroDivisionError Traceback (most recent call last)
<ipython-input-15-8b172672db5a> in <module>
1 try:
----> 2 print(1 / 0)
3 finally:
ZeroDivisionError: division by zero
During handling of the above exception, another exception occurred:
RuntimeError Traceback (most recent call last)
<ipython-input-15-8b172672db5a> in <module>
2 print(1 / 0)
3 finally:
----> 4 raise RuntimeError("Something bad happened")
RuntimeError: Something bad happened