什么是__return__？

Question

我在Python 3.1中调试脚本并发现了这个:

(Pdb)p locals(){'count':264,'self':,'depth':1,'offset':0 , '__ return__':无, 'blkno':4,'size':264}

我发现延迟的PEP提到它,而其他一点也没有.

什么是__return__？什么时候加入？它有用吗？

Answer 1

它是在评估返回命令后pdb调试器停止时函数调用的返回值.对于具有任何副作用的返回表达式非常重要(无法再现,例如从管道中读取一行).

(Pdb) ...                       # stop somewhere in the debugger ...
> test.py(3)f()
-> return x + 1
(Pdb) l                         # list source: I'm just before return
1      def f():
2        x = 7
3  ->    return x + 1
(Pdb) '__return__' in locals()  # __return__ is still undefined
False
(Pdb) s
--Return--
> test.py(3)f()->8              # This printed 8 is a simple case, but frequently
(Pdb) '__return__' in locals()  # the value is an object or line shortened to 80 ch.
True                            # __return__ has the value after return
(Pdb) __return__
8

如果函数退出而不执行return命令则__return__ == None每次都是.

Answer 2

该__return__关键字仅出现在调试器代码中:

matt@stanley:~/src/Python-3.2$ grep -R __return__ .
./Lib/pdb.py:        frame.f_locals['__return__'] = return_value
./Lib/pdb.py:        if '__return__' in self.curframe_locals:
./Lib/pdb.py:            self.message(repr(self.curframe_locals['__return__']))
./Lib/bdb.py:        if '__return__' in frame.f_locals:
./Lib/bdb.py:            rv = frame.f_locals['__return__']