python - 为什么 os.popen 的返回对象不支持 next() 调用

Question

Python 文档:os.popen:

Open a pipe to or from command. The return value is an open file object connected to the pipe, which can be read or written depending on whether mode is 'r' (default) or 'w'.

我可以使用 next 方法 X.__next__()/X.next() (2.X) 但是不是 next(x) 调用，

• __next__ 方法和 next(x) 不是一样的吗？
• 为什么我们不能将 next(x) 用于 os.popen 的对象？

最后但同样重要的是，next() 和 next method 是如何工作的？

Answer 1

查看源代码(Python 3.4)似乎 __next__ 方法没有在 _wrap_close 类中实现，所以 next()调用失败，因为它无法在类中找到 __next__ 方法。由于覆盖了 __getattr__ 方法，显式 __next__ 调用有效。

相关source code :

def popen(cmd, mode="r", buffering=-1):
    if not isinstance(cmd, str):
        raise TypeError("invalid cmd type (%s, expected string)" % type(cmd))
    if mode not in ("r", "w"):
        raise ValueError("invalid mode %r" % mode)
    if buffering == 0 or buffering is None:
        raise ValueError("popen() does not support unbuffered streams")
    import subprocess, io
    if mode == "r":
        proc = subprocess.Popen(cmd,
                                shell=True,
                                stdout=subprocess.PIPE,
                                bufsize=buffering)
        return _wrap_close(io.TextIOWrapper(proc.stdout), proc)
    else:
        proc = subprocess.Popen(cmd,
                                shell=True,
                                stdin=subprocess.PIPE,
                                bufsize=buffering)
        return _wrap_close(io.TextIOWrapper(proc.stdin), proc)

# Helper for popen() -- a proxy for a file whose close waits for the process
class _wrap_close:
    def __init__(self, stream, proc):
        self._stream = stream
        self._proc = proc
    def close(self):
        self._stream.close()
        returncode = self._proc.wait()
        if returncode == 0:
            return None
        if name == 'nt':
            return returncode
        else:
            return returncode << 8  # Shift left to match old behavior
    def __enter__(self):
        return self
    def __exit__(self, *args):
        self.close()
    def __getattr__(self, name):
        return getattr(self._stream, name)
    def __iter__(self):
        return iter(self._stream)