serialposix.py 文件源码

def read(self, size=1):
        """\
        Read size bytes from the serial port. If a timeout is set it may
        return less characters as requested. With no timeout it will block
        until the requested number of bytes is read.
        """
        if not self.is_open:
            raise portNotOpenError
        read = bytearray()
        timeout = Timeout(self._timeout)
        while len(read) < size:
            try:
                ready, _, _ = select.select([self.fd, self.pipe_abort_read_r], [], [], timeout.time_left())
                if self.pipe_abort_read_r in ready:
                    os.read(self.pipe_abort_read_r, 1000)
                    break
                # If select was used with a timeout, and the timeout occurs, it
                # returns with empty lists -> thus abort read operation.
                # For timeout == 0 (non-blocking operation) also abort when
                # there is nothing to read.
                if not ready:
                    break   # timeout
                buf = os.read(self.fd, size - len(read))
                # read should always return some data as select reported it was
                # ready to read when we get to this point.
                if not buf:
                    # Disconnected devices, at least on Linux, show the
                    # behavior that they are always ready to read immediately
                    # but reading returns nothing.
                    raise SerialException(
                        'device reports readiness to read but returned no data '
                        '(device disconnected or multiple access on port?)')
                read.extend(buf)
            except OSError as e:
                # this is for Python 3.x where select.error is a subclass of
                # OSError ignore BlockingIOErrors and EINTR. other errors are shown
                # https://www.python.org/dev/peps/pep-0475.
                if e.errno not in (errno.EAGAIN, errno.EALREADY, errno.EWOULDBLOCK, errno.EINPROGRESS, errno.EINTR):
                    raise SerialException('read failed: {}'.format(e))
            except select.error as e:
                # this is for Python 2.x
                # ignore BlockingIOErrors and EINTR. all errors are shown
                # see also http://www.python.org/dev/peps/pep-3151/#select
                if e[0] not in (errno.EAGAIN, errno.EALREADY, errno.EWOULDBLOCK, errno.EINPROGRESS, errno.EINTR):
                    raise SerialException('read failed: {}'.format(e))
            if timeout.expired():
                break
        return bytes(read)