Lib/_pyio.py (part 6)

Source:

cpython 3.14 @ ab2d84fe1023/Lib/_pyio.py ↗

This annotation covers the raw file I/O layer. See modules_io7_detail for TextIOWrapper.readline, seek, and reconfigure.

Map

Lines	Symbol	Role
1-80	RawIOBase	ABC for raw binary I/O
81-160	FileIO.__init__	Open a file by path or fd
161-260	FileIO.read	Read up to n bytes
261-360	FileIO.readinto	Read into an existing buffer
361-500	FileIO.write / seek / tell	Write, position control

Reading

FileIO.__init__

# CPython: Lib/_pyio.py:1280 FileIO.__init__
def __init__(self, file, mode='r', closefd=True, opener=None):
    """Open a file."""
    if isinstance(file, (str, bytes, PathLike)):
        if not closefd:
            raise ValueError('Cannot use closefd=False with file name')
        if opener is None:
            fd = os.open(file, flags, 0o666)
        else:
            fd = opener(file, flags)
    elif isinstance(file, int):
        fd = file
    ...
    self._fd = fd
    self._closefd = closefd

FileIO accepts a path (opens the OS file) or an integer fd (wraps an existing open fd). closefd=False prevents close() from closing an externally-owned fd.

FileIO.readinto

# CPython: Lib/_pyio.py:1380 readinto
def readinto(self, b):
    """Read up to len(b) bytes into b.
    Returns number of bytes read (0 for EOF, -1 if non-blocking and no data)."""
    self._checkClosed()
    self._checkReadable()
    length = len(b)
    buf = (ctypes.c_char * length).from_buffer(b)
    n = os.readinto(self._fd, buf)  # or use os.read and copy
    return n

readinto avoids an extra copy: the OS writes directly into the provided buffer (a bytearray or memoryview). The C implementation in Modules/_io/fileio.c calls read(fd, buf, len) directly.

RawIOBase.read

# CPython: Lib/_pyio.py:480 RawIOBase.read
class RawIOBase(IOBase):
    def read(self, size=-1):
        """Read and return up to size bytes, where size is an int.
        Returns an empty bytes object at EOF, or None if the object is
        set not to block and has no data to read."""
        if size is None:
            size = -1
        if size < 0:
            return self.readall()
        b = bytearray(size.__index__())
        n = self.readinto(b)
        if n is None:
            return None
        del b[n:]
        return bytes(b)

RawIOBase.read is implemented in terms of readinto for concrete subclasses that only implement readinto. This is the ABC mixin pattern: implement one method, get the other for free.

gopy notes

FileIO is module/io.FileIO in module/io/module.go. It wraps os.File. readinto calls file.Read(buf) where buf is a Go slice derived from objects.BufferProtocol. RawIOBase.read delegates to readinto via the method resolution order.