Lib/io.py (part 3)

Source:

cpython 3.14 @ ab2d84fe1023/Lib/io.py ↗

Lib/io.py is a minimal shim that imports from the C extension _io and exposes the public API. The actual implementation is Modules/_io/.

Map

Lines	Symbol	Role
1-30	Imports from _io	FileIO, BytesIO, StringIO, BufferedReader, etc.
31-60	text_encoding	Default encoding helper (PEP 597)
61-100	open alias, DEFAULT_BUFFER_SIZE	open = _io.open, DEFAULT_BUFFER_SIZE = 8192

Reading

text_encoding

# CPython: Lib/io.py:52 text_encoding
def text_encoding(encoding, stacklevel=2, /):
    if encoding is None or encoding == "locale":
        if _bootlocale.getpreferredencoding(False) == 'UTF-8':
            encoding = 'utf-8'
        else:
            encoding = 'locale'
        if sys.flags.warn_default_encoding:
            import warnings
            warnings.warn(
                "'encoding' argument not specified. The default encoding will be "
                "UTF-8 in a future Python version.",
                EncodingWarning, stacklevel + 1
            )
    return encoding

text_encoding was added in Python 3.10 (PEP 597). It warns when code opens a text file without specifying encoding=, since the default will change from the locale encoding to UTF-8.

ABC hierarchy

IOBase
├── RawIOBase     (FileIO)
├── BufferedIOBase (BytesIO, StringIO, Buffered*)
└── TextIOBase    (TextIOWrapper, StringIO for text mode)

IOBase defines close, fileno, flush, readable, readline, readlines, seek, seekable, tell, truncate, writable, writelines. All are implemented in C with Python stubs for the abstract methods.

DEFAULT_BUFFER_SIZE

# CPython: Lib/io.py:75
DEFAULT_BUFFER_SIZE = 8 * 1024   # 8192 bytes

Used as the default buffering argument to open() and BufferedReader.

gopy notes

io.open is the primary entry point for all file I/O in Python. In gopy io.open delegates to module/io/open.go which creates the appropriate wrapper stack (FileIO + Buffered + TextIOWrapper). DEFAULT_BUFFER_SIZE = 8192 is used by the buffered layers. text_encoding must emit EncodingWarning when appropriate (requires warnings module).