RustPython/RustPython · error · ValueError

binary mode doesn't take a newline argument

Error message

binary mode doesn't take a newline argument

What it means

ValueError raised when newline is passed together with a binary mode. newline controls line-ending translation ('\n' vs os.linesep), which is a text-layer concept; binary streams must see bytes exactly as they are. Notably the common csv recipe open(path, 'w', newline='') is text-mode and legal - the error only appears when 'b' is combined with a non-None newline.

Source

Thrown at Lib/_pyio.py:227

    creating = "x" in modes
    reading = "r" in modes
    writing = "w" in modes
    appending = "a" in modes
    updating = "+" in modes
    text = "t" in modes
    binary = "b" in modes
    if text and binary:
        raise ValueError("can't have text and binary mode at once")
    if creating + reading + writing + appending > 1:
        raise ValueError("can't have read/write/append mode at once")
    if not (creating or reading or writing or appending):
        raise ValueError("must have exactly one of read/write/append mode")
    if binary and encoding is not None:
        raise ValueError("binary mode doesn't take an encoding argument")
    if binary and errors is not None:
        raise ValueError("binary mode doesn't take an errors argument")
    if binary and newline is not None:
        raise ValueError("binary mode doesn't take a newline argument")
    if binary and buffering == 1:
        import warnings
        warnings.warn("line buffering (buffering=1) isn't supported in binary "
                      "mode, the default buffer size will be used",
                      RuntimeWarning, 2)
    raw = FileIO(file,
                 (creating and "x" or "") +
                 (reading and "r" or "") +
                 (writing and "w" or "") +
                 (appending and "a" or "") +
                 (updating and "+" or ""),
                 closefd, opener=opener)
    result = raw
    try:
        line_buffering = False
        if buffering == 1 or buffering < 0 and raw._isatty_open_only():
            buffering = -1
            line_buffering = True

View on GitHub (pinned to aaeab4f754)

Solutions

1. Remove newline from binary opens
2. Keep newline='' only with text modes: open(path, 'w', newline='', encoding='utf-8')
3. Strip text-only keys from shared kwargs when 'b' in mode

Example fix

# before
with open(path, 'wb', newline='') as f:
    writer = csv.writer(f)      # binary + newline -> ValueError

# after
with open(path, 'w', newline='', encoding='utf-8') as f:
    writer = csv.writer(f)

def open_any(path, mode='r', newline=None):
    kwargs = {} if 'b' in mode else ({'newline': newline} if newline is not None else {})
    return open(path, mode, **kwargs)

def takes_text_kwargs(mode) -> bool:
    return 'b' not in mode

When it happens

Trigger: open(path, 'wb', newline='') copied from the csv.writer recipe after switching to binary; wrappers that always pass newline='' to avoid universal-newline surprises; a commit that flipped 'w' to 'wb' while leaving newline in place.

Common situations: CSV/text exporters migrated to binary formats; a shared open-kwargs dict reused across text and binary paths; documentation examples adapted incompletely.

Related errors

• binary mode doesn't take an encoding argument
• binary mode doesn't take an errors argument
• can't have text and binary mode at once
• can't have read/write/append mode at once
• must have exactly one of read/write/append mode

AI-assisted analysis of RustPython/RustPython@aaeab4f754 (2026-08-17). Data as JSON: /api/errors/e299119551b731f7. Report an issue: GitHub.