python/cpython · error · ValueError

binary mode doesn't take an errors argument

Error message

binary mode doesn't take an errors argument

What it means

Raised by io.open() as a ValueError when binary mode ('b') is combined with a non-None errors argument. The errors handler governs text encode/decode failures; a binary stream performs no codec work, so the argument is rejected.

Source

Thrown at Lib/_pyio.py:225

    if modes - set("axrwb+t") or len(mode) > len(modes):
        raise ValueError("invalid mode: %r" % mode)
    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():

View on GitHub (pinned to bc6749cc3b)

Solutions

1. Remove the errors argument for binary opens.
2. Conditionally build kwargs: include errors (and encoding) only when 'b' not in mode.
3. If you wanted lenient decoding, you are handling text — use text mode with errors='replace'.

Example fix

// before
with open(path, 'rb', errors='ignore') as f:   # ValueError
    data = f.read()

// after
with open(path, 'rb') as f:
    data = f.read()
# or, if text was intended:
with open(path, 'r', encoding='utf-8', errors='ignore') as f:
    text = f.read()

def open_any(path, mode='r', encoding=None, errors=None, newline=None):
    if 'b' in mode:
        return open(path, mode)   # binary: no text kwargs at all
    return open(path, mode, encoding=encoding, errors=errors, newline=newline)

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

When it happens

Trigger: open('f.bin', 'wb', errors='replace'), or wrappers that unconditionally forward errors=... while the mode is binary. Often appears together with error 170 when both encoding and errors are threaded through every call.

Common situations: Shared open helpers with errors='replace' defaults; migrating a text-mode call to binary for speed while keeping the error-handler kwarg; copy-pasted open() lines edited from 'r' to 'rb' but leaving the kwargs.

Related errors

• can't have text and binary mode at once
• binary mode doesn't take an encoding argument
• binary mode doesn't take a newline argument
• invalid errors: %r
• can't have read/write/append mode at once

AI-assisted analysis of python/cpython@bc6749cc3b (2026-08-14). Data as JSON: /api/errors/2bd9f3b56456f6fe. Report an issue: GitHub.