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Improve WebP encoder error handling #10039

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17 changes: 4 additions & 13 deletions Tests/test_file_webp.py
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Original file line number Diff line number Diff line change
Expand Up @@ -2,7 +2,6 @@

import io
import re
import sys
import warnings
from typing import Any

Expand Down Expand Up @@ -158,21 +157,13 @@ def test_write_unsupported_mode_P(self, tmp_path: Path) -> None:

self._roundtrip(tmp_path, "P", 50.0)

@pytest.mark.skipif(sys.maxsize <= 2**32, reason="Requires 64-bit system")
def test_write_encoding_error_message(self, tmp_path: Path) -> None:
im = Image.new("RGB", (15000, 15000))
with pytest.raises(ValueError, match="encoding error 6"):
im.save(tmp_path / "temp.webp", method=0)

@pytest.mark.skipif(sys.maxsize <= 2**32, reason="Requires 64-bit system")
def test_write_encoding_error_bad_dimension(self, tmp_path: Path) -> None:
im = Image.new("L", (16384, 1))
with pytest.raises(ValueError) as e:
with pytest.raises(
ValueError,
match="encoding error 5: image size exceeds WebP limit of 16383 pixels",
):
im.save(tmp_path / "temp.webp")
assert (
str(e.value)
== "encoding error 5: Image size exceeds WebP limit of 16383 pixels"
)

def test_WebPEncode_with_invalid_args(self) -> None:
"""
Expand Down
6 changes: 6 additions & 0 deletions docs/releasenotes/13.0.0.rst
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Expand Up @@ -134,6 +134,12 @@ escape sequences.
Other changes
=============

WebP encoding errors now include a description
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

When libwebp fails to encode an image, the raised :py:exc:`ValueError` now
describes the error rather than only reporting a numeric code.

@radarhere radarhere Sep 23, 2026

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#8322 was already doing this for one error, so could we include the word 'consistently' in here?


PNG palettes are no longer padded when saving with the ``bits`` argument
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Expand Down
42 changes: 31 additions & 11 deletions src/_webp.c
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Expand Up @@ -73,6 +73,36 @@ HandleMuxError(WebPMuxError err, char *chunk) {
return NULL;
}

/* -------------------------------------------------------------------- */
/* WebP Encoder Error Handling */
/* -------------------------------------------------------------------- */

#define WEBP_STR_HELPER(x) #x
#define WEBP_STR(x) WEBP_STR_HELPER(x)

static const char *const kEncoderErrorMessages[VP8_ENC_ERROR_LAST] = {

@akx akx Sep 22, 2026

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See https://github.com/webmproject/libwebp/blob/13977a505203ae4e3641ce71b6a60738ad66c564/src/webp/encode.h#L284-L297

"ok",
"out of memory allocating objects",
"out of memory re-allocating byte buffer",
"NULL parameter passed to function",
"configuration is invalid",
"image size exceeds WebP limit of " WEBP_STR(WEBP_MAX_DIMENSION) " pixels",

@radarhere radarhere Sep 22, 2026

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What is the story with WEBP_STR and WEBP_STR_HELPER? Why do they need to be defined separately, and why are there two of them?

@akx akx Sep 22, 2026 •
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That's just C preprocessor expansion rules and standard procedure:

a.c

#define WEBP_MAX_DIMENSION 123

// Doesn't work like we want
#define WEBP_STR_1(x) #x
WEBP_STR_1(WEBP_MAX_DIMENSION)

// Works like we want
#define WEBP_STR_HELPER(x) #x
#define WEBP_STR_2(x) WEBP_STR_HELPER(x)
WEBP_STR_2(WEBP_MAX_DIMENSION)

Preprocessing:

$ gcc -E a.c
# 1 "a.c"
# 1 "<built-in>" 1
# 1 "<built-in>" 3
# 466 "<built-in>" 3
# 1 "<command line>" 1
# 1 "<built-in>" 2
# 1 "a.c" 2



"WEBP_MAX_DIMENSION"




"123"

This shows up in Pillow e.g. here (#4700):

Pillow/src/_imaging.c

Lines 4372 to 4374 in 4b03987

#define tostr1(a) #a
#define tostr(a) tostr1(a)
PyObject *v = PyUnicode_FromString(tostr(LIBJPEG_TURBO_VERSION));

"partition #0 is bigger than 512K",
"partition is bigger than 16M",
"picture writer returned an I/O error",
"file would be bigger than 4G",
"encoding aborted by user"
};

static PyObject *
HandleEncoderError(WebPEncodingError error_code) {
const char *message = error_code > VP8_ENC_OK && error_code < VP8_ENC_ERROR_LAST
? kEncoderErrorMessages[error_code]
: "unknown error";
PyErr_Format(PyExc_ValueError, "encoding error %d: %s", error_code, message);
return NULL;
}

/* -------------------------------------------------------------------- */
/* Frame import */
/* -------------------------------------------------------------------- */
Expand Down Expand Up @@ -647,18 +677,8 @@ WebPEncode_wrapper(PyObject *self, PyObject *args) {
ret_size = writer.size;

if (!ok) {
int error_code = (&pic)->error_code;
char message[50] = "";
if (error_code == VP8_ENC_ERROR_BAD_DIMENSION) {
sprintf(
message,
": Image size exceeds WebP limit of %d pixels",
WEBP_MAX_DIMENSION
);
}
PyErr_Format(PyExc_ValueError, "encoding error %d%s", error_code, message);
free(output);
return NULL;
return HandleEncoderError(pic.error_code);

@radarhere radarhere Sep 22, 2026

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Why does HandleEncoderError need to be a separate function?

@akx akx Sep 22, 2026

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It doesn't, but it follows the form of HandleMuxError, above. Would you prefer it inlined?

@radarhere radarhere Sep 23, 2026

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Thanks for pointing that out.

It could go either way, but my preference would be to inline it, since HandleMuxError is repeatedly called, and this isn't.

}

{
Expand Down
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