Getting "cannot write mode P as JPEG" while operating on JPG image

This answer is quite old, however, I thought I will put a better way to do the same by checking for the mode before doing the conversion:

if img.mode != 'RGB':
    img = img.convert('RGB')

This is required to save your image in JPEG format.

summary 1 and 2:

• backgroud
  • JPG not support alpha = transparency
  • RGBA, P has alpha = transparency
    • RGBA= Red Green Blue Alpha
• result
  • cannot write mode RGBA as JPEG
  • cannot write mode P as JPEG
• solution
  • before save to JPG, discard alpha = transparency
    • such as: convert Image to RGB
  • then save to JPG
• your code

if im.mode == "JPEG":
    im.save("xxx.jpg")
    # in most case, resulting jpg file is resized small one
elif rgba_or_p_im.mode in ["RGBA", "P"]:
    rgb_im = rgba_or_p_im.convert("RGB")
    rgb_im.save("xxx.jpg")
    # some minor case, resulting jpg file is larger one, should meet your expectation

• do more for you:

about resize imgae file, I have implement a function, for your refer:

from PIL import Image, ImageDraw
cfgDefaultImageResample = Image.BICUBIC # Image.LANCZOS

def resizeImage(inputImage,
                newSize,
                resample=cfgDefaultImageResample,
                outputFormat=None,
                outputImageFile=None
                ):
    """
        resize input image
        resize normally means become smaller, reduce size
    :param inputImage: image file object(fp) / filename / binary bytes
    :param newSize: (width, height)
    :param resample: PIL.Image.NEAREST, PIL.Image.BILINEAR, PIL.Image.BICUBIC, or PIL.Image.LANCZOS
        https://pillow.readthedocs.io/en/stable/reference/Image.html#PIL.Image.Image.thumbnail
    :param outputFormat: PNG/JPEG/BMP/GIF/TIFF/WebP/..., more refer:
        https://pillow.readthedocs.io/en/stable/handbook/image-file-formats.html
        if input image is filename with suffix, can omit this -> will infer from filename suffix
    :param outputImageFile: output image file filename
    :return:
        input image file filename: output resized image to outputImageFile
        input image binary bytes: resized image binary bytes
    """
    openableImage = None
    if isinstance(inputImage, str):
        openableImage = inputImage
    elif CommonUtils.isFileObject(inputImage):
        openableImage = inputImage
    elif isinstance(inputImage, bytes):
        inputImageLen = len(inputImage)
        openableImage = io.BytesIO(inputImage)

    if openableImage:
        imageFile = Image.open(openableImage)
    elif isinstance(inputImage, Image.Image):
        imageFile = inputImage
    # <PIL.PngImagePlugin.PngImageFile image mode=RGBA size=3543x3543 at 0x1065F7A20>
    imageFile.thumbnail(newSize, resample)
    if outputImageFile:
        # save to file
        imageFile.save(outputImageFile)
        imageFile.close()
    else:
        # save and return binary byte
        imageOutput = io.BytesIO()
        # imageFile.save(imageOutput)
        outputImageFormat = None
        if outputFormat:
            outputImageFormat = outputFormat
        elif imageFile.format:
            outputImageFormat = imageFile.format
        imageFile.save(imageOutput, outputImageFormat)
        imageFile.close()
        compressedImageBytes = imageOutput.getvalue()
        compressedImageLen = len(compressedImageBytes)
        compressRatio = float(compressedImageLen)/float(inputImageLen)
        print("%s -> %s, resize ratio: %d%%" % (inputImageLen, compressedImageLen, int(compressRatio * 100)))
        return compressedImageBytes

latest code can found here:

https://github.com/crifan/crifanLibPython/blob/master/python3/crifanLib/thirdParty/crifanPillow.py

Faced with the same error but after the quantize() method.

The solution the same: convert to RGB

image.quantize(colors=256, method=2).convert('RGB')

JPEG does not support alpha = transparency
So before saving to JPEG discard alpha = transparency

You can check the extension and specifically use covert for JPEG if you work with different formats.

extension = str(imgName).split('.')[-1]

if extension == "png":
    bg_image.save(imgName, "PNG")
else:
    if bg_image.mode in ("RGBA", "P"):
        bg_image = bg_image.convert("RGB")
    bg_image.save(imgName, "JPEG")