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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def reduce_opacity(img, opacity):
 """ Returns an image with reduced opacity. """

    assert opacity >= 0 and opacity <= 1

    if img.mode != 'RGBA':
        img = img.convert('RGBA')
    else:
        img = img.copy()

    alpha = img.split()[3]
    alpha = ImageEnhance.Brightness(alpha).enhance(opacity)
    img.putalpha(alpha)

    return img