title: Python PIL reduce alpha level, but do not change transparent background
body: <p>I want to make the visible part of image more transparent, but also do not change the alpha-level of fully-transparent background.</p>
  <p>Here's the image: <a href="https://i.stack.imgur.com/AHfL0.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/AHfL0.png" alt="enter image description here" /></a></p>
  <p>and I do it like this:</p>
  <pre class="lang-py prettyprint-override"><code>from PIL import Image
  img = Image.open('image_with_transparent_background.png')
  img.putalpha(128)
  img.save('half_transparent_image_with_preserved_background.png')
  </code></pre>
  <p>And here is what I get: <a href="https://i.stack.imgur.com/LrfO7.png" rel="nofollow noreferrer">half_transparent_image_with_preserved_background.png</a></p>
  <p>How do I achieve exactly what I want - so, without changing the background?</p>