title: How to install libavformat.so.56 along with libavformat.so.57?
body: <p>I used pip to install opencv3 with the following command on an Anaconda virtual environment. </p>
  
  <pre><code>install opencv-python
  </code></pre>
  
  <p>I successfully installed it because I can see the package in pip list.</p>
  
  <pre><code>(olfatf)rover@rover_pi:/usr/lib/arm-linux-gnueabihf $ pip list | grep opencv
  opencv-python          3.4.0.14  
  </code></pre>
  
  <p>However, when I do import cv2, I get the following error:</p>
  
  <pre><code>(olfatf)rover@rover_pi:/usr/lib/arm-linux-gnueabihf $ python
  Python 3.4.3 |Continuum Analytics, Inc.| (default, Aug 21 2015, 00:53:08) 
  [GCC 4.6.3] on linux
  Type "help", "copyright", "credits" or "license" for more information.
  &gt;&gt;&gt; import cv2
  Traceback (most recent call last):
    File "&lt;stdin&gt;", line 1, in &lt;module&gt;
    File "/home/pi/.conda/envs/olfatf/lib/python3.4/site-packages/cv2/__init__.py", line 3, in &lt;module&gt;
      from .cv2 import *
  ImportError: libavformat.so.56: cannot open shared object file: No such file or directory
  </code></pre>
  
  <p>I checked the <code>/usr/lib/arm-linux-gnueabihf</code> directory and I found: </p>
  
  <pre><code>(olfatf)rover@rover_pi:/usr/lib/arm-linux-gnueabihf $ ls | grep libavformat
  libavformat.a
  libavformat.so
  libavformat.so.57
  libavformat.so.57.56.101
  </code></pre>
  
  <p>I you can see there, I have the version 57 but the version 56 is missing.
  I did research online and people suggested that I should install libavformat-dev </p>
  
  <pre><code>(olfatf)rover@rover_pi:/usr/lib/arm-linux-gnueabihf $ sudo apt-get install libavformat-dev
  Reading package lists... Done
  Building dependency tree       
  Reading state information... Done
  libavformat-dev is already the newest version (7:3.2.10-1~deb9u1+rpt1).
  </code></pre>
  
  <p>But it says that the package already exists. I also tried installing ffmpeg.</p>
  
  <pre><code>(olfatf)rover@rover_pi:/usr/lib/arm-linux-gnueabihf $ sudo apt-get install ffmpeg
  Reading package lists... Done
  Building dependency tree       
  Reading state information... Done
  ffmpeg is already the newest version (7:3.2.10-1~deb9u1+rpt1).
  </code></pre>
  
  <p>But it says that the package already exists.</p>
  
  <p>How to get the 56 version?
  I use Raspberry Pi Strech</p>