title: Add python project as a dependency of another
body: <p>I have a git repository with 4 folders, each of them is a python project. Since some of them want to be able to use functions/class that are in another of the projects but I don't know how to do it.</p>
  <pre><code>repository folder
    |&gt; project 1
    |   |&gt; class1.py
    |   |&gt; class2.py
    |&gt; project 2
    |   |&gt; class10.py
    |   |&gt; class20.py
    |&gt; project 3
    |   |&gt; class11.py
    |   |&gt; class21.py
    |&gt; project 4
    |   |&gt; class110.py
    |   |&gt; class210.py
  </code></pre>
  <p>For example, how I can add <code>project 1</code> as dependency in <code>project 4</code>?</p>
  <p>EDIT: a new folder structure:</p>
  <pre><code>repository_folder
    |&gt; project_1
    |   |&gt; class1.py
    |   |&gt; class2.py
    |&gt; project_2
    |   |&gt; class10.py
    |   |&gt; class20.py
    |&gt; project_3
    |   |&gt; class11.py
    |   |&gt; class21.py
    |&gt; project_4
    |   |&gt; class110.py
    |   |&gt; class210.py
    |   |&gt; folder_1
    |   |   |&gt; class_3.py
    |   |&gt; tests
    |   |   |&gt; test_1.py
    |   |   |&gt; test_2.py
  </code></pre>
  <p>Do I need a <code>__init__.py</code> file into <code>project_4/folder_1</code> folder if I need code from <code>project_1</code>? and into <code>project_4/tests</code> folder?</p>
  <p>What about <code>__all__</code>?</p>
  <p>Can I have a <code>__init__.py</code> and <code>from/import</code> example?</p>