title: Python3 import modules from folder to another folder
body: <p>my structure dictionary is</p>
  
  <pre><code>mainFolder    
      folder1
         __init__.py
         file1.py
         file2.py    
      folder2
         __init__.py
         file3.py
         file4.py    
      setup.py    
      __init__.py
  </code></pre>
  
  <p>i need import file4.py from folder2 to folder1/file1.py 
  <br>file1.py:</p>
  
  <blockquote>
    <p>from ..folder2.file4 import MyClass</p>
  </blockquote>
  
  <p>and i gets:</p>
  
  <blockquote>
    <p>SystemError: Parent module '' not loaded, cannot perform relative
    import</p>
  </blockquote>
  
  <p>how to fix that ?</p>