ImportError: Failed to import pydot. You must install pydot and graphviz for `pydotprint` to work.

I met the error in Windows.

It occurs though I have installed pydot & graphviz.

Root cause: pydot can't find executable graphviz in targeted folders, inclusing system PATH.

Solution: manually download graphviz from http://www.graphviz.org/Download_windows.php and install it, then add path like 'C:\Program Files (x86)\Graphviz2.38\bin' to PATH. 

Note: if your are using virtual env of python, exit it then re-activate to validate the updated PATH.