With typical Python usage, the PYTHONPATH environment variable (or IRONPYTHONPATH, etc.) provides the default search path for module files. That is, when you use an from <name> import... or import <name> statement, Python searches the following locations in order for a matching name:

1. Python's built-in modules.
2. The folder containing the Python code you're running.
3. The "module search path" as defined by the applicable environment variable. (See The Module Search Path and Environment variables in the core Python documentation.)

Visual Studio ignores the search path environment variable, however, even when the variable is set for the entire system. It's ignored, in fact, precisely because it's set for the entire system and thus raises certain questions that cannot be answered automatically: Are the referenced modules meant for Python 2.7 or Python 3.6? Are they going to override standard library modules? Is the developer aware of this behavior or is it a malicious hijacking attempt?

Visual Studio thus provides a means to specify search paths directly in both environments and projects. Code that you run or debug in Visual Studio receives search paths in the value of PYTHONPATH (and other equivalent variables). By adding search paths, Visual Studio inspects the libraries in those locations and builds IntelliSense databases for them when needed (Visual Studio 2017 version 15.5 and earlier; constructing the database may take some time depending on the number of libraries).

To add a search path, right-click on the Search Paths item in Solution Explorer, select Add Folder to Search Path..., and select the folder to include. This path is used for any environment associated with the project. (You may see errors if the environment is based on Python 3 and you attempt to add a search path to Python 2.7 modules.)

Files with a .zip or .egg extension can also be added as search paths by selecting Add Zip Archive to Search Path.... As with folders, the contents of these files are scanned and made available to IntelliSense.

If you are regularly using the same search paths and the contents do not often change, it may be more efficient to install it into your site-packages folder. The search path is then analyzed and stored in the IntelliSense database, is always be associated with the intended environment, and does not require a search path to be added to each project.

• Managing Python environments in Visual Studio
• Select an interpreter for a project
• Using requirements.txt for dependencies
• Python Environments window reference