You know what has worked for me really well on windows.

My Computer > Properties > Advanced System Settings > Environment Variables >

Just add the path as C:\Python27 (or wherever you installed python)

OR

Then under system variables I create a new Variable called PythonPath. In this variable I have C:\Python27\Lib;C:\Python27\DLLs;C:\Python27\Lib\lib-tk;C:\other-folders-on-the-path

This is the best way that has worked for me which I hadn't found in any of the docs offered.

EDIT: For those who are not able to get it, Please add

C:\Python27;

along with it. Else it will never work.

The easiest way to do that successfully, is to run the python installer again (after the first installation) and then:

1. choose Modify.
2. check the optional features which you want and click Next.
3. here we go, in "Advanced Options" step you must see an option saying "Add Python to environment variables". Just check that option and click Install. When the installation is completed, python environment variables are added and you can easily use python everywhere.

The PYTHONPATH environment variable is used by Python to specify a list of directories that modules can be imported from on Windows. When running, you can inspect the sys.path variable to see which directories will be searched when you import something.

To set this variable from the Command Prompt, use: set PYTHONPATH=list;of;paths.

To set this variable from PowerShell, use: $env:PYTHONPATH=’list;of;paths’ just before you launch Python.

Setting this variable globally through the Environment Variables settings is not recommended, as it may be used by any version of Python instead of the one that you intend to use. Read more in the Python on Windows FAQ docs.

This question needs a proper answer:

Just use the standard package site, which was made for this job!

and here is how (plagiating my own answer to my own question on the very same topic):

1. Open a Python prompt and type

>>> import site
>>> site.USER_SITE
'C:\\Users\\ojdo\\AppData\\Roaming\\Python\\Python37\\site-packages'
...

(Alternatively, call python -m site --user-site for the same effect.)

2. Create this folder if it does not exist yet:

...
>>> import os
>>> os.makedirs(site.USER_SITE)
...

(Or, in Bash, your preferred variant of makedirs -p $(python -m site --user-site).)

3. Create a file sitecustomize.py (with exactly this filename, or it won't work) in this folder containing the content of FIND_MY_PACKAGES, either manually or using something like the following code. Of course, you have to change C:\My_Projects to the correct path to your custom import location.

...
>>> FIND_MY_PACKAGES = """
import site
site.addsitedir(r'C:\My_Projects')
"""
>>> filename = os.path.join(site.USER_SITE, 'sitecustomize.py')
>>> with open(filename, 'w') as outfile:
...     print(FIND_MY_PACKAGES, file=outfile)

And the next time you start Python, C:\My_Projects is present in your sys.path, without having to touch system-wide settings. Bonus: the above steps work on Linux, too!

Why does this work?

From the documentation of standard library package site:

[Then] an attempt is made to import a module named sitecustomize, which can perform arbitrary site-specific customizations. [...].

So if you create a module named sitecustomize anywhere in PYTHONPATH, package site will execute it at Python startup. And by calling site.addsitedir, the sys.path can be safely extended to your liking.

I got it worked in Windows 10 by following below steps.

Under environment variables, you should only add it under PATH of "System Variables" and not under "User Variables". This is a great confusion and eats time if we miss it.

Also, just try to navigate to the path where you got Python installed in your machine and add it to PATH. This just works and no need to add any other thing in my case.I added just below path and it worked.

C:\Users\YourUserName\AppData\Local\Programs\Python\Python37-32

Most important, close command prompt, re-open and then re-try typing "python" to see the version details. You need to restart command prompt to see the version after setting up the path in environment variables.

After restarting, you should be able to see the python prompt and below info when typing python in command prompt:

While this question is about the 'real' Python, it did come up in a websearch for 'Iron Python PYTHONPATH'. For Iron Python users as confused as I was: It turns out that Iron Python looks for an environment variable called IRONPYTHONPATH.

Linux/Mac/POSIX users: Don't forget that not only does Windows use \ as path separators, but it also uses ; as path delimiters, not :.

You can set the path variable for easily by command prompt.

1. Open run and write cmd

2. In the command window write the following: set path=%path%;C:\python36

3. press enter.
4. to check write python and enter. You will see the python version as shown in the picture.

To make sure Python can find code based on the directory you are executing this code from, if not already there, add to your system environment variable: key PYTHONPATH, value ..