Run Wolfram Language code from external programs written in C++, C#, Python, Java, JavaScript and other languages.

Create an APIFunction

Create a simple APIFunction that adds 3 to an integer argument:

Deploy the APIFunction to the Wolfram Cloud

Deploy the function to the cloud with Permissions->"Public" to make the API available to anyone:

Get the embed code for the APIFunction

Get the code for calling the API from Python with EmbedCode. Click Copy to Clipboard to copy the embed code to the clipboard:

Paste the embed code into source code

Paste the embed code into a Python script:

Run the program, calling the Wolfram Language API

Run the Python script, which calls Wolfram Language code to add 3 to 6:

$ ./APITest.py 
9

Notes

Calls to APIs deployed in your account use your Cloud Credits.
To call an API function programmatically from the Wolfram Language, use URLExecute and provide the API arguments in a list:
Out[4]=