In Python, how can I test if I'm in Google App Engine SDK?

Based on the same trick, I use this function in my code:

def isLocal():
    return os.environ["SERVER_NAME"] in ("localhost", "www.lexample.com")

I have customized my /etc/hosts file in order to be able to access the local version by prepending a “l” to my domain name, that way it is really easy to pass from local to production.

Example:

• production url is www.example.com
• development url is www.lexample.com

I just check the httplib (which is a wrapper around appengine fetch)

def _is_gae():
   import httplib
   return 'appengine' in str(httplib.HTTP)

A more general solution
A more general solution, which does not imply to be on a Google server, detects if the code is running on your local machine.
I am using the code below regardless the hosting server:

import socket

if socket.gethostname() == "your local computer name":
    DEBUG = True
    ALLOWED_HOSTS = ["127.0.0.1", "localhost", ]
    ...
else:
    DEBUG = False
    ALLOWED_HOSTS = [".your_site.com",]
    ...

If you use macOS you could write a more generic code:

if socket.gethostname().endswith(".local"): # True in your local computer
    ...

Django developers must put this sample code in the file settings.py of the project.

EDIT:
According to Jeff O’Neill in macOS High Sierra socket.gethostname() returns a string ending in “.lan”.

In app.yaml, you can add IS_APP_ENGINE environment variable

env_variables:
  IS_APP_ENGINE: 1

and in your Python code check if it has been set

if os.environ.get("IS_APP_ENGINE"):
    print("The app is being run in App Engine")
else:
    print("The app is being run locally")

Update on October 2020:
I tried using os.environ["SERVER_SOFTWARE"] and os.environ["APPENGINE_RUNTIME"] but both didn’t work so I just logged all keys from the results from os.environ.
In these keys, there was GAE_RUNTIME which I used to check if I was in the local environment or cloud environment.
The exact key might change or you could add your own in app.yaml but the point is, log os.environ, perhaps by adding to a list in a test webpage, and use its results to check your environment.

The current suggestion from Google Cloud documentation is:

if os.getenv('GAE_ENV', '').startswith('standard'):
  # Production in the standard environment
else:
  # Local execution.