With the recent experimental release of Python 2.7 support, many people are starting to move their apps from the 2.5 runtime to 2.7. In this series of posts, I'll go over the various considerations for migrating your app in detail, starting with the most immediately obvious - and arguably biggest impact - of them all: threadsafe and multithreaded Python apps.
As you're probably aware, the 2.7 runtime supports making your Python app multithreaded, meaning a single instance of the app may service multiple user requests at the same time. Due to the Global Interpreter Lock, a multithreaded Python app still has limited concurrency, but since most of the wallclock time of a typical App Engine app is spent waiting for RPCs - during which the GIL is not held - the parallelism, and corresponding improvements in utilization, can be substantial.
Moving to Python 2.7 and enabling multithreading is pretty straightforward. First, you have to update your app.yaml. Suppose the start of your app.yaml looks something like this:
application: myapp version: main runtime: python api_version: 1
To switch to Python 2.7, you change it to this (changes are highlighted):
application: myapp version: main runtime: python27 ...