The Python runtime is the software stack responsible for installing your application code and dependencies, and then running that application in the flexible environment.
Python versions
Python 3.12 uses buildpacks. For the full list of supported Python versions, and their corresponding Ubuntu version, see the Runtime support schedule.
To use a supported Python version, you must:
Include the
runtime_config
andoperating_system
settings in yourapp.yaml
file to specify an operating system.Install
gcloud
CLI version 420.0.0 or later. You can update your CLI tooling by running thegcloud components update
command. To view your installed version, run thegcloud version
command.Optionally, you can specify a runtime version by including the
runtime_version
setting in yourapp.yaml
file. By default, the latest Python version is used if theruntime_version
setting is not specified.
Examples
To specify Python 3.12 on Ubuntu 22:
runtime: python env: flex entrypoint: gunicorn -b :$PORT main:app runtime_config: operating_system: "ubuntu22" runtime_version: "3.12"
To specify the latest supported Python version on Ubuntu 22:
runtime: python env: flex entrypoint: gunicorn -b :$PORT main:app runtime_config: operating_system: "ubuntu22"
See the app.yaml
reference page for more information.
Previous runtime versions
For Python version 3.7 and earlier, you specify a version using the
runtime_config
and python_version
settings in your application's app.yaml
file.
Example
runtime: python
env: flex
entrypoint: gunicorn -b :$PORT main:app
runtime_config:
python_version: 3.7
For Python versions 3.7 and earlier, the default interpreter is
Python 2.7.12 if
runtime_config
or python_version
are omitted. For example, you can use
the default runtime by specifying runtime: python
in your app.yaml
file:
runtime: python
env: flex
See the app.yaml
reference page for more information.
The interpreters that are deployed for each version setting are shown in the following table:
python_version setting |
Deployed interpreter | Runtime ID | app.yaml example |
---|---|---|---|
2 (default) |
2.7.12 | python2 |
runtime_config: python_version: 2 |
3.4 |
3.4.8 | python34 |
runtime_config: python_version: 3.4 |
3.5 |
3.5.9 | python35 |
runtime_config: python_version: 3.5 |
3 or 3.6 |
3.6.10 | python36 |
runtime_config: python_version: 3 |
3.7 |
3.7.9 | python37 |
runtime_config: python_version: 3.7 |
Support for other Python runtimes
If you need to use a Python version that isn't supported, you can create a custom runtime and select a valid base image with the Python version you need.
For Google-supplied base images or Docker Python base images, see Building custom runtimes.
To further investigate containerizing App Engine apps for Cloud Run, see the migration guide.
Dependencies
The runtime looks for a
requirements.txt
file in your application's source directory and uses pip
to install any dependencies before starting your application. For more
information on declaring and managing packages, see
Using Python Libraries.
If your app requires private dependencies, you need to use a custom runtime based on the Python runtime to install the appropriate packages.
Application startup
The runtime starts your application using the entrypoint
defined in your
app.yaml
file. The entrypoint
should start a process that responds to HTTP requests on the port defined by the
environment variable PORT
.
Most web applications use a WSGI server such as Gunicorn, uWSGI or Waitress.
Before you can use one of these servers, you must add them as a dependency in your application's requirements.txt
.
If you use gunicorn
for your Flask application, ensure that your application's
Python version is compatible with gunicorn
.
The runtime ensures that all dependencies are installed before your entrypoint is called.
Flask==2.0.2
gunicorn==20.1.0
An example entrypoint using gunicorn for a Flask application:
entrypoint: gunicorn -b :$PORT main:app
An example entrypoint using gunicorn for a Django application:
entrypoint: gunicorn -b :$PORT mydjangoapp:wsgi
Gunicorn is the recommended WSGI server, but it's completely possible to use any other WSGI server. For example, here is an entrypoint that uses uWSGI with Flask:
entrypoint: uwsgi --http :$PORT --wsgi-file main.py --callable app
For applications that can handle requests without a WSGI server, you can just execute a Python script:
entrypoint: python main.py
Recommended Gunicorn configuration
The basic entrypoint examples shown above are intended to be starting points
and may work for your web applications. Most applications, however, will need to
further configure the WSGI server. Instead of specifying all of the settings on
the entrypoint, create a gunicorn.conf.py
file in your project root directory,
where your app.yaml
file is located, and specify it in your entrypoint:
entrypoint: gunicorn -c gunicorn.conf.py -b :$PORT main:app
You can read about all of Gunicorn's configuration values in its documentation.
Workers
Gunicorn uses workers to handle requests. By default, Gunicorn uses sync workers. This worker class is compatible with all web applications, but each worker can only handle one request at a time. By default, gunicorn only uses one of these workers. This can often cause your instances to be underutilized and increase latency in applications under high load.
We recommend setting the number of workers to 2-4 times the number of CPU cores for your instance plus one. You can specify this in gunicorn.conf.py
as:
import multiprocessing
workers = multiprocessing.cpu_count() * 2 + 1
Additionally, some web applications that are mostly I/O bound can see a
performance improvement by using a different worker class.
If your worker class requires additional dependencies such as gevent or tornado, those dependencies will need to be declared in your application's requirements.txt
.
HTTPS and forwarding proxies
App Engine terminates the HTTPS connection at the load balancer and forwards the
request to your application. Most applications do not need to know if the
request was sent over HTTPS or not, but applications that do need this
information should configure Gunicorn to trust the App Engine proxy in their
gunicorn.conf.py
:
forwarded_allow_ips = '*'
secure_scheme_headers = {'X-FORWARDED-PROTO': 'https'}
Gunicorn will now ensure that the wsgi.url_scheme
to 'https'
, which most web
frameworks will use as indication of the request is secure. If your WSGI server
or framework doesn't support this, just check the value of the
X-Forwarded-Proto
header manually.
Some applications also need to ascertain the user's IP address. This is
available in the X-Forwarded-For
header.
Note that the secure_scheme_headers
setting in gunicorn.conf.py
should be
upper case, like X-FORWARDED-PROTO
, but the headers that your code can read
will be in mixed case, like X-Forwarded-Proto
.
Extending the runtime
The flexible environment Python runtime can be used to create a custom runtime. See Customizing the Python for more information.
Environment variables
The following environment variables are set by the runtime environment:
Environment Variable | Description |
---|---|
GAE_INSTANCE |
The name of the current instance. |
GAE_MEMORY_MB |
The amount of memory available to the application process. |
GAE_SERVICE |
The service name specified in your application's app.yaml
file, or if no service name is specified, it is set to default . |
GAE_VERSION |
The version label of the current application. |
GOOGLE_CLOUD_PROJECT |
The Project ID associated with your application, which is visible in the Google Cloud console |
PORT |
The port that will receive HTTP requests. |
You can set additional environment variables in the app.yaml
file.
Metadata server
Each instance of your application can use the Compute Engine metadata server to query information about the instance, including its host name, external IP address, instance ID, custom metadata, and service account information. App Engine does not allow you to set custom metadata for each instance, but you can set project-wide custom metadata and read it from your App Engine and Compute Engine instances.
This example function uses the metadata server to get the external IP address of the instance: