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dev_appserver-python3-windows

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What

This is a patch to allow you run Python 3 Apps on Windows using dev_appserver.py (ONLY for GAE Standard Environment).

Tested on:

Windows 10 Home Edition, Google Cloud SDK 470.0.0, app-engine-python 1.9.111 🆕

Note: Google Cloud SDK 470.0.0 requires a minimum of Python 3.9
Windows 10 Home Edition, Google Cloud SDK 429.0.0, app-engine-python 1.9.103, app-engine-python-extras 1.9.96
Windows 10 Home Edition, Google Cloud SDK 407.0.0, app-engine-python 1.9.101, app-engine-python-extras 1.9.96
Windows 10 Home Edition, Google Cloud SDK 391.0.0, app-engine-python 1.9.100, app-engine-python-extras 1.9.96

Why

According to Google

The dev_appserver tool does not support development of Python 3 apps on Windows.

How

A high level summary of the changes/code in the patch

If your App doesn't include an 'entrypoint', dev_appserver.py will add the default entrypoint, gunicorn -b :${PORT} main:app. This means dev_appserver.py runs your App with gunicorn because this is what Google uses in Production.

Since gunicorn doesn't run on Windows, the Patch replaces it with another WSGI server, waitress, when it detects you're running Windows and uses the default entrypoint, waitress-serve --listen=*:${PORT} main:app. Note that when your App is deployed to production, it will still be run with gunicorn.
Windows uses the Script folder instead of bin folder for storing python executables. dev_appserver.py included bin folder in the paths to executable files. The Patch uses Script folder when it detects you're running Windows. This has been fixed by Google
dev_appserver.py first creates a copy of your requirements file via the command tempfile.NamedTemporaryFile(), adds gunicorn to the bottom of the copy and then sends this copy to a function which re-opens the file, reads its contents and installs the requirements.

However, Windows doesn't allow reopening of a temporary file via its filename while the file is still open (reference)

The Patch solves this problem by not creating a copy of the requirements file. Instead, it installs the contents of the original requirements file, after which it then installs waitress.
The Patch added the environment variable PIP_USER and set it to False because calling pip -m install <package_name> on Windows via subprocess.Popen() can sometimes lead to the error

'[WinError 5] Access is denied: Consider using the --user option or check the permissions'.

If you then run pip -m --user install <package_name>, you get another error -

Can not perform a '--user' install. User site-packages are not visible in this virtualenv.

Setting PIP_USER = False solves all of the above error i.e. it allows you to run pip -m install <package_name> (reference).

Changed Files

instance_factory.py

Location:

<SDK_INSTALL_PATH>\Cloud SDK\google-cloud-sdk\platform\google_appengine\google\appengine\tools\devappserver2\python\
http_runtime.py

Location:

<SDK_INSTALL_PATH>\Cloud SDK\google-cloud-sdk\platform\google_appengine\google\appengine\tools\devappserver2\

Note:

SDK_INSTALL_PATH = The path to Google Cloud SDK/CLI installation on your machine
If you don't see the path/directory google_appengine\google\appengine\tools\devappserver2\python\, it probaly means you don't have app-engine-python-extras installed. Run the command gcloud components install app-engine-python-extras and it should create the missing path/directories

Installation

Versions

In the src folder, pick the folder which matches your Google Cloud SDK version

For Google Cloud SDK Version 470.0.0 and above, choose gcloud_sdk_70.0.0+
For Google Cloud SDK Version 427.0.0 and above, choose gcloud_sdk_427.0.0+ (Google Cloud SDK Version 427.0.0 introduced a breaking change)
For Google Cloud SDK Version below 427.0.0, choose gcloud_sdk_426.0.0-

For each of the files listed under 'Changed Files',

Navigate to the location
Create a backup of the file
Download our copy of the file from the 'src' folder and overwrite the original file

When done, run your app with the dev_appserver.py command as usual i.e.

dev_appserver.py --runtime_python_path=<PYTHON3_PATH> --application=<PROJECT_ID> app.yaml --port=<PORT_NO>

Note:

Don't include your Python2 Path in the values for the flag --runtime_python_path
For Cloud SDK 427.0.0 and above, don't forget to set the environment variable CLOUDSDK_DEVAPPSERVER_PYTHON to the path of your Python 2 interpreter. If you don't, you'll get an error when trying to run your App with dev_appserver.py. For more details, see Google documentation

Roadmap

Flag to reuse existing virtual environment:

This is now officially supported in gcloud CLI version 422.0.0 via the --python_virtualenv_path flag

Each time you run dev_appserver.py, it creates & activates a new virtual environment (in your temp folder) and installs your requirements.txt file. This can slow down your application startup (even if it's installing the libraries from cache). In addition, dev_appserver.py doesn't delete the previously created temp folders. This means that over time (especially when you're debugging which leads to multiple app restarts), your temp folder becomes littered with temp virtual environments

~~The plan is to create a flag to tell dev_appserver.py to use an existing virtual environment like the venv folder which you would typically create in your project root.~~
Flag to delete the virtual environment created in temp when the application shuts down

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