Import error while using dev_appserver.py in virtualenv

Question

When I run dev_appserver.py . (in the directory containing the app.py) while being in an virtualenv with Python 2.7.12, I get this error and output:

(.venv)$ dev_appserver.py .                                                              
INFO     2017-02-21 18:54:47,250 devappserver2.py:764] Skipping SDK update check.
INFO     2017-02-21 18:54:47,273 api_server.py:268] Starting API server at: http://localhost:35473
INFO     2017-02-21 18:54:47,276 dispatcher.py:199] Starting module "default" running at: http://localhost:8080
INFO     2017-02-21 18:54:47,276 admin_server.py:116] Starting admin server at: http://localhost:8000
Traceback (most recent call last):
  File "/opt/gcloud/google-cloud-sdk/platform/google_appengine/_python_runtime.py", line 101, in <module>
_run_file(__file__, globals())
  File "/opt/gcloud/google-cloud-sdk/platform/google_appengine/_python_runtime.py", line 97, in _run_file
execfile(_PATHS.script_file(script_name), globals_)
  File "/opt/gcloud/google-cloud-sdk/platform/google_appengine/google/appengine/tools/devappserver2/python/runtime.py", line 185, in <module>
main()
  File "/opt/gcloud/google-cloud-sdk/platform/google_appengine/google/appengine/tools/devappserver2/python/runtime.py", line 165, in main
sandbox.enable_sandbox(config)
  File "/opt/gcloud/google-cloud-sdk/platform/google_appengine/google/appengine/tools/devappserver2/python/sandbox.py", line 198, in enable_sandbox
__import__('%s.threading' % dist27.__name__)
  File "/opt/gcloud/google-cloud-sdk/platform/google_appengine/google/appengine/dist27/threading.py", line 11, in <module>
import warnings
  File "/opt/gcloud/google-cloud-sdk/platform/google_appengine/google/appengine/tools/devappserver2/python/sandbox.py", line 1001, in load_module
raise ImportError('No module named %s' % fullname)
ImportError: No module named warnings

Works fine when I'm not in the virtualenv. The warnings module is part of the python standard library as I understand it, so not sure what to to do here. Running pip install warnings doesn't help.

How do I get dev_appserver.py to run in the virtualenv?

Answer 1

I was not working with dev_appserver.py and virtualenv in a proper way. Virtualenv is not to be used here. Relevant documentation on using third-party libraries is found here.

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In short, to include third party libraries with dev_appserver.py:

Instruct pip to store libraries in a folder with the -t flag:

$ pip install -t lib/ <library name>

Create a file named appengine_config.py in the same folder as app, and include the following code:

from google.appengine.ext import vendor

# Add any libraries install in the "lib" folder.
vendor.add('lib')

Now run dev_appserver.py as usual: $ dev_appserver.py app.yaml

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Keep in mind that you can only use Python libraries that use pure Python code (e.g. the bcrypt library cannot be used.).