Opened 4 years ago

Closed 4 years ago

Last modified 4 years ago

#18518 closed Bug (fixed) does not overwrite DJANGO_SETTINGS_MODULE in apache prefork

Reported by: schaefer@… Owned by: nobody
Component: Uncategorized Version: 1.4
Severity: Normal Keywords:
Cc: schaefer@…, grahamd, leho@…, benjaoming@… Triage Stage: Design decision needed
Has patch: no Needs documentation: no
Needs tests: no Patch needs improvement: no
Easy pickings: no UI/UX: no


The Django WSGI application shipped with Django 1.4 contains the following piece of code:

os.environ.setdefault("DJANGO_SETTINGS_MODULE", "{{ project_name }}.settings")

(Obviously adapted to the respective project)

Using setdefault() here is wrong. When using a preforked Apache, different WSGI-application can be run consecutively in the same process. If they use different setting modules, the second application will not overwrite the settings module of the first, resulting in the wrong Django application to be shown.


  • Create two Django projects
  • Host each of them on a different virtual host
  • Reload one a number of times to "seed" the processes
  • Reload the other one. Notice that (randomly) it shows the *first* project instead of the second


Replace the aforementioned line with the following:

os.environ["DJANGO_SETTINGS_MODULE"] = "{{ project_name }}.settings"

There is no reason to try and avoid overwriting the environment variable.

Change History (16)

comment:1 Changed 4 years ago by Carl Meyer

Needs documentation: unset
Needs tests: unset
Patch needs improvement: unset
Resolution: wontfix
Status: newclosed

There is a reason to try to avoid overwriting the environment variable - there are quite a few use cases where you might want to set the environment variable yourself externally, if you are using multiple different settings files in your project (a common practice). The value hardcoded in the wsgi file is intended only as a fallback default. In general, hardcoded values should not override values set explicitly in the environment.

Running multiple different Django sites in the same process under preforked Apache is an edge case, and it's easy enough to change the default wsgi file in your project (or use SetEnv in the virtualhost) if you are in that situation.

comment:2 Changed 4 years ago by Graham.Dumpleton@…

You cannot use SetEnv to do anything. That does not affect process wide environment variables when using mod_wsgi, only per request WSGI environ which Django doesn't as far as I know pay attention to for this because it is still stuck with this global variable to define what the settings are.

The root trigger for this problem has been documented in mod_wsgi documentation for years:

Use of multiple sub interpreters is not an edge case, it is actually the default use case in mod_wsgi in that for every distinct WSGIScriptAlias, each runs in a separate sub interpreter of the same process unless you deliberately work out how to use daemon mode and force each WSGI application into a separate daemon process group. Because the majority of people don't read documentation, they don't usually use daemon mode and so will be hit with this problem as soon as they try and host two Django applications.

comment:3 Changed 4 years ago by Carl Meyer

Thanks for clarifying that SetEnv is not useful here. I still think that, since this whole issue is based on the peculiarities of mod_wsgi, it does not provide justification for Django to ship a that overrides an explicitly-set environment variable. It might, however, provide a justification for a patch to the "hosting with mod_wsgi" documentation, explaining why this issue occurs and how one can address it by modifying the default (Yes, I know you said people don't read documentation. At some point, that's their problem.)

comment:4 Changed 4 years ago by Graham.Dumpleton@…

See ticket #18518 for documentation change.

comment:5 Changed 4 years ago by Graham.Dumpleton@…

Silly cut and paste. See ticket #18559.

comment:6 Changed 4 years ago by grahamd

Cc: grahamd added

comment:7 Changed 4 years ago by Russell Keith-Magee

Resolution: wontfix
Status: closedreopened
Triage Stage: UnreviewedDesign decision needed

Discussion on Twitter, and a duplicate as #18559 makes it clear that there is some contention over this wontfix. At the very least, the limitations that exist with prefork need to be documented (as suggested on #18559); a strong recommendation to use daemon mode (obviating the "default config is prefork" problem) might also be appropriate.

comment:8 Changed 4 years ago by Leho Kraav

Cc: leho@… added

comment:9 Changed 4 years ago by schaefer@…

What, exactly, would be the use case for setting DJANGO_SETTINGS_MODULE prior to seeing it? SetEnv does not work, so you can not set the environment variable from the Apache config. Where, exactly, would a user set it? It seems to me that setting D_S_M outside of would be the niche case, in which case the user can edit for his needs?

comment:10 Changed 4 years ago by benjaoming

mod_wsgi is very important to Django, and an abundance of Django server platforms are running it with Apache prefork. So it's a bit bizarre to introduce a default behaviour that does not support it, nor even mention the incompatibility. I did not expect it and walked right into a trap with hours of debugging, because I run older (working!) WSGI scripts mixed with a broken Django 1.4 auto-generated script, and because there are 30 server processes, I only had these random processes with an incorrect DJANGO_SETTINGS_MODULE.

I suggest a commented-out example of how to directly set DJANGO_SETTINGS_MODULE from so the user is aware and can take action.

# Default behaviour is to respect the outside environment
os.environ.setdefault('DJANGO_SETTINGS_MODULE', 'mysite.settings')

# Enable the below line if you are running multiple sites sharing the same environment.
# This is necessary with for instance Apache prefork + mod_wsgi.
#os.environ['DJANGO_SETTINGS_MODULE'] = 'mysite.settings'

I would much prefer having the second line commented in, and the setdefault as an example. I agree with schaefer that setting the environment outside of the script is probably a niche -- since is included in a django project, then it's already a project-specific script!

I could live with a big warning box in the documentation about how to properly configure the environment and an example of how to customize -- but having out-of-the-box good behaviour is my preference :)

comment:11 Changed 4 years ago by benjaoming

Cc: benjaoming@… added

comment:12 Changed 4 years ago by Carl Meyer

This boils down to an issue of how much Django wants to privilege the peculiar needs of mod_wsgis unusual Python execution environment over other WSGI app servers that execute Python in a more usual way (one environment, one process). If you're using one of the latter (gunicorn, for instance), it's pretty natural to, for instance, have separate "development" and "production" settings modules and choose one by setting DJANGO_SETTINGS_MODULE explicitly, for instance DJANGO_SETTINGS_MODULE=myproject.settings.production gunicorn ..., and it would be quite surprising (even anti-social) for a Django script to presume to unilaterally override DJANGO_SETTINGS_MODULE to a fixed value, disregarding what is set in the environment.

That said, I recognize that there are a lot of mod_wsgi users, and the current behavior interacts quite badly with its default settings. I am still opposed to changing the default Django wsgi script to accommodate thate, but I'd certainly be in favor of a strong documentation fix, even including a comment directly above that line in the file.

comment:13 Changed 4 years ago by Aymeric Augustin

FWIW I agree with Carl's last comment. I would be extremely surprised if DJANGO_SETTINGS_MODULE was arbitrarily overridden.

comment:14 Changed 4 years ago by benjaoming

I've suggested a commented out example of how things should look with WSGI. Would that work together with a comment in the documentation?

comment:15 Changed 4 years ago by Carl Meyer <carl@…>

Resolution: fixed
Status: reopenedclosed

In 751a7d0c32746dc6774f1b561db523b25365148a:

Fixed #18518 -- Add warning re mod_wsgi and environ handling.

comment:16 Changed 4 years ago by Carl Meyer

Ah shoot, I forgot to thank people appropriately in the commit message; my apologies. Thanks to schaefer for the report and Graham and benjaoming for further discussion and clarification. I've closed the ticket with this prominent documentation warning (including a comment in the generated, but that doesn't rule out the possibility of someone opening a new ticket or pull request (or reopening this one) if there's a brilliant idea for how to better support mod_wsgi sub-interpreters directly, without sacrificing the ability to set DJANGO_SETTINGS_MODULE externally.

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