Configure Python web apps for IIS

Articolo
01/12/2022

Applies to: Visual Studio Visual Studio for Mac

Note

This article applies to Visual Studio 2017. If you're looking for the latest Visual Studio documentation, see Visual Studio documentation. We recommend upgrading to the latest version of Visual Studio. Download it here

When using Internet Information Services (IIS) as a web server on a Windows computer (including Windows virtual machines on Azure, Python apps must include specific settings in their web.config files so that IIS can properly process Python code. The computer itself must also have Python installed along with any packages the web app requires.

Note

The previous article, is still available on Managing App Service on Windows with the Python extensions.

Install Python on Windows

To run a web app, first install your required version of Python directly on the Windows host machine as described on Install Python interpreters.

Record the location of the python.exe interpreter for later steps. For convenience, you can add that location to your PATH environment variable.

Install packages

When using a dedicated host, you can use the global Python environment to run your app rather than a virtual environment. Accordingly, you can install all of your app's requirements into the global environment simply by running pip install -r requirements.txt at a command prompt.

Set web.config to point to the Python interpreter

Your app's web.config file instructs the IIS (7+) web server running on Windows about how it should handle Python requests through either HttpPlatform (recommended) or FastCGI. Visual Studio versions 2015 and earlier make these modifications automatically. When using Visual Studio 2017 and later, you must modify web.config manually.

Configure the HttpPlatform handler

The HttpPlatform module passes socket connections directly to a standalone Python process. This pass-through allows you to run any web server you like, but requires a startup script that runs a local web server. You specify the script in the <httpPlatform> element of web.config, where the processPath attribute points to the site extension's Python interpreter and the arguments attribute points to your script and any arguments you want to provide:

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <system.webServer>
    <handlers>
      <add name="PythonHandler" path="*" verb="*" modules="httpPlatformHandler" resourceType="Unspecified"/>
    </handlers>
    <httpPlatform processPath="c:\python36-32\python.exe"
                  arguments="c:\home\site\wwwroot\runserver.py --port %HTTP_PLATFORM_PORT%"
                  stdoutLogEnabled="true"
                  stdoutLogFile="c:\home\LogFiles\python.log"
                  startupTimeLimit="60"
                  processesPerApplication="16">
      <environmentVariables>
        <environmentVariable name="SERVER_PORT" value="%HTTP_PLATFORM_PORT%" />
      </environmentVariables>
    </httpPlatform>
  </system.webServer>
</configuration>

The HTTP_PLATFORM_PORT environment variable shown here contains the port that your local server should listen on for connections from localhost. This example also shows how to create another environment variable, if desired, in this case SERVER_PORT.

Configure the FastCGI handler

FastCGI is an interface that works at the request level. IIS receives incoming connections and forwards each request to a WSGI app running in one or more persistent Python processes.

To use it, first install and configure the wfastcgi package as described on pypi.org/project/wfastcgi/.

Next, modify your app's web.config file to include the full paths to python.exe and wfastcgi.py in the PythonHandler key. The steps below assume that Python is installed in c:\python36-32 and that your app code is in c:\home\site\wwwroot; adjust for your paths accordingly:

Modify the PythonHandler entry in web.config so that the path matches the Python install location (see IIS Configuration Reference (iis.net) for exact details).

<system.webServer>
  <handlers>
    <add name="PythonHandler" path="*" verb="*" modules="FastCgiModule"
        scriptProcessor="c:\python36-32\python.exe|c:\python36-32\wfastcgi.py"
        resourceType="Unspecified" requireAccess="Script"/>
  </handlers>
</system.webServer>

Within the <appSettings> section of web.config, add keys for WSGI_HANDLER, WSGI_LOG (optional), and PYTHONPATH:
```
<appSettings>
  <add key="PYTHONPATH" value="c:\home\site\wwwroot"/>
  
  <add key="WSGI_HANDLER" value="app.wsgi_app()"/>
  <add key="WSGI_LOG" value="c:\home\LogFiles\wfastcgi.log"/>
</appSettings>
```
These <appSettings> values are available to your app as environment variables:
- The value for PYTHONPATH may be freely extended but must include the root of your app.
- WSGI_HANDLER must point to a WSGI app importable from your app.
- WSGI_LOG is optional but recommended for debugging your app.
Set the WSGI_HANDLER entry in web.config as appropriate for the framework you're using:
- Bottle: make sure you have parentheses after app.wsgi_app as shown below. This is necessary because that object is a function (see app.py) rather than a variable:
```

<add key="WSGI_HANDLER" value="app.wsgi_app()"/>
```
- Flask: Change the WSGI_HANDLER value to <project_name>.app where <project_name> matches the name of your project. You can find the exact identifier by looking at the from <project_name> import app statement in the runserver.py. For example, if the project is named "FlaskAzurePublishExample", the entry would appear as follows:
```

<add key="WSGI_HANDLER" value="FlaskAzurePublishExample.app"/>
```
- Django: Two changes are needed to web.config for Django projects. First, change the WSGI_HANDLER value to django.core.wsgi.get_wsgi_application() (the object is in the wsgi.py file):
```

<add key="WSGI_HANDLER" value="django.core.wsgi.get_wsgi_application()"/>
```
  Second, add the following entry below the one for WSGI_HANDLER, replacing DjangoAzurePublishExample with the name of your project:
```
<add key="DJANGO_SETTINGS_MODULE" value="django_iis_example.settings" />
```
Django apps only: In the Django project's settings.py file, add your site URL domain or IP address to ALLOWED_HOSTS as shown below, replacing '1.2.3.4' with your URL or IP address, of course:
```
# Change the URL or IP address to your specific site
ALLOWED_HOSTS = ['1.2.3.4']
```
Failure to add your URL to the array results in the error DisallowedHost at / Invalid HTTP_HOST header: '<site URL>'. You may need to add '<site URL>' to ALLOWED_HOSTS.

When the array is empty, Django automatically allows 'localhost' and '127.0.0.1', but adding your production URL removes those capabilities. For this reason, you might want to maintain separate development and production copies of settings.py, or use environment variables to control the runtime values.

Deploy to IIS or a Windows VM

With the correct web.config file in your project, you can publish to the computer running IIS by using the Publish command on the project's context menu in Solution Explorer, and selecting the option, IIS, FTP, etc.. In this case, Visual Studio simply copies the project files to the server; you're responsible for all server-side configuration.

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