[Tutor] Environment variables and Flask

[Cameron Simpson]

On 28Jun2019 09:34, Alan Gauld <alan.gauld at yahoo.co.uk> wrote:
>Environment variable have fallen out of favour for user settings
>and config files are now preferred. But some things are a bit
>easier via en environment variable - especially where you spawn
>new sub-processes and don't want the sub-process to have
>to re-read the config file each time.

This is something of a simplification. Most programmes consult a few 
places for configuration information.

A programme may want to run in different ways (different places to write 
files, different language settings or timezones, etc). Environment 
variables are a convenient and inheritable way to indicate a specific 
way to run, because they get inherited (as a copy) from parent programme 
to child programmes and so on.

So a flask application will usually be invoked from within a web server, 
and various things about how it should run _may_ be indicated by 
environment variables set by the web server.

A flexible programme may decide how to run from several places, in a 
specific order (to ensure predictable controllable behaviour).

A normal order would be: command line arguments, environment variables, 
personal config file ($HOME/.thingrc), system configfile (/etc/thingrc), 
inbuilt defaults within the programme.

The idea here is that this is a simple hierachy of defaults. Anything 
can be overridden by a command line option. If not supplied, an 
environment variable may be consulted. Otherwise the personal config 
file. Otherwise the system default. Otherwise some default within the 
programme.

Programmatically you don't go: for each setting, look at these things in 
the order above. Instead you has some "settings" structure in the 
programme, initially filled out with some internal defaults. You read 
the system file to override the builtin defaults.  Then you read the 
personal file to override that.  Then you consult the environment to 
override that.  Then you process the command line and have it override 
various things. A single pass across all this stuff.  Any of it may be 
missing.

Returning to Flask and the environment: because a Flask app is often 
invoked from within a web server instead of directly, it isn't feasible 
to pass it "command line" arguments to control it. So the environment 
becomes the most convenient place for ad hoc special settings.

Cheers,
Cameron Simpson <cs at cskk.id.au>

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