[Tutor] Environment variables and Flask

[Mayo Adams]

Many thanks to some very bright and helpful gentlemen.

On Fri, Jun 28, 2019 at 9:24 AM Mats Wichmann <mats at wichmann.us> wrote:

> On 6/27/19 11:24 PM, Mayo Adams wrote:
> > I have for some time been flummoxed as to the significance of setting
> > environment variables, for example in order to run a Flask application.
> > What are these environment variables, exactly, and why is it necessary to
> > set them? "Googling" here simply leads me into more arcana, and doesn't
> > really help.
>
> As others have noted, it's a way to pass information from one process to
> another at startup time.   Since this is a Python list, I thought it
> might be instructive to show how it works. In Python, you access these
> environment variables through a dictionary in the os module, called
> environ, which "survives" across the call-another-process boundary,
> unlike any normal variables you might set in your program. Here's a
> trivial Python app that is able to recognize those environment variables
> that begin with MYENV_.  That points up one issue with environment
> variables right away: it's a namespace you share with everybody, and
> there's a chance someone accidentally is using a variable you think is
> important - because it's important to them in their context, not yours.
> So tricks like special naming conventions may be useful.
>
> In this snip, we build a dictionary from os.environ, using only the keys
> that seem to be "for us":
>
>
> === child.py ===
> import os
>
> myenv = { k: v for k, v in os.environ.items() if "MYENV_" in k }
>
> print("child found these settings:", myenv)
> ======
>
> Now write another script which sets a value, then calls the child
> script; then sets a different value, then calls the child script.
>
> === parent.py ===
> import os
> import subprocess
>
> print("Calling with MYENV_foo set")
> os.environ['MYENV_foo'] = "Yes"
> subprocess.run(["python", "child.py"])
>
> print("Calling with MYENV_bar set")
> os.environ['MYENV_bar'] = "1"
> subprocess.run(["python", "child.py"])
> ======
>
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-- 
Mayo Adams

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