How to avoid repetition in a ternary operator assignment?

Question

When fetching a number of config values from os.environ, it's nice to have defaults in the python code to easily allow the application to start in a number of contexts. A typical django settings.py has a number of

SOME_SETTING = os.environ.get('SOME_SETTING')

lines.

To provide sensible defaults we opted for

SOME_SETTING = os.environ.get('SOME_SETTING') or "theValue"

However, this is error prone because calling the application with

SOME_SETTING=""

manage.py

will lead SOME_SETTING to be set to theValue instead of the explicitly defined ""

Is there a way to assign values in python using the ternary a = b if b else d without repeating b or assigning it to a shorthand variable before?

this becomes obvious if we look at

SOME_VERY_LONG_VAR_NAME = os.environ.get('SOME_VERY_LONG_VAR_NAME') if os.environ.get('SOME_VERY_LONG_VAR_NAME') else 'meh'

It would be much nicer to be able to do something like

SOME_VERY_LONG_VAR_NAME = if os.environ.get('SOME_VERY_LONG_VAR_NAME') else 'meh'

iBug · Accepted Answer · 2018-12-26T12:03:48.763

5

Just like Python's built-in mapping class dict, os.environ.get has a second argument, and it seems like you want it:

SOME_SETTING = os.environ.get('SOME_SETTING', "theValue")

This is the same as

try:
    SOME_SETTING = os.environ['SOME_SETTING']
except KeyError:
    SOME_SETTING = "theValue"

edited Dec 26 '18 at 12:03

answered Dec 26 '18 at 12:00

iBug

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Beat me by a couple seconds - but you should edit your post to mention that the `default` argument is not specific to `os.environ.get`, it's for just any dict or dict-like (mapping) types. – bruno desthuilliers Dec 26 '18 at 12:03
@brunodesthuilliers Yes, thank you for the heads up. – iBug Dec 26 '18 at 12:04

score 2 · Answer 2 · answered Dec 26 '18 at 12:01

2

If you read dict.get()'s doc, you'll find out the method's signature is get(self, key, default=None). The default argument is what gets returned if the key is not found in the dict (and default to a sensible None). So you can use this second argument instead of doing an erroneous boolean test:

SOME_SETTING = os.environ.get('SOME_SETTING', "theValue")

answered Dec 26 '18 at 12:01

bruno desthuilliers

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Trivia: `os.environ` is *not* a dict, though it behaves almost identically. – iBug Dec 26 '18 at 12:02
Indeed - but it's documented as a "dict-like", which means it supports the same API as a dict ;-) – bruno desthuilliers Dec 26 '18 at 12:04

How to avoid repetition in a ternary operator assignment?

2 Answers2