This use of python ternary operator fails - not sure why?

Question

In my function, if a dictionary exists (passed in as a parameter) then add another dictionary to it, or else use it as the dictionary. (here context is the relevant dictionary)

def some_view(request, form_class, template, success_url, context=None):
    ..........
    if context is not None:
        context.update({'form': form})
    else:
        context = {'form': form}
    return render(request, template, context)

This works fine but using

context = context.update({'form': form}) if context is not None else {'form': form}

fails for some reason as context is return as None?

context.update() returns None but you're using it in an assignment — Cory Kramer
– Cory Kramer, Commented Sep 26, 2017 at 13:57
This is because when you update an dictionary in python, it does not return anything. The update happens inplace. — rrawat
– rrawat, Commented Sep 26, 2017 at 14:00
Thanks that's the issue, it updated in place so doesn't return anything, so can't be used for an assignment. I was thinking of it like evaluating an expression, so context could be assigned to it. — Yunti
– Yunti, Commented Sep 26, 2017 at 14:13

chepner · Accepted Answer · 2017-09-26 13:58:16Z

4

The idiom you are looking for is simply

if context is None:
    context = {}
context.update({'form': form})

answered Sep 26, 2017 at 13:58

chepner

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2 Comments

user2390182 Over a year ago

context = context or {} is even shorter (even though not exactly the same).

chepner Over a year ago

I prefer to be explicit.

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