How to say $a=$b->{"A"} ||={} in Python?

[beginner]

Hi All.

I'd like to do the following in more succint code:

if k in b:
    a=b[k]
else:
    a={}
    b[k]=a

a['A']=1


In perl it is just one line: $a=$b->{"A"} ||={}.

Thanks,
Geoffrey