expanding a variable to a dict

[idle]

I've got a variable in a loop that I'm trying to expand/translate/
readdress as an existing dict so as to add some keys into it..

eg; I have a set of existing dicts: dictFoo, dictBar, dictFrotz (names
changed to protect the innocent)

now I'd like to check them all for the existence of certain default
keys; ie, if the dicts don't contain the keys, add them in with
default values.

so, I've got:

for a in ['dictFoo','dictBar','dictFrotz']:
    if hasattr(a,'srcdir') == False:
        a['srcdir']='/usr/src'

the error I get (which I expect) is 'str' object doesn't support item
assignment.

what incantation do I cast on 'a' to make the interpreter parse it as
'dictFoo' on the first iteration, 'dictBar' on the second, and so
forth?

and/or less importantly, what is such a transformation called, to help
me target my searching?

thanks