PEP 8 kinda predates list comprehensions. I usually break these up over multiple lines at logical locations:
memberdef_list = [elem for elem in from_cache(classname, 'memberdefs')
if elem.argsstring != '[]' and
'std::string' in null2string(elem.vartype)]
Mostly though, I'd forgo the involved test there in the first place:
def stdstring_args(elem):
if elem.argstring == '[]':
return False
return 'std::string' in null2string(elem.vartype)
memberdef_list = [elem for elem in from_cache(classname, 'memberdefs')
if stdstring_args(elem)]
