myDict = { 0 : ['a','b','c'],
           1 : ['d','e','f']}

any('a' in lst for lst in myDict.values())

for x in y:
    if condition:
        return True
return False
# return any(condition for x in y)

myDict = { 0 : ['a','b','c'],
           1 : ['d','e','f']}

rset = {x for v in myDict.values() for x in v}

print(rset)

{'b', 'e', 'c', 'd', 'a', 'f'}

'a' in rset

from itertools import chain

if 'a' in chain.from_iterable(myDict.values()):
    # do something
    pass

if 'a' in myDict[0]:
    # do something
    pass

# will not error, but False in case key does not exists
if 'a' in myDict.get(0, ()):
    # do something
    pass