appending to dict

[bucket79]

Hi
is there anyway appending to dictionary?
list has this feature

>>>a = []
>>>a.append(1)
>>>print a
[1]

but dictionary can't
i wanna do like this thing
>>>a = {1, 2}
>>>a.append(3, 4) -> This is just my idea :@
>>>print a
{1:2, 3:4}

so i try this feature like
>>>a = {1, 2}
>>>a = dict(zip(a.keys().append(3), a.values().append(4)))
but this make's error
because a.keys().append(3) makes 'None' (i can't understand this :( )

so i try 
>>>import copy
>>>a = dict(zip(copy.copy(a.keys()).append(3),
copy.copy(a.values()).append(4)))
and 
a = dict(zip(copy.deepcopy(a.keys()).append(3),
copy.deepcopy(a.values()).append(4)))
but this make's error, too 

why a.keys().append(3) makes 'None' ?
and is there anyway appending to dictionary?

any post will be appreciated :)