>>> import dis
>>> def f(): s=''
>>> dis.dis(f)
  2           0 LOAD_CONST               1 ('')
              2 STORE_FAST               0 (s)
              4 LOAD_CONST               0 (None)
              6 RETURN_VALUE

>>> def f(): s=str()
>>> dis.dis(f)
  2           0 LOAD_GLOBAL              0 (str)
              2 CALL_FUNCTION            0
              4 STORE_FAST               0 (s)
              6 LOAD_CONST               0 (None)
              8 RETURN_VALUE

>>> def f(): d={}
>>> dis.dis(f)
  2           0 BUILD_MAP                0
              2 STORE_FAST               0 (d)
              4 LOAD_CONST               0 (None)
              6 RETURN_VALUE

>>> def f(): d=dict()
>>> dis.dis(f)
  2           0 LOAD_GLOBAL              0 (dict)
              2 CALL_FUNCTION            0
              4 STORE_FAST               0 (d)
              6 LOAD_CONST               0 (None)
              8 RETURN_VALUE