def f():
    a = 'zzz'
    locals()[a] = 'xxx'
    print(a)
f()

def locals():
    import sys
    frame = sys._getframe()
    return frame.f_locals

>>> def f():
...     a = 'zzz'
...     locals()[a] = 'xxx'
...     print a
...
>>> f.func_code.co_varnames
('a',)
>>> import dis
>>> dis.dis(f)
  2           0 LOAD_CONST               1 ('zzz')
              3 STORE_FAST               0 (a)
...
  4          19 LOAD_FAST                0 (a)
             22 PRINT_ITEM

case LOAD_FAST:
    x = GETLOCAL(oparg);
    if (x != NULL) {
        Py_INCREF(x);
        PUSH(x);
        goto fast_next_opcode;
    }
    format_exc_check_arg(PyExc_UnboundLocalError, UNBOUNDLOCAL_ERROR_MSG, PyTuple_GetItem(co->co_varnames, oparg));
    break;

#define GETLOCAL(i)<--->(fastlocals[i])

fastlocals = f->f_localsplus;

typedef struct _frame {
...
    PyObject *f_localsplus[1];<>/* locals+stack, dynamically sized */
} PyFrameObject;

case STORE_FAST:
    v = POP();
    SETLOCAL(oparg, v);
    goto fast_next_opcode;

#define SETLOCAL(i, value)    do { PyObject *tmp = GETLOCAL(i); \
    GETLOCAL(i) = value; \
    Py_XDECREF(tmp); } while (0)