I sure love allocating PyObjects!

a.pyx

"""Tested with:

- cython==0.27.3
- cython==0.25.2
"""

cdef class PairWithConstructor:
    cdef int a;
    cdef int b;

    def __cinit__(PairWithConstructor self, int a, int b):
        self.a = a
        self.b = b

cdef class PairWithoutConstructor:
    cdef int a;
    cdef int b;


cdef pair_without_constructor(int a, int b):
    cdef PairWithoutConstructor ret = PairWithoutConstructor()
    ret.a = a
    ret.b = b
    return ret



cpdef example():
    cdef int a = 1
    cdef int b = 2

    cdef PairWithConstructor boxed = PairWithConstructor(a, b)
    cdef PairWithoutConstructor not_boxed = pair_without_constructor(a, b)

example.c

/* ... */  

 /* "a.pyx":26
 *     cdef int b = 2
 * 
 *     cdef PairWithConstructor boxed = PairWithConstructor(a, b)             # <<<<<<<<<<<<<<
 *     cdef PairWithoutConstructor not_boxed = pair_without_constructor(a, b)
 */
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  __Pyx_GOTREF(__pyx_t_2);
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  __pyx_t_2 = 0;
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  __pyx_v_boxed = ((struct __pyx_obj_1a_PairWithConstructor *)__pyx_t_2);
  __pyx_t_2 = 0;

  /* "a.pyx":27
 * 
 *     cdef PairWithConstructor boxed = PairWithConstructor(a, b)
 *     cdef PairWithoutConstructor not_boxed = pair_without_constructor(a, b)             # <<<<<<<<<<<<<<
 */
  __pyx_t_2 = __pyx_f_1a_pair_without_constructor(__pyx_v_a, __pyx_v_b); if (unlikely(!__pyx_t_2)) __PYX_ERR(1, 27, __pyx_L1_error)
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  __pyx_v_not_boxed = ((struct __pyx_obj_1a_PairWithoutConstructor *)__pyx_t_2);
  __pyx_t_2 = 0;

/* ... */

pair_without_constructor.c

/* "a.pyx":14
 * 
 * 
 * cdef pair_without_constructor(int a, int b):             # <<<<<<<<<<<<<<
 *     cdef PairWithoutConstructor ret = PairWithoutConstructor()
 *     ret.a = a
 */

static PyObject *__pyx_f_1a_pair_without_constructor(int __pyx_v_a, int __pyx_v_b) {
  struct __pyx_obj_1a_PairWithoutConstructor *__pyx_v_ret = 0;
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  __Pyx_RefNannyDeclarations
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  /* "a.pyx":15
 * 
 * cdef pair_without_constructor(int a, int b):
 *     cdef PairWithoutConstructor ret = PairWithoutConstructor()             # <<<<<<<<<<<<<<
 *     ret.a = a
 *     ret.b = b
 */
  __pyx_t_1 = __Pyx_PyObject_Call(((PyObject *)__pyx_ptype_1a_PairWithoutConstructor), __pyx_empty_tuple, NULL); if (unlikely(!__pyx_t_1)) __PYX_ERR(1, 15, __pyx_L1_error)
  __Pyx_GOTREF(__pyx_t_1);
  __pyx_v_ret = ((struct __pyx_obj_1a_PairWithoutConstructor *)__pyx_t_1);
  __pyx_t_1 = 0;

  /* "a.pyx":16
 * cdef pair_without_constructor(int a, int b):
 *     cdef PairWithoutConstructor ret = PairWithoutConstructor()
 *     ret.a = a             # <<<<<<<<<<<<<<
 *     ret.b = b
 *     return ret
 */
  __pyx_v_ret->a = __pyx_v_a;

  /* "a.pyx":17
 *     cdef PairWithoutConstructor ret = PairWithoutConstructor()
 *     ret.a = a
 *     ret.b = b             # <<<<<<<<<<<<<<
 *     return ret
 * 
 */
  __pyx_v_ret->b = __pyx_v_b;

  /* "a.pyx":18
 *     ret.a = a
 *     ret.b = b
 *     return ret             # <<<<<<<<<<<<<<
 * 
 * 
 */
  __Pyx_XDECREF(__pyx_r);
  __Pyx_INCREF(((PyObject *)__pyx_v_ret));
  __pyx_r = ((PyObject *)__pyx_v_ret);
  goto __pyx_L0;

  /* "a.pyx":14
 * 
 * 
 * cdef pair_without_constructor(int a, int b):             # <<<<<<<<<<<<<<
 *     cdef PairWithoutConstructor ret = PairWithoutConstructor()
 *     ret.a = a
 */

  /* function exit code */
  __pyx_L1_error:;
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  __pyx_r = 0;
  __pyx_L0:;
  __Pyx_XDECREF((PyObject *)__pyx_v_ret);
  __Pyx_XGIVEREF(__pyx_r);
  __Pyx_RefNannyFinishContext();
  return __pyx_r;
}

cdef pair_without_constructor(int a, int b):
    cdef PairWithoutConstructor ret = PairWithoutConstructor.__new__(PairWithoutConstructor)
    ret.a = a
    ret.b = b
    return ret