Comment by maksimum

Compared to __slots__ (also Python 3.7.4)

Using your definition of class X

  %timeit x.w()
  313 ns ± 18.3 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

Add __slots__

  class X:
    __slots__ = ('a')
    def w(z):
      a = z.a
      return a+a+a+a+a

  %timeit x.w()
  271 ns ± 7.13 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

About 14% less time.