In [35]: def a(i):
    ...:     return i * 2
    ...:

In [36]: def b():
    ...:     return a(i)
    ...:

In [37]: def c():
    ...:     def a(i):
    ...:         return i * 2
    ...:     return a(2)
    ...:

In [38]: def d():
    ...:     a = lambda i: i * 2
    ...:     return a(2)
    ...:

In [39]: def e():
    ...:     i = 2
    ...:     return i * 2
    ...:

In [47]: %timeit b()
177 ns ± 1.06 ns per loop (mean ± std. dev. of 7 runs, 10000000 loops each)

In [48]: %timeit c()
261 ns ± 2.54 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

In [49]: %timeit d()
260 ns ± 1.05 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

In [50]: %timeit e()
120 ns ± 0.13 ns per loop (mean ± std. dev. of 7 runs, 10000000 loops each)

In [59]: class C:
    ...:     def a(self, i):
    ...:         return i * 2
    ...:     def b(self):
    ...:         return a(2)
    ...:

In [60]: class D:
    ...:     def b(self):
    ...:         def a(i):
    ...:             return i * 2
    ...:         return a(2)
    ...:

In [61]: obj_c = C()

In [62]: obj_d = D()

In [63]: %timeit obj_c.b()
202 ns ± 0.628 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

In [64]: %timeit obj_d.b()
286 ns ± 1.12 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)

In [65]: dis.dis("""def a(i):
    ...:     return i * 2
    ...: def b():
    ...:     return a(2)
    ...: """)
  1           0 LOAD_CONST               0 (<code object a at 0x106ad45d0, file "<dis>", line 1>)
              2 LOAD_CONST               1 ('a')
              4 MAKE_FUNCTION            0
              6 STORE_NAME               0 (a)

  3           8 LOAD_CONST               2 (<code object b at 0x1071e44b0, file "<dis>", line 3>)
             10 LOAD_CONST               3 ('b')
             12 MAKE_FUNCTION            0
             14 STORE_NAME               1 (b)
             16 LOAD_CONST               4 (None)
             18 RETURN_VALUE

Disassembly of <code object a at 0x106ad45d0, file "<dis>", line 1>:
  2           0 LOAD_FAST                0 (i)
              2 LOAD_CONST               1 (2)
              4 BINARY_MULTIPLY
              6 RETURN_VALUE

Disassembly of <code object b at 0x1071e44b0, file "<dis>", line 3>:
  4           0 LOAD_GLOBAL              0 (a)
              2 LOAD_CONST               1 (2)
              4 CALL_FUNCTION            1
              6 RETURN_VALUE

In [66]: dis.dis("""def b():
    ...:     def a(i):
    ...:         return i * 2
    ...:     return a(2)
    ...: """)
  1           0 LOAD_CONST               0 (<code object b at 0x1071e4420, file "<dis>", line 1>)
              2 LOAD_CONST               1 ('b')
              4 MAKE_FUNCTION            0
              6 STORE_NAME               0 (b)
              8 LOAD_CONST               2 (None)
             10 RETURN_VALUE

Disassembly of <code object b at 0x1071e4420, file "<dis>", line 1>:
  2           0 LOAD_CONST               1 (<code object a at 0x106ad45d0, file "<dis>", line 2>)
              2 LOAD_CONST               2 ('b.<locals>.a')
              4 MAKE_FUNCTION            0
              6 STORE_FAST               0 (a)

  4           8 LOAD_FAST                0 (a)
             10 LOAD_CONST               3 (2)
             12 CALL_FUNCTION            1
             14 RETURN_VALUE

Disassembly of <code object a at 0x106ad45d0, file "<dis>", line 2>:
  3           0 LOAD_FAST                0 (i)
              2 LOAD_CONST               1 (2)
              4 BINARY_MULTIPLY
              6 RETURN_VALUE