(Using CPython commit c95f8bc270.)

This program has extra bytecodes:

    def f():
        for i in range(10):
            break
        return 17

The dis output is:

      1           0 LOAD_CONST               0 (<code object f at 0x109cf7450, file "afterbreak.py", line 1>)
                  2 LOAD_CONST               1 ('f')
                  4 MAKE_FUNCTION            0
                  6 STORE_NAME               0 (f)
                  8 LOAD_CONST               2 (None)
                 10 RETURN_VALUE

    Disassembly of <code object f at 0x109cf7450, file "afterbreak.py", line 1>:
      2           0 LOAD_GLOBAL              0 (range)
                  2 LOAD_CONST               1 (10)
                  4 CALL_FUNCTION            1
                  6 GET_ITER
                  8 FOR_ITER                 8 (to 18)
                 10 STORE_FAST               0 (i)

      3          12 POP_TOP

      4          14 LOAD_CONST               2 (17)
                 16 RETURN_VALUE
            >>   18 LOAD_CONST               2 (17)
                 20 RETURN_VALUE

The break has something to do with it, because if I change the Python to:

    def f():
        for i in range(10):
            a = 1
        return 17

then the dis output is:

      1           0 LOAD_CONST               0 (<code object f at 0x10bf8f450, file "afterbreak.py", line 1>)
                  2 LOAD_CONST               1 ('f')
                  4 MAKE_FUNCTION            0
                  6 STORE_NAME               0 (f)
                  8 LOAD_CONST               2 (None)
                 10 RETURN_VALUE

    Disassembly of <code object f at 0x10bf8f450, file "afterbreak.py", line 1>:
      2           0 LOAD_GLOBAL              0 (range)
                  2 LOAD_CONST               1 (10)
                  4 CALL_FUNCTION            1
                  6 GET_ITER
            >>    8 FOR_ITER                 8 (to 18)
                 10 STORE_FAST               0 (i)

      3          12 LOAD_CONST               2 (1)
                 14 STORE_FAST               1 (a)
                 16 JUMP_ABSOLUTE            8

      4     >>   18 LOAD_CONST               3 (17)
                 20 RETURN_VALUE

My guess is that we are changing the CFG after computing reachability leaving unreachable blocks marked as reachable.

Our reachability analysis is correct (in this case at least).
There is no unreachable code here.

In theory we could merge the two basic blocks at the end, but searching for identical blocks and merging them is potentially quite expensive (and fiddly).

What might be better is to be change the order of optimizations, so that we duplicate BBs after we have performed jump-to-jump elimination.

Is this a problem in practice?

This isn't a problem for me.  I noticed it and figured I'd mention it.

> Is this a problem in practice?

It's just a hint that code generation has imperfections.

From a teacher's or author's point of view, it is something we have to explain away when demonstrating dis().  It leaves the impression that Python is kludgy.

In what way is it "kludgy"?
There is a mathematical theorem that states that generated code will be imperfect, 
so we will have to live with that :)
https://en.wikipedia.org/wiki/Full_employment_theorem

3.10a produces more efficient bytecode than 3.9.

3.9:

  2           0 LOAD_GLOBAL              0 (range)
              2 LOAD_CONST               1 (10)
              4 CALL_FUNCTION            1
              6 GET_ITER
        >>    8 FOR_ITER                 8 (to 18)
             10 STORE_FAST               0 (i)

  3          12 POP_TOP
             14 JUMP_ABSOLUTE           18
             16 JUMP_ABSOLUTE            8

  4     >>   18 LOAD_CONST               2 (17)
             20 RETURN_VALUE

3.10a:

  2           0 LOAD_GLOBAL              0 (range)
              2 LOAD_CONST               1 (10)
              4 CALL_FUNCTION            1
              6 GET_ITER
              8 FOR_ITER                 8 (to 18)
             10 STORE_FAST               0 (i)

  3          12 POP_TOP

  4          14 LOAD_CONST               2 (17)
             16 RETURN_VALUE
        >>   18 LOAD_CONST               2 (17)
             20 RETURN_VALUE