Xavier de Gaye added the comment:
The difference lies in the line numbers. When tracing lines, the interpreter
operates on the basis of physical lines, not logical lines
(https://docs.python.org/3/reference/lexical_analysis.html#line-structure).
The local trace function is called on the first instruction of each physical
line (see the comment in maybe_call_line_trace() in Python/ceval.c).
In the first case of as_context.py, the 'zero' case, we have:
25 74 SETUP_EXCEPT 49 (to 126)
27 77 LOAD_NAME 6 (Context)
80 CALL_FUNCTION 0 (0 positional, 0 keyword pair)
83 SETUP_WITH 20 (to 106)
86 STORE_NAME 7 (zero)
28 89 LOAD_NAME 8 (print)
92 LOAD_CONST 5 ('zero')
95 LOAD_NAME 7 (zero)
...
After 'CALL_FUNCTION' has been run and the trace function set up, the __raise()
local trace function is invoked for the first time upon running the 'LOAD_NAME'
instruction, since 'SETUP_WITH' and 'STORE_NAME' are not the first instruction
of line 27.
In the 'two' case, we have:
41 >> 214 SETUP_EXCEPT 49 (to 266)
42 217 LOAD_NAME 6 (Context)
220 CALL_FUNCTION 0 (0 positional, 0 keyword pair)
223 SETUP_WITH 20 (to 246)
43 226 STORE_NAME 11 (two)
44 229 LOAD_NAME 8 (print)
232 LOAD_CONST 9 ('two')
235 LOAD_NAME 11 (two)
....
Here the __raise() local trace function is invoked for the first time upon
running the 'STORE_NAME' instruction.
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue25474>
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