5          16 LOAD_CONST               2 (2)		      19 PRINT_ITEM		      20 PRINT_NEWLINE		 >>   21 LOAD_CONST               0 (None)		      24 RETURN_VALUE 	   If 'a' is false, execution will jump to instruction at offset	   15 and the co_lnotab will claim that execution has moved to	   line 3.  This is at best misleading.  In this case we could	   associate the POP_TOP with line 4, but that doesn't make	   sense in all cases (I think). 	   What we do is only call the line trace function if the co_lnotab	   indicates we have jumped to the *start* of a line, i.e. if the	   current instruction offset matches the offset given for the	   start of a line by the co_lnotab. 	   This also takes care of the situation where 'a' is true.	   Execution will jump from instruction offset 12 to offset 21.	   Then the co_lnotab would imply that execution has moved to line	   5, which is again misleading. 	   Why do we set f_lineno when tracing?  Well, consider the code	   above when 'a' is true.  If stepping through this with 'n' in	   pdb, you would stop at line 1 with a "call" type event, then	   line events on lines 2 and 3, then a "return" type event -- but	   you would be shown line 5 during this event.  This is a change	   from the behaviour in 2.2 and before, and I've found it	   confusing in practice.  By setting and using f_lineno when	   tracing, one can report a line number different from that	   suggested by f_lasti on this one occasion where it's desirable.	*/  int PyCode_CheckLineNumber(PyCodeObject* co, int lasti, PyAddrPair *bounds){        int size, addr, line;        unsigned char* p;         p = (unsigned char*)PyString_AS_STRING(co->co_lnotab);        size = PyString_GET_SIZE(co->co_lnotab) / 2;         addr = 0;        line = co->co_firstlineno;        assert(line > 0);         /* possible optimization: if f->f_lasti == instr_ub           (likely to be a common case) then we already know           instr_lb -- if we stored the matching value of p           somwhere we could skip the first while loop. */         /* see comments in compile.c for the description of           co_lnotab.  A point to remember: increments to p           should come in pairs -- although we don't care about           the line increments here, treating them as byte           increments gets confusing, to say the least. */         bounds->ap_lower = 0;        while (size > 0) {                if (addr + *p > lasti)                        break;                addr += *p++;                if (*p)                         bounds->ap_lower = addr;                line += *p++;                --size;        }         /* If lasti and addr don't match exactly, we don't want to           change the lineno slot on the frame or execute a trace           function.  Return -1 instead.        */        if (addr != lasti)                line = -1;                if (size > 0) {                while (--size >= 0) {                        addr += *p++;                        if (*p++)                                break;                }                bounds->ap_upper = addr;        }        else {                bounds->ap_upper = INT_MAX;        }         return line;}

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