«So, by simply copying that one and `ast.py`, modifying the latter so it
uses my modified syntax checker, and modifying the `setup.py` file, *I should be
done*». Famous last words.

I was completely wrong. The `_ast` module imported by `ast.py` is not implemented
in `Python/ast.c` but in `Python/Python-ast.c`. Understanding this, I reread [this
post](http://eli.thegreenplace.net/2010/06/30/python-internals-adding-a-new-statement-to-python/)
I linked to last time and finally[^1] understood that changing the syntax would
not be so easy. Another eye-opener was the stack trace I got when `ast.c`'s
`ast_for_call()` (the function I'm planning to change) is called when a function
call is found in the source. Here, have a look:

    #0  ast_for_call (c=0x7fffffffa300, n=0x7ffff700d918, func=0x9ccf40) at Python/ast.c:2470
    #1  0x00000000005bfe73 in ast_for_trailer (c=0x7fffffffa300, n=0x7ffff700db98, left_expr=0x9ccf40) at Python/ast.c:2105
    #2  0x00000000005c03c0 in ast_for_power (c=0x7fffffffa300, n=0x7ffff6ca4238) at Python/ast.c:2217
    #3  0x00000000005c0b01 in ast_for_expr (c=0x7fffffffa300, n=0x7ffff6ca4238) at Python/ast.c:2406
    #4  0x00000000005c1108 in ast_for_testlist (c=0x7fffffffa300, n=0x7ffff6ca4030) at Python/ast.c:2562
    #5  0x00000000005c118e in ast_for_expr_stmt (c=0x7fffffffa300, n=0x7ffff6facfa8) at Python/ast.c:2584
    #6  0x00000000005c3e84 in ast_for_stmt (c=0x7fffffffa300, n=0x7ffff6facfa8) at Python/ast.c:3586
    #7  0x00000000005bc94c in PyAST_FromNodeObject (n=0x7ffff6face18, flags=0x7fffffffa4e0, filename=0x7ffff6d56370, arena=0x8c8780) at Python/ast.c:709
    #8  0x000000000050c9e8 in PyParser_ASTFromStringObject (s=0x7ffff6ca37a0 "foo (a=3, b)", filename=0x7ffff6d56370, start=257, flags=0x7fffffffa4e0, arena=0x8c8780) at Python/pythonrun.c:2160
    #9  0x000000000050c6be in Py_CompileStringObject (str=0x7ffff6ca37a0 "foo (a=3, b)", filename=0x7ffff6d56370, start=257, flags=0x7fffffffa4e0, optimize=-1) at Python/pythonrun.c:2068
    #10 0x00000000004c6e54 in builtin_compile (self=0x7ffff70f9c28, args=0x7ffff6ca04f8, kwds=0x0) at Python/bltinmodule.c:671

The `compile()` builtin (#10) passes through a couple of obscure functions till
it reaches the AST build/syntax check code (all the `ast_for_*()` functions), and
we quickly reach it from `ast.parse()`:

```python
def parse(source, filename='<unknown>', mode='exec'):
    return compile(source, filename, mode, PyCF_ONLY_AST)
```

In a desperate move, I though of hacking a modified version of just that function
and do some `LD_PRELOAD` jedi moves, but the Python interpreter binary (`/usr/bin/python3`)
is statically linked against `libpython3`. Future versions[^2] of the Python
interpreter `main()` function[^3] [will be really simple](http://hg.python.org/cpython/file/tip/Modules/python.c):

```c
/* Minimal main program -- everything is loaded from the library */

#include "Python.h"

#ifdef __FreeBSD__
#include <floatingpoint.h>
#endif

int
main(int argc, char **argv)
{
        /* 754 requires that FP exceptions run in "no stop" mode by default,
         * and until C vendors implement C99's ways to control FP exceptions,
         * Python requires non-stop mode.  Alas, some platforms enable FP
         * exceptions by default.  Here we disable them.
         */
#ifdef __FreeBSD__
        fp_except_t m;

        m = fpgetmask();
        fpsetmask(m & ~FP_X_OFL);
#endif
        return Py_Main(argc, argv);
}
```

So it's not so crazy to think of creating a new interpreter that uses my version
of `ast_for_call()`, but I haven't found the linking command that makes that
possible. So this part of the project is in the freezer.

Meanwhile, I spent a couple of hours with the code and managed to implement piping
and redirection, even with some tests! You can have an idea of how it works reading
[the README](https://github.com/StyXman/ayrton#piping-redirection)[^4]. Also, expect
a release soon.

As for replacing `sh`, no efforts (besides burning some glucose and firing some
neurons) have been done.

[^1]: Also famous last words.

[^2]: Python-3.3 has a more complex version which has to handle making two copies
      of `argv` while dealing with the system's encoding.

[^3]: It's weird, the source code of the executable resides in `Modules/python.c`.
      Go figure.

[^4]: Some day I'll convert that file to reST, make it the tutorial, and have it
      with the rest of the documentation.