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Using custom functions in Rebuild #1065

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franzhaas opened this issue Jan 18, 2024 · 1 comment · May be fixed by #1069
Open

Using custom functions in Rebuild #1065

franzhaas opened this issue Jan 18, 2024 · 1 comment · May be fixed by #1069

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@franzhaas
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franzhaas commented Jan 18, 2024 •
edited

This minimal test shows how using a custom function to rebuild fails in the compiled case, but works in the interpreted case.

def getlen(this):
    return 2

template = Struct("count" / Rebuild(Byte, getlen), "my_items" / Byte[this.count])
for d  in [template, template.compile()]:
    assert d.parse(b"\x02ab") == Container(count=2, my_items=[97,98])
    assert d.build(dict(count=None,my_items=[255,255])) == b"\x02\xff\xff"
    assert d.build(dict(count=2,my_items=[255,255])) == b"\x02\xff\xff"
    assert d.build(dict(my_items=[255,255])) == b"\x02\xff\xff"
@franzhaas
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franzhaas commented Jan 28, 2024 •
edited

I learned more about how to use the "this" mechanics. If you encounter this problem, check if you can make do without a function, just a this expression. In my case it was possible.

I find it more readable, the compiled code is faster...

I still believe that this improvement is worth it. Alternatively, we can detect the situation and raise a helpful Exception.

@franzhaas franzhaas linked a pull request Feb 4, 2024 that will close this issue
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Allow for user supplied functions in compiled mode franzhaas/construct
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@franzhaas