IfThenElse inside Select using context not working as intended

[guilloulouis]

Hello, I looked for a bit but was unable to find anything related to this.
I have this code snippet :

from construct import *

b1 = IfThenElse(lambda ctx: ctx._params.get('ctx') == 1, Int8ul, Int16ul)
a = Struct(
    "1" / Int8ul,
    "2" / Select(b1, Bit)
)
context = {"ctx": 1}

b = a.build({"1": 1, "2": 2}, **context)
print(len(b))  # Output: 3

I would expect to have the length of b to be 2 as my ctx provided is 1 but not it goes to the else part of the IfThenElse statement.
The behavior is correct when I remove the Select.

[arekbulski]

Not sure why it is so but you cannot use Bit class outside of Bitwise context.

[guilloulouis]

I added the Bit to make sure that the subcon would trigger an exception, removing it still shows the issue :

from construct import *

a = Struct(
    "1" / Int8ul,
    "2" / Select(IfThenElse(lambda ctx: ctx._params.get('ctx') == 1, Int8ul, Int16ul))
)
context = {"ctx": 1}

b = a.build({"1": 1, "2": 2}, **context)
print(len(b))  # Output: 3

[arekbulski]

I cannot for the love of god find what is wrong with it. I added following test case. I also managed to workaround the issue by just using different construction:

@xfail(reason="unknown problem with Select")
def test_select_issue_1038():
    s = Struct(
        "value" / Select(IfThenElse(this._params.ctx == 1, Byte, Short)),
    )
    assert s.build(dict(value=9), ctx=1) == b"\x09"

def test_select_issue_1038_fixed():
    s = Struct(
        "value" / Select(If(this._.ctx == 1, Byte), If(this._.ctx == 2, Short)),
    )
    assert s.build(dict(value=9), ctx=1) == b"\x09"

[franzhaas]

I looked at it, and using the compile feature yielded the same issue. This is expected as the compiled code just calls the uncompiled feature.

I added compilation feature.: https://github.com/franzhaas/construct and with that it works.

The changes are quite significant (among other things you cant use "1" as a name for a field anymore), and currently I have no idea why the testsuite doesn't run at all on github actions... but just for this bug... it looks promising.

from construct import *

a = Struct(
    "a" / Int8ul,
    "b" / Select(IfThenElse(lambda ctx: ctx._params.get('ctx') == 1, Int8ul, Int16ul))
)
context = {"ctx": 1}
s = a.compile()
b = s.build({"a": 1, "b": 2}, **context)
print(len(b))  # Output: 2
context = {"ctx": 0}
s = a.compile()
b = s.build({"a": 1, "b": 2}, **context)
print(len(b))  # Output: 3

Update.:

• tests did not run due to pytest 8.0.0 update
• one unit test is failing, repr/str mixup probably...

[franzhaas]

while my aproach changed a lot, the statements here remain valid...