Errors when initialising a struct with partial keywords or arguments

[yunzheng]

When initialising a struct with partial keywords or arguments the other fields become None resulting in exceptions, for example when printing the instance of the struct.

When initialising without any arguments it works and all values are 0 as expected. But when you only initialise some of the fields it breaks, example:

from dissect.cstruct import cstruct

C_DEF = """"
struct FooBar {
    uint32  foo;
    uint32  bar;
};
"""

cs = cstruct().load(C_DEF)

print(cs.FooBar())  # works as expected: <FooBar foo=0x0 bar=0x0>
print(cs.FooBar(1, 1)) # works as expected: <FooBar foo=0x1 bar=0x1>

print(cs.FooBar(1)) # breaks, bar=None
print(cs.FooBar(bar=1)) # breaks, foo=None

The exception:

$ python3 kwargs_init.py 
<FooBar foo=0x0 bar=0x0>
<FooBar foo=0x1 bar=0x1>
Traceback (most recent call last):
  File "/workspaces/dissect.cstruct/examples/kwargs_init.py", line 14, in <module>
    print(cs.FooBar(1)) # breaks, bar=None
    ^^^^^^^^^^^^^^^^^^^
  File "/workspaces/dissect.cstruct/dissect/cstruct/types/structure.py", line 388, in __repr__
    value = hex(value)
            ^^^^^^^^^^
TypeError: 'NoneType' object cannot be interpreted as an integer

[Schamper]

I think it's more the repr that breaks, not the initialization, right? I think this is actually a bug introduced with #101 😄

[yunzheng]

But if you don't initialise with any arguments it works. So it's more the initialisation behaviour i think.

[yunzheng]

The bug also happens before merging #101, in the 4.1 tagged release:

$ python3 kwargs_init.py 
<FooBar foo=0x0 bar=0x0>
<FooBar foo=0x1 bar=0x1>
Traceback (most recent call last):
  File "/workspaces/dissect.cstruct/examples/kwargs_init.py", line 14, in <module>
    print(cs.FooBar(1)) # breaks, bar=None
    ^^^^^^^^^^^^^^^^^^^
  File "/workspaces/dissect.cstruct/dissect/cstruct/types/structure.py", line 384, in __repr__
    f"{k}={hex(self[k]) if (issubclass(f.type, int) and not issubclass(f.type, Pointer)) else repr(self[k])}"
           ^^^^^^^^^^^^
TypeError: 'NoneType' object cannot be interpreted as an integer

To be clear in the example, the following works:

print(cs.FooBar())
print(cs.FooBar(foo=1, bar=2))

The following breaks, only one field initialised from the two fields:

print(cs.FooBar(foo=1))

[Schamper]

We looked at the code together and decided it’s best to change the default initialization behavior of None to be the default for that type instead.

[yunzheng]

I've currently used the following workaround, which is to initialise it empty first without any arguments, then set the partial values via attributes afterwards, eg:

>>> c = cs.FooBar()
>>> c.foo = 0x1337
>>> print(c)
<FooBar foo=0x1337 bar=0x0>