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Pass setval calls through SubstructureField to the contained Structure #31

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Pass setval calls through SubstructureField to the contained Structure #31

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0403e71
Pass setval calls through SubstructureField to the contained Structur…
Dec 16, 2015
5421adc
Handle None as a Structure assignment to align with the original beha…
Dec 16, 2015
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7 changes: 7 additions & 0 deletions suitcase/fields.py
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Original file line number Diff line number Diff line change
Expand Up @@ -8,6 +8,7 @@
import six
from suitcase.exceptions import SuitcaseChecksumException, SuitcaseProgrammingError, \
SuitcaseParseError, SuitcaseException, SuitcasePackStructException
import suitcase
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Stylistically, I think I would prefer:

from suitcase.structure import Structure

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I agree that it would look nicer, but it introduces a circular dependency between fields.py and structure.py.

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That could be resolved by adding from suitcase.structure import Structure to the setval method.

from six import BytesIO, StringIO


Expand Down Expand Up @@ -831,6 +832,12 @@ def unpack(self, data, **kwargs):
self._value = self.substructure()
return self._value.unpack(data, **kwargs)

def setval(self, value):
if isinstance(value, suitcase.structure.Structure):
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Perhaps I am thinking slowly today. Is there some instance where this would not be the case? I think this change makes sense, but I'm wondering if it might make more sense to do something like this instead:

def setval(self, value):
    if not isinstance(value, self.substructure):
        raise TypeError("SubstructureField expected %r instance, got %r" % (self.substructure, type(value)))
    else:
        self._value = value

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Ok, I was thinking slowly. I see what you are trying to do here.

I've been thinking for awhile about having a way to instantiate new Structures without having to assign to each field individually. I think that could help with this problem as well. This would look something like this:

# Arguments for creating a new structure with fields initialized
Structure(*positional_field_values, **field_values_by_key)

# Assign to members of existing structure in memory
structure.set(*positional_field_values, **field_values_by_key)

The .set variant could be used in this context. I don't think overloading the normal assignment operator makes sense as many structures have multiple fields.

Does this sound like an acceptable approach?

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So what I was thinking was:

  1. Assigning a non-Structure in place of an arbitrary Structure should raise an exception, rather than overwriting it. Admittedly, there are other ways to do that.
  2. A user is less likely to want to assigned directly to a larger, multi-field structure.
  3. Smaller structures (like strings or RGB colors, etc.) could be more convenient to work with if the Structure subclass handled converting from a Python-native data structure to one that will be packed.

Anyway, maybe it's a little too magical, but I definitely think an exception should be thrown for overwriting a Structure with a non-Structure.

BaseField.setval(self, value)
else:
self._value.setval(value)


class FieldArray(BaseField):
"""Field which contains a list of some other field.
Expand Down