AssertionError: assert(len(cstruct) == len(field.sequence_fields))

Running the following program results in an AssertionError in deform/widget.py.

Uncommenting either of the two commented out lines prevents the problem.

import colander
from colander import SchemaNode, Integer
import deform
from deform.widget import Select2Widget


class TitleSchema(colander.MappingSchema):
    @colander.instantiate()
    class xs(colander.SequenceSchema):
        x = SchemaNode(Integer())

    y = SchemaNode(Integer())



def test_stackoverflow():
    schema = TitleSchema()
    form = deform.Form(schema)

    items = [
        ('__start__', 'xs:sequence'), ('x', '1'), ('__end__', 'xs:sequence'),
        # ('y', '2'),
    ]

    try:
        title = form.validate(items)
    except deform.ValidationFailure as e:
        title = {}

    # form = deform.Form(schema)
    form.render(title)

test_stackoverflow()

$ python ~/deform-stackoverflow.py
Traceback (most recent call last):
  File "/home/thomas/.virtualenvs/vodsuite/lib/python3.6/site-packages/chameleon/template.py", line 171, in render
    self._render(stream, econtext, rcontext)
  File "mapping_item_8ef2b9ece625905633682437b4262fd4.py", line 527, in render
  File "/home/thomas/.virtualenvs/vodsuite/lib/python3.6/site-packages/deform/field.py", line 512, in serialize
    return self.widget.serialize(**values)
  File "/home/thomas/.virtualenvs/vodsuite/lib/python3.6/site-packages/deform/widget.py", line 1524, in serialize
    assert(len(cstruct) == len(field.sequence_fields))
AssertionError

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/thomas/deform-stackoverflow.py", line 34, in <module>
    test_stackoverflow()
  File "/home/thomas/deform-stackoverflow.py", line 31, in test_stackoverflow
    rendered_form = form.render(title)
  File "/home/thomas/.virtualenvs/vodsuite/lib/python3.6/site-packages/deform/field.py", line 551, in render
    html = self.serialize(cstruct, **kw)
  File "/home/thomas/.virtualenvs/vodsuite/lib/python3.6/site-packages/deform/field.py", line 512, in serialize
    return self.widget.serialize(**values)
  File "/home/thomas/.virtualenvs/vodsuite/lib/python3.6/site-packages/deform/widget.py", line 1369, in serialize
    return field.renderer(template, **values)
  File "/home/thomas/.virtualenvs/vodsuite/lib/python3.6/site-packages/deform/template.py", line 85, in __call__
    return self.load(template_name)(**kw)
  File "/home/thomas/.virtualenvs/vodsuite/lib/python3.6/site-packages/chameleon/template.py", line 124, in __call__
    return self.render(**kwargs)
  File "/home/thomas/.virtualenvs/vodsuite/lib/python3.6/site-packages/chameleon/zpt/template.py", line 297, in render
    return super(PageTemplate, self).render(**_kw)
  File "/home/thomas/.virtualenvs/vodsuite/lib/python3.6/site-packages/chameleon/template.py", line 171, in render
    self._render(stream, econtext, rcontext)
  File "form_69db4de6f2356e2bc79900fcf934ee09.py", line 634, in render
  File "/home/thomas/.virtualenvs/vodsuite/lib/python3.6/site-packages/deform/field.py", line 741, in render_template
    return self.renderer(template, **values)
  File "/home/thomas/.virtualenvs/vodsuite/lib/python3.6/site-packages/deform/template.py", line 85, in __call__
    return self.load(template_name)(**kw)
  File "/home/thomas/.virtualenvs/vodsuite/lib/python3.6/site-packages/chameleon/template.py", line 124, in __call__
    return self.render(**kwargs)
  File "/home/thomas/.virtualenvs/vodsuite/lib/python3.6/site-packages/chameleon/zpt/template.py", line 297, in render
    return super(PageTemplate, self).render(**_kw)
  File "/home/thomas/.virtualenvs/vodsuite/lib/python3.6/site-packages/chameleon/template.py", line 191, in render
    raise_with_traceback(exc, tb)
  File "/home/thomas/.virtualenvs/vodsuite/lib/python3.6/site-packages/chameleon/utils.py", line 75, in raise_with_traceback
    raise exc
  File "/home/thomas/.virtualenvs/vodsuite/lib/python3.6/site-packages/chameleon/template.py", line 171, in render
    self._render(stream, econtext, rcontext)
  File "mapping_item_8ef2b9ece625905633682437b4262fd4.py", line 527, in render
  File "/home/thomas/.virtualenvs/vodsuite/lib/python3.6/site-packages/deform/field.py", line 512, in serialize
    return self.widget.serialize(**values)
  File "/home/thomas/.virtualenvs/vodsuite/lib/python3.6/site-packages/deform/widget.py", line 1524, in serialize
    assert(len(cstruct) == len(field.sequence_fields))
AssertionError: <unprintable AssertionError object>

 - Expression: "field.serialize(cstruct).strip()"
 - Filename:   ... python3.6/site-packages/deform/templates/mapping_item.pt
 - Location:   (line 29: col 34)
 - Source:     ... place="structure field.serialize(cstruct).strip()"
                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 - Expression: "child.render_template(item_template)"
 - Filename:   ... ite/lib/python3.6/site-packages/deform/templates/form.pt
 - Location:   (line 46: col 32)
 - Source:     ... ace="structure child.render_template(item_template)"/>
                                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 - Arguments:  field: <Field - at 0x7f9579ebd668>
               cstruct: <_null - at 0x7f957be26b00>
               target_language: <NoneType - at 0x9beb80>
               repeat: {...} (0)
               error_class: <NoneType - at 0x9beb80>
               description:
               title: Xs
               oid: deformField1
               hidden: False
               category: default
               structural: False
               required: True
               input_prepend: <NoneType - at 0x9beb80>
               input_append: <NoneType - at 0x9beb80>

...

Versions used:

• python3.4 and python3.6
• deform 2.0.4
• colander 1.4
• Chameleon: 3.2

Start with the supplied Python reproduction and trace deform/widget.py at the Select2Widget.serialize assertion. Compare the sequence data when y is omitted with the case where it is included, then inspect the existing widget tests for a suitable regression-test location. Done means form.render(title) completes without AssertionError for the reported input while existing tests pass.