Package: xfw
ContentsFeaturesMissing features / bugsExamplexfw is an eXtensible Fixed-Width file handling module.Featuresfield types (integers, strings, dates) are declared independently of
file structure, and can be extended through subclassing. (BaseField
subclasses)multi-field structure declaration (FieldList class)non-homogeneous file file structure declaration (FieldListFile)checksum/hash computation helpers (ChecksumedFile subclasses)does not depend on line notion (file may not contain CR/LF chars at all
between successive field sets)Missing features / bugsstring trucating is multi-byte (UTF-8, …) agnostic, and will mindlessly cut
in the middle of any entity
If your fields are defined in number of characters of some encoding, just use
provide xfw with unicode objects, and do the transcoding outside it. Seecodecsstandard module.proper interface declarationfields (IntegerField, DateTimeField) should cast by default when parsingFieldList total length should be made optional, and only used to
auto-generate annonymous padding at record end when longer that the sum of
individual fields lengths.ExampleDislaimer: give file format is purely hypothetical, does not some from any spec
I know of, should not be taken as a guideline but just as a showcase of xfw
capabilities.Let’s assume a file composed of a general header, containing some
constant-value 5-char identifier, a 3-char integer giving the number of records
contained, and an optional 20-char comment. It is followed by records, composed
of a header itself composed of a date (YYYYMMDD), a row type (2-char integer)
and number of rows (2-char integer), and followed by rows. Row types all start
with a time (HHMMSS), followed by fields which depend on row type:type 1: a 10-char stringtype 2: a 2-char integer, 8 chars of padding, a 1-char integerTo run the following code as a doctest, run:python -m doctest README.rstDeclare all file structures:>>> import xfw
>>> ROOT_HEADER = xfw.FieldList([
...     (xfw.StringField(5), True, 'header_id'),
...     (xfw.IntegerField(3, cast=True), True, 'block_count'),
...     (xfw.StringField(15), False, 'comment'),
... ], 23, fixed_value_dict={
...     'header_id': 'HEAD1',
... })
>>> BLOCK_HEADER = xfw.FieldList([
...     (xfw.DateTimeField('%Y%m%d', cast=True), True, 'date'),
...     (xfw.IntegerField(2, cast=True), True, 'row_type'),
...     (xfw.IntegerField(2, cast=True), True, 'row_count'),
... ], 12)
>>> ROW_BASE = xfw.FieldList([
...     (xfw.DateTimeField('%H%M%S', cast=True), True, 'time'),
... ], 6)
>>> ROW_TYPE_DICT = {
...     1: xfw.FieldList([
...         ROW_BASE,
...         (xfw.StringField(10), True, 'description'),
...     ], 16),
...     2: xfw.FieldList([
...         ROW_BASE,
...         (xfw.IntegerField(2, cast=True), True, 'some_value'),
...         (xfw.StringField(8), False, None), # annonymous padding
...         (xfw.IntegerField(1, cast=True), True, 'another_value'),
...     ], 17),
... }
>>> def blockCallback(head, item_list=None):
...     if item_list is None:
...         row_count = head['row_count']
...     else:
...         row_count = len(item_list)
...     return row_count, ROW_TYPE_DICT[head['row_type']]
>>> FILE_STRUCTURE = xfw.ConstItemTypeFile(
...     ROOT_HEADER,
...     'block_count',
...     xfw.FieldListFile(
...         BLOCK_HEADER,
...         blockCallback,
...         separator='\n',
...     ),
...     separator='\n',
... )Parse sample file through a hash helper wrapper (SHA1):>>> from cStringIO import StringIO
>>> sample_file = StringIO(
...     'HEAD1002blah           \n'
...     '201112260101\n'
...     '115500other str \n'
...     '201112260201\n'
...     '11550099        8'
... )
>>> from datetime import datetime
>>> checksumed_wrapper = xfw.SHA1ChecksumedFile(sample_file)
>>> parsed_file = FILE_STRUCTURE.parseStream(checksumed_wrapper)
>>> parsed_file == \
... (
...     {
...         'header_id': 'HEAD1',
...         'block_count': 2,
...         'comment': 'blah',
...     },
...     [
...         (
...             {
...                 'date': datetime(2011, 12, 26, 0, 0),
...                 'row_type': 1,
...                 'row_count': 1,
...             },
...             [
...                 {
...                     'time': datetime(1900, 1, 1, 11, 55),
...                     'description': 'other str',
...                 },
...             ]
...         ),
...         (
...             {
...                 'date': datetime(2011, 12, 26, 0, 0),
...                 'row_type': 2,
...                 'row_count': 1,
...             },
...             [
...                 {
...                     'time': datetime(1900, 1, 1, 11, 55),
...                     'some_value': 99,
...                     'another_value': 8,
...                 },
...             ]
...         ),
...     ],
... )
TrueVerify SHA1 was properly accumulated:>>> import hashlib
>>> hashlib.sha1(sample_file.getvalue()).hexdigest() == checksumed_wrapper.getHexDigest()
TrueGenerate a file from parsed data (as it was verified correct above):>>> generated_stream = StringIO()
>>> FILE_STRUCTURE.generateStream(generated_stream, parsed_file)
>>> generated_stream.getvalue() == sample_file.getvalue()
TrueLikewise, using unicode objects and producing streams of different binary
length, although containing the same number of entities. Note that
fixed-values defined in format declaration are optional (ex:header_id),
and dependent values are automaticaly computed (ex:block_count).Generate with unicode chars fitting in single UTF-8-encoded bytes:>>> import codecs
>>> encoded_writer = codecs.getwriter('UTF-8')
>>> input_data = (
...    {
...        'comment': u'Just ASCII',
...    },
...    [],
... )
>>> sample_file = StringIO()
>>> FILE_STRUCTURE.generateStream(encoded_writer(sample_file), input_data)
>>> sample_file.getvalue()
'HEAD1000Just ASCII     '
>>> len(sample_file.getvalue())
23Generate again, with chars needing more bytes when encoded, and demonstrating
checksum generation:>>> wide_input_data = (
...    {
...        'comment': u'\u3042\u3044\u3046\u3048\u304a\u304b\u304d\u304f\u3051\u3053\u3055\u3057\u3059\u305b\u305d',
...    },
...    [],
... )
>>> wide_sample_file = StringIO()
>>> checksumed_wrapper = xfw.SHA1ChecksumedFile(wide_sample_file)
>>> FILE_STRUCTURE.generateStream(encoded_writer(checksumed_wrapper), wide_input_data)
>>> wide_sample_file.getvalue()
'HEAD1000\xe3\x81\x82\xe3\x81\x84\xe3\x81\x86\xe3\x81\x88\xe3\x81\x8a\xe3\x81\x8b\xe3\x81\x8d\xe3\x81\x8f\xe3\x81\x91\xe3\x81\x93\xe3\x81\x95\xe3\x81\x97\xe3\x81\x99\xe3\x81\x9b\xe3\x81\x9d'
>>> len(wide_sample_file.getvalue())
53
>>> hashlib.sha1(wide_sample_file.getvalue()).hexdigest() == checksumed_wrapper.getHexDigest()
TrueStill, both parse to their respective original data:>>> encoded_reader = codecs.getreader('UTF-8')
>>> FILE_STRUCTURE.parseStream(encoded_reader(StringIO(sample_file.getvalue())))[0]['comment']
u'Just ASCII'
>>> FILE_STRUCTURE.parseStream(encoded_reader(StringIO(wide_sample_file.getvalue())))[0]['comment']
u'\u3042\u3044\u3046\u3048\u304a\u304b\u304d\u304f\u3051\u3053\u3055\u3057\u3059\u305b\u305d'