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eXtensible Fixed-Width file handling module

Project description

xfw is an eXtensible Fixed-Width file handling module.

Features

  • field 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 / bugs

  • string 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. See codecs standard module.
  • proper interface declaration
  • fields (IntegerField, DateTimeField) should cast by default when parsing
  • FieldList 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.

Example

Dislaimer: 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 string
  • type 2: a 2-char integer, 8 chars of padding, a 1-char integer

To run the following code as a doctest, run:

python -m doctest README.rst

Declare 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,
...                 },
...             ]
...         ),
...     ],
... )
True

Verify SHA1 was properly accumulated:

>>> import hashlib
>>> hashlib.sha1(sample_file.getvalue()).hexdigest() == checksumed_wrapper.getHexDigest()
True

Generate 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()
True

Likewise, 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())
23

Generate 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()
True

Still, 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'

Project details


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