/
objectify.txt
1409 lines (1064 loc) · 43.2 KB
/
objectify.txt
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
==============
lxml.objectify
==============
:Authors:
Stefan Behnel, Holger Joukl
lxml supports an alternative API similar to the Amara_ bindery or
gnosis.xml.objectify_ through a `custom Element implementation`_. The main idea
is to hide the usage of XML behind normal Python objects, sometimes referred
to as data-binding. It allows you to use XML as if you were dealing with a
normal Python object hierarchy.
Accessing the children of an XML element deploys object attribute access. If
there are multiple children with the same name, slicing and indexing can be
used. Python data types are extracted from XML content automatically and made
available to the normal Python operators.
.. contents::
..
1 The lxml.objectify API
1.1 Element access through object attributes
1.2 Creating objectify trees
1.3 Tree generation with the E-factory
1.4 Namespace handling
2 Asserting a Schema
3 ObjectPath
4 Python data types
4.1 Recursive tree dump
4.2 Recursive string representation of elements
5 How data types are matched
5.1 Type annotations
5.2 XML Schema datatype annotation
5.3 The DataElement factory
5.4 Defining additional data classes
5.5 Advanced element class lookup
6 What is different from lxml.etree?
.. _Amara: http://uche.ogbuji.net/tech/4suite/amara/
.. _gnosis.xml.objectify: http://gnosis.cx/download/
.. _`benchmark page`: performance.html#lxml-objectify
.. _`custom Element implementation`: element_classes.html
To set up and use ``objectify``, you need both the ``lxml.etree``
module and ``lxml.objectify``:
.. sourcecode:: pycon
>>> from lxml import etree
>>> from lxml import objectify
The objectify API is very different from the ElementTree API. If it
is used, it should not be mixed with other element implementations
(such as trees parsed with ``lxml.etree``), to avoid non-obvious
behaviour.
The `benchmark page`_ has some hints on performance optimisation of
code using lxml.objectify.
To make the doctests in this document look a little nicer, we also use
this:
.. sourcecode:: pycon
>>> import lxml.usedoctest
Imported from within a doctest, this relieves us from caring about the exact
formatting of XML output.
..
>>> try: from StringIO import StringIO
... except ImportError:
... from io import BytesIO # Python 3
... def StringIO(s):
... if isinstance(s, str): s = s.encode('UTF-8')
... return BytesIO(s)
..
>>> import sys
>>> from lxml import etree as _etree
>>> if sys.version_info[0] >= 3:
... class etree_mock(object):
... def __getattr__(self, name): return getattr(_etree, name)
... def tostring(self, *args, **kwargs):
... s = _etree.tostring(*args, **kwargs)
... if isinstance(s, bytes) and bytes([10]) in s: s = s.decode("utf-8") # CR
... if s[-1] == '\n': s = s[:-1]
... return s
... else:
... class etree_mock(object):
... def __getattr__(self, name): return getattr(_etree, name)
... def tostring(self, *args, **kwargs):
... s = _etree.tostring(*args, **kwargs)
... if s[-1] == '\n': s = s[:-1]
... return s
>>> etree = etree_mock()
The lxml.objectify API
======================
In ``lxml.objectify``, element trees provide an API that models the behaviour
of normal Python object trees as closely as possible.
Element access through object attributes
----------------------------------------
The main idea behind the ``objectify`` API is to hide XML element access
behind the usual object attribute access pattern. Asking an element for an
attribute will return the sequence of children with corresponding tag names:
.. sourcecode:: pycon
>>> root = objectify.Element("root")
>>> b = objectify.SubElement(root, "b")
>>> print(root.b[0].tag)
b
>>> root.index(root.b[0])
0
>>> b = objectify.SubElement(root, "b")
>>> print(root.b[0].tag)
b
>>> print(root.b[1].tag)
b
>>> root.index(root.b[1])
1
For convenience, you can omit the index '0' to access the first child:
.. sourcecode:: pycon
>>> print(root.b.tag)
b
>>> root.index(root.b)
0
>>> del root.b
Iteration and slicing also obey the requested tag:
.. sourcecode:: pycon
>>> x1 = objectify.SubElement(root, "x")
>>> x2 = objectify.SubElement(root, "x")
>>> x3 = objectify.SubElement(root, "x")
>>> [ el.tag for el in root.x ]
['x', 'x', 'x']
>>> [ el.tag for el in root.x[1:3] ]
['x', 'x']
>>> [ el.tag for el in root.x[-1:] ]
['x']
>>> del root.x[1:2]
>>> [ el.tag for el in root.x ]
['x', 'x']
If you want to iterate over all children or need to provide a specific
namespace for the tag, use the ``iterchildren()`` method. Like the other
methods for iteration, it supports an optional tag keyword argument:
.. sourcecode:: pycon
>>> [ el.tag for el in root.iterchildren() ]
['b', 'x', 'x']
>>> [ el.tag for el in root.iterchildren(tag='b') ]
['b']
>>> [ el.tag for el in root.b ]
['b']
XML attributes are accessed as in the normal ElementTree API:
.. sourcecode:: pycon
>>> c = objectify.SubElement(root, "c", myattr="someval")
>>> print(root.c.get("myattr"))
someval
>>> root.c.set("c", "oh-oh")
>>> print(root.c.get("c"))
oh-oh
In addition to the normal ElementTree API for appending elements to trees,
subtrees can also be added by assigning them to object attributes. In this
case, the subtree is automatically deep copied and the tag name of its root is
updated to match the attribute name:
.. sourcecode:: pycon
>>> el = objectify.Element("yet_another_child")
>>> root.new_child = el
>>> print(root.new_child.tag)
new_child
>>> print(el.tag)
yet_another_child
>>> root.y = [ objectify.Element("y"), objectify.Element("y") ]
>>> [ el.tag for el in root.y ]
['y', 'y']
The latter is a short form for operations on the full slice:
.. sourcecode:: pycon
>>> root.y[:] = [ objectify.Element("y") ]
>>> [ el.tag for el in root.y ]
['y']
You can also replace children that way:
.. sourcecode:: pycon
>>> child1 = objectify.SubElement(root, "child")
>>> child2 = objectify.SubElement(root, "child")
>>> child3 = objectify.SubElement(root, "child")
>>> el = objectify.Element("new_child")
>>> subel = objectify.SubElement(el, "sub")
>>> root.child = el
>>> print(root.child.sub.tag)
sub
>>> root.child[2] = el
>>> print(root.child[2].sub.tag)
sub
Note that special care must be taken when changing the tag name of an element:
.. sourcecode:: pycon
>>> print(root.b.tag)
b
>>> root.b.tag = "notB"
>>> root.b
Traceback (most recent call last):
...
AttributeError: no such child: b
>>> print(root.notB.tag)
notB
Creating objectify trees
------------------------
As with ``lxml.etree``, you can either create an ``objectify`` tree by
parsing an XML document or by building one from scratch. To parse a
document, just use the ``parse()`` or ``fromstring()`` functions of
the module:
.. sourcecode:: pycon
>>> fileobject = StringIO('<test/>')
>>> tree = objectify.parse(fileobject)
>>> print(isinstance(tree.getroot(), objectify.ObjectifiedElement))
True
>>> root = objectify.fromstring('<test/>')
>>> print(isinstance(root, objectify.ObjectifiedElement))
True
To build a new tree in memory, ``objectify`` replicates the standard
factory function ``Element()`` from ``lxml.etree``:
.. sourcecode:: pycon
>>> obj_el = objectify.Element("new")
>>> print(isinstance(obj_el, objectify.ObjectifiedElement))
True
After creating such an Element, you can use the `usual API`_ of
lxml.etree to add SubElements to the tree:
.. sourcecode:: pycon
>>> child = objectify.SubElement(obj_el, "newchild", attr="value")
.. _`usual API`: tutorial.html#the-element-class
New subelements will automatically inherit the objectify behaviour
from their tree. However, all independent elements that you create
through the ``Element()`` factory of lxml.etree (instead of objectify)
will not support the ``objectify`` API by themselves:
.. sourcecode:: pycon
>>> subel = objectify.SubElement(obj_el, "sub")
>>> print(isinstance(subel, objectify.ObjectifiedElement))
True
>>> independent_el = etree.Element("new")
>>> print(isinstance(independent_el, objectify.ObjectifiedElement))
False
Tree generation with the E-factory
----------------------------------
To simplify the generation of trees even further, you can use the E-factory:
.. sourcecode:: pycon
>>> E = objectify.E
>>> root = E.root(
... E.a(5),
... E.b(6.1),
... E.c(True),
... E.d("how", tell="me")
... )
>>> print(etree.tostring(root, pretty_print=True))
<root xmlns:py="http://codespeak.net/lxml/objectify/pytype">
<a py:pytype="int">5</a>
<b py:pytype="float">6.1</b>
<c py:pytype="bool">true</c>
<d py:pytype="str" tell="me">how</d>
</root>
This allows you to write up a specific language in tags:
.. sourcecode:: pycon
>>> ROOT = objectify.E.root
>>> TITLE = objectify.E.title
>>> HOWMANY = getattr(objectify.E, "how-many")
>>> root = ROOT(
... TITLE("The title"),
... HOWMANY(5)
... )
>>> print(etree.tostring(root, pretty_print=True))
<root xmlns:py="http://codespeak.net/lxml/objectify/pytype">
<title py:pytype="str">The title</title>
<how-many py:pytype="int">5</how-many>
</root>
``objectify.E`` is an instance of ``objectify.ElementMaker``. By default, it
creates pytype annotated Elements without a namespace. You can switch off the
pytype annotation by passing False to the ``annotate`` keyword argument of the
constructor. You can also pass a default namespace and an ``nsmap``:
.. sourcecode:: pycon
>>> myE = objectify.ElementMaker(annotate=False,
... namespace="http://my/ns", nsmap={None : "http://my/ns"})
>>> root = myE.root( myE.someint(2) )
>>> print(etree.tostring(root, pretty_print=True))
<root xmlns="http://my/ns">
<someint>2</someint>
</root>
Namespace handling
------------------
During tag lookups, namespaces are handled mostly behind the scenes.
If you access a child of an Element without specifying a namespace,
the lookup will use the namespace of the parent:
.. sourcecode:: pycon
>>> root = objectify.Element("{http://ns/}root")
>>> b = objectify.SubElement(root, "{http://ns/}b")
>>> c = objectify.SubElement(root, "{http://other/}c")
>>> print(root.b.tag)
{http://ns/}b
Note that the ``SubElement()`` factory of ``lxml.etree`` does not
inherit any namespaces when creating a new subelement. Element
creation must be explicit about the namespace, and is simplified
through the E-factory as described above.
Lookups, however, inherit namespaces implicitly:
.. sourcecode:: pycon
>>> print(root.b.tag)
{http://ns/}b
>>> print(root.c)
Traceback (most recent call last):
...
AttributeError: no such child: {http://ns/}c
To access an element in a different namespace than its parent, you can
use ``getattr()``:
.. sourcecode:: pycon
>>> c = getattr(root, "{http://other/}c")
>>> print(c.tag)
{http://other/}c
For convenience, there is also a quick way through item access:
.. sourcecode:: pycon
>>> c = root["{http://other/}c"]
>>> print(c.tag)
{http://other/}c
The same approach must be used to access children with tag names that are not
valid Python identifiers:
.. sourcecode:: pycon
>>> el = objectify.SubElement(root, "{http://ns/}tag-name")
>>> print(root["tag-name"].tag)
{http://ns/}tag-name
>>> new_el = objectify.Element("{http://ns/}new-element")
>>> el = objectify.SubElement(new_el, "{http://ns/}child")
>>> el = objectify.SubElement(new_el, "{http://ns/}child")
>>> el = objectify.SubElement(new_el, "{http://ns/}child")
>>> root["tag-name"] = [ new_el, new_el ]
>>> print(len(root["tag-name"]))
2
>>> print(root["tag-name"].tag)
{http://ns/}tag-name
>>> print(len(root["tag-name"].child))
3
>>> print(root["tag-name"].child.tag)
{http://ns/}child
>>> print(root["tag-name"][1].child.tag)
{http://ns/}child
or for names that have a special meaning in lxml.objectify:
.. sourcecode:: pycon
>>> root = objectify.XML("<root><text>TEXT</text></root>")
>>> print(root.text.text)
Traceback (most recent call last):
...
AttributeError: 'NoneType' object has no attribute 'text'
>>> print(root["text"].text)
TEXT
Asserting a Schema
==================
When dealing with XML documents from different sources, you will often
require them to follow a common schema. In lxml.objectify, this
directly translates to enforcing a specific object tree, i.e. expected
object attributes are ensured to be there and to have the expected
type. This can easily be achieved through XML Schema validation at
parse time. Also see the `documentation on validation`_ on this
topic.
.. _`documentation on validation`: validation.html
First of all, we need a parser that knows our schema, so let's say we
parse the schema from a file-like object (or file or filename):
.. sourcecode:: pycon
>>> f = StringIO('''\
... <xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
... <xsd:element name="a" type="AType"/>
... <xsd:complexType name="AType">
... <xsd:sequence>
... <xsd:element name="b" type="xsd:string" />
... </xsd:sequence>
... </xsd:complexType>
... </xsd:schema>
... ''')
>>> schema = etree.XMLSchema(file=f)
When creating the validating parser, we must make sure it `returns
objectify trees`_. This is best done with the ``makeparser()``
function:
.. sourcecode:: pycon
>>> parser = objectify.makeparser(schema = schema)
.. _`returns objectify trees`: #advance-element-class-lookup
Now we can use it to parse a valid document:
.. sourcecode:: pycon
>>> xml = "<a><b>test</b></a>"
>>> a = objectify.fromstring(xml, parser)
>>> print(a.b)
test
Or an invalid document:
.. sourcecode:: pycon
>>> xml = "<a><b>test</b><c/></a>"
>>> a = objectify.fromstring(xml, parser)
Traceback (most recent call last):
lxml.etree.XMLSyntaxError: Element 'c': This element is not expected.
Note that the same works for parse-time DTD validation, except that
DTDs do not support any data types by design.
ObjectPath
==========
For both convenience and speed, objectify supports its own path language,
represented by the ``ObjectPath`` class:
.. sourcecode:: pycon
>>> root = objectify.Element("{http://ns/}root")
>>> b1 = objectify.SubElement(root, "{http://ns/}b")
>>> c = objectify.SubElement(b1, "{http://ns/}c")
>>> b2 = objectify.SubElement(root, "{http://ns/}b")
>>> d = objectify.SubElement(root, "{http://other/}d")
>>> path = objectify.ObjectPath("root.b.c")
>>> print(path)
root.b.c
>>> path.hasattr(root)
True
>>> print(path.find(root).tag)
{http://ns/}c
>>> find = objectify.ObjectPath("root.b.c")
>>> print(find(root).tag)
{http://ns/}c
>>> find = objectify.ObjectPath("root.{http://other/}d")
>>> print(find(root).tag)
{http://other/}d
>>> find = objectify.ObjectPath("root.{not}there")
>>> print(find(root).tag)
Traceback (most recent call last):
...
AttributeError: no such child: {not}there
>>> find = objectify.ObjectPath("{not}there")
>>> print(find(root).tag)
Traceback (most recent call last):
...
ValueError: root element does not match: need {not}there, got {http://ns/}root
>>> find = objectify.ObjectPath("root.b[1]")
>>> print(find(root).tag)
{http://ns/}b
>>> find = objectify.ObjectPath("root.{http://ns/}b[1]")
>>> print(find(root).tag)
{http://ns/}b
Apart from strings, ObjectPath also accepts lists of path segments:
.. sourcecode:: pycon
>>> find = objectify.ObjectPath(['root', 'b', 'c'])
>>> print(find(root).tag)
{http://ns/}c
>>> find = objectify.ObjectPath(['root', '{http://ns/}b[1]'])
>>> print(find(root).tag)
{http://ns/}b
You can also use relative paths starting with a '.' to ignore the actual root
element and only inherit its namespace:
.. sourcecode:: pycon
>>> find = objectify.ObjectPath(".b[1]")
>>> print(find(root).tag)
{http://ns/}b
>>> find = objectify.ObjectPath(['', 'b[1]'])
>>> print(find(root).tag)
{http://ns/}b
>>> find = objectify.ObjectPath(".unknown[1]")
>>> print(find(root).tag)
Traceback (most recent call last):
...
AttributeError: no such child: {http://ns/}unknown
>>> find = objectify.ObjectPath(".{http://other/}unknown[1]")
>>> print(find(root).tag)
Traceback (most recent call last):
...
AttributeError: no such child: {http://other/}unknown
For convenience, a single dot represents the empty ObjectPath (identity):
.. sourcecode:: pycon
>>> find = objectify.ObjectPath(".")
>>> print(find(root).tag)
{http://ns/}root
ObjectPath objects can be used to manipulate trees:
.. sourcecode:: pycon
>>> root = objectify.Element("{http://ns/}root")
>>> path = objectify.ObjectPath(".some.child.{http://other/}unknown")
>>> path.hasattr(root)
False
>>> path.find(root)
Traceback (most recent call last):
...
AttributeError: no such child: {http://ns/}some
>>> path.setattr(root, "my value") # creates children as necessary
>>> path.hasattr(root)
True
>>> print(path.find(root).text)
my value
>>> print(root.some.child["{http://other/}unknown"].text)
my value
>>> print(len( path.find(root) ))
1
>>> path.addattr(root, "my new value")
>>> print(len( path.find(root) ))
2
>>> [ el.text for el in path.find(root) ]
['my value', 'my new value']
As with attribute assignment, ``setattr()`` accepts lists:
.. sourcecode:: pycon
>>> path.setattr(root, ["v1", "v2", "v3"])
>>> [ el.text for el in path.find(root) ]
['v1', 'v2', 'v3']
Note, however, that indexing is only supported in this context if the children
exist. Indexing of non existing children will not extend or create a list of
such children but raise an exception:
.. sourcecode:: pycon
>>> path = objectify.ObjectPath(".{non}existing[1]")
>>> path.setattr(root, "my value")
Traceback (most recent call last):
...
TypeError: creating indexed path attributes is not supported
It is worth noting that ObjectPath does not depend on the ``objectify`` module
or the ObjectifiedElement implementation. It can also be used in combination
with Elements from the normal lxml.etree API.
Python data types
=================
The objectify module knows about Python data types and tries its best to let
element content behave like them. For example, they support the normal math
operators:
.. sourcecode:: pycon
>>> root = objectify.fromstring(
... "<root><a>5</a><b>11</b><c>true</c><d>hoi</d></root>")
>>> root.a + root.b
16
>>> root.a += root.b
>>> print(root.a)
16
>>> root.a = 2
>>> print(root.a + 2)
4
>>> print(1 + root.a)
3
>>> print(root.c)
True
>>> root.c = False
>>> if not root.c:
... print("false!")
false!
>>> print(root.d + " test !")
hoi test !
>>> root.d = "%s - %s"
>>> print(root.d % (1234, 12345))
1234 - 12345
However, data elements continue to provide the objectify API. This means that
sequence operations such as ``len()``, slicing and indexing (e.g. of strings)
cannot behave as the Python types. Like all other tree elements, they show
the normal slicing behaviour of objectify elements:
.. sourcecode:: pycon
>>> root = objectify.fromstring("<root><a>test</a><b>toast</b></root>")
>>> print(root.a + ' me') # behaves like a string, right?
test me
>>> len(root.a) # but there's only one 'a' element!
1
>>> [ a.tag for a in root.a ]
['a']
>>> print(root.a[0].tag)
a
>>> print(root.a)
test
>>> [ str(a) for a in root.a[:1] ]
['test']
If you need to run sequence operations on data types, you must ask the API for
the *real* Python value. The string value is always available through the
normal ElementTree ``.text`` attribute. Additionally, all data classes
provide a ``.pyval`` attribute that returns the value as plain Python type:
.. sourcecode:: pycon
>>> root = objectify.fromstring("<root><a>test</a><b>5</b></root>")
>>> root.a.text
'test'
>>> root.a.pyval
'test'
>>> root.b.text
'5'
>>> root.b.pyval
5
Note, however, that both attributes are read-only in objectify. If you want
to change values, just assign them directly to the attribute:
.. sourcecode:: pycon
>>> root.a.text = "25"
Traceback (most recent call last):
...
TypeError: attribute 'text' of 'StringElement' objects is not writable
>>> root.a.pyval = 25
Traceback (most recent call last):
...
TypeError: attribute 'pyval' of 'StringElement' objects is not writable
>>> root.a = 25
>>> print(root.a)
25
>>> print(root.a.pyval)
25
In other words, ``objectify`` data elements behave like immutable Python
types. You can replace them, but not modify them.
Recursive tree dump
-------------------
To see the data types that are currently used, you can call the module level
``dump()`` function that returns a recursive string representation for
elements:
.. sourcecode:: pycon
>>> root = objectify.fromstring("""
... <root xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
... <a attr1="foo" attr2="bar">1</a>
... <a>1.2</a>
... <b>1</b>
... <b>true</b>
... <c>what?</c>
... <d xsi:nil="true"/>
... </root>
... """)
>>> print(objectify.dump(root))
root = None [ObjectifiedElement]
a = 1 [IntElement]
* attr1 = 'foo'
* attr2 = 'bar'
a = 1.2 [FloatElement]
b = 1 [IntElement]
b = True [BoolElement]
c = 'what?' [StringElement]
d = None [NoneElement]
* xsi:nil = 'true'
You can freely switch between different types for the same child:
.. sourcecode:: pycon
>>> root = objectify.fromstring("<root><a>5</a></root>")
>>> print(objectify.dump(root))
root = None [ObjectifiedElement]
a = 5 [IntElement]
>>> root.a = 'nice string!'
>>> print(objectify.dump(root))
root = None [ObjectifiedElement]
a = 'nice string!' [StringElement]
* py:pytype = 'str'
>>> root.a = True
>>> print(objectify.dump(root))
root = None [ObjectifiedElement]
a = True [BoolElement]
* py:pytype = 'bool'
>>> root.a = [1, 2, 3]
>>> print(objectify.dump(root))
root = None [ObjectifiedElement]
a = 1 [IntElement]
* py:pytype = 'int'
a = 2 [IntElement]
* py:pytype = 'int'
a = 3 [IntElement]
* py:pytype = 'int'
>>> root.a = (1, 2, 3)
>>> print(objectify.dump(root))
root = None [ObjectifiedElement]
a = 1 [IntElement]
* py:pytype = 'int'
a = 2 [IntElement]
* py:pytype = 'int'
a = 3 [IntElement]
* py:pytype = 'int'
Recursive string representation of elements
-------------------------------------------
Normally, elements use the standard string representation for str() that is
provided by lxml.etree. You can enable a pretty-print representation for
objectify elements like this:
.. sourcecode:: pycon
>>> objectify.enable_recursive_str()
>>> root = objectify.fromstring("""
... <root xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
... <a attr1="foo" attr2="bar">1</a>
... <a>1.2</a>
... <b>1</b>
... <b>true</b>
... <c>what?</c>
... <d xsi:nil="true"/>
... </root>
... """)
>>> print(str(root))
root = None [ObjectifiedElement]
a = 1 [IntElement]
* attr1 = 'foo'
* attr2 = 'bar'
a = 1.2 [FloatElement]
b = 1 [IntElement]
b = True [BoolElement]
c = 'what?' [StringElement]
d = None [NoneElement]
* xsi:nil = 'true'
This behaviour can be switched off in the same way:
.. sourcecode:: pycon
>>> objectify.enable_recursive_str(False)
How data types are matched
==========================
Objectify uses two different types of Elements. Structural Elements (or tree
Elements) represent the object tree structure. Data Elements represent the
data containers at the leafs. You can explicitly create tree Elements with
the ``objectify.Element()`` factory and data Elements with the
``objectify.DataElement()`` factory.
When Element objects are created, lxml.objectify must determine which
implementation class to use for them. This is relatively easy for tree
Elements and less so for data Elements. The algorithm is as follows:
1. If an element has children, use the default tree class.
2. If an element is defined as xsi:nil, use the NoneElement class.
3. If a "Python type hint" attribute is given, use this to determine the element
class, see below.
4. If an XML Schema xsi:type hint is given, use this to determine the element
class, see below.
5. Try to determine the element class from the text content type by trial and
error.
6. If the element is a root node then use the default tree class.
7. Otherwise, use the default class for empty data classes.
You can change the default classes for tree Elements and empty data Elements
at setup time. The ``ObjectifyElementClassLookup()`` call accepts two keyword
arguments, ``tree_class`` and ``empty_data_class``, that determine the Element
classes used in these cases. By default, ``tree_class`` is a class called
``ObjectifiedElement`` and ``empty_data_class`` is a ``StringElement``.
Type annotations
----------------
The "type hint" mechanism deploys an XML attribute defined as
``lxml.objectify.PYTYPE_ATTRIBUTE``. It may contain any of the following
string values: int, long, float, str, unicode, NoneType:
.. sourcecode:: pycon
>>> print(objectify.PYTYPE_ATTRIBUTE)
{http://codespeak.net/lxml/objectify/pytype}pytype
>>> ns, name = objectify.PYTYPE_ATTRIBUTE[1:].split('}')
>>> root = objectify.fromstring("""\
... <root xmlns:py='%s'>
... <a py:pytype='str'>5</a>
... <b py:pytype='int'>5</b>
... <c py:pytype='NoneType' />
... </root>
... """ % ns)
>>> print(root.a + 10)
510
>>> print(root.b + 10)
15
>>> print(root.c)
None
Note that you can change the name and namespace used for this
attribute through the ``set_pytype_attribute_tag(tag)`` module
function, in case your application ever needs to. There is also a
utility function ``annotate()`` that recursively generates this
attribute for the elements of a tree:
.. sourcecode:: pycon
>>> root = objectify.fromstring("<root><a>test</a><b>5</b></root>")
>>> print(objectify.dump(root))
root = None [ObjectifiedElement]
a = 'test' [StringElement]
b = 5 [IntElement]
>>> objectify.annotate(root)
>>> print(objectify.dump(root))
root = None [ObjectifiedElement]
a = 'test' [StringElement]
* py:pytype = 'str'
b = 5 [IntElement]
* py:pytype = 'int'
XML Schema datatype annotation
------------------------------
A second way of specifying data type information uses XML Schema types as
element annotations. Objectify knows those that can be mapped to normal
Python types:
.. sourcecode:: pycon
>>> root = objectify.fromstring('''\
... <root xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
... xmlns:xsd="http://www.w3.org/2001/XMLSchema">
... <d xsi:type="xsd:double">5</d>
... <i xsi:type="xsd:int" >5</i>
... <s xsi:type="xsd:string">5</s>
... </root>
... ''')
>>> print(objectify.dump(root))
root = None [ObjectifiedElement]
d = 5.0 [FloatElement]
* xsi:type = 'xsd:double'
i = 5 [IntElement]
* xsi:type = 'xsd:int'
s = '5' [StringElement]
* xsi:type = 'xsd:string'
Again, there is a utility function ``xsiannotate()`` that recursively
generates the "xsi:type" attribute for the elements of a tree: