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/Doc/library/xml.etree.elementtree.rst

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  1
  2:mod:`xml.etree.ElementTree` --- The ElementTree XML API
  3========================================================
  4
  5.. module:: xml.etree.ElementTree
  6   :synopsis: Implementation of the ElementTree API.
  7.. moduleauthor:: Fredrik Lundh <fredrik@pythonware.com>
  8
  9
 10.. versionadded:: 2.5
 11
 12The Element type is a flexible container object, designed to store hierarchical
 13data structures in memory. The type can be described as a cross between a list
 14and a dictionary.
 15
 16Each element has a number of properties associated with it:
 17
 18* a tag which is a string identifying what kind of data this element represents
 19  (the element type, in other words).
 20
 21* a number of attributes, stored in a Python dictionary.
 22
 23* a text string.
 24
 25* an optional tail string.
 26
 27* a number of child elements, stored in a Python sequence
 28
 29To create an element instance, use the Element or SubElement factory functions.
 30
 31The :class:`ElementTree` class can be used to wrap an element structure, and
 32convert it from and to XML.
 33
 34A C implementation of this API is available as :mod:`xml.etree.cElementTree`.
 35
 36See http://effbot.org/zone/element-index.htm for tutorials and links to other
 37docs. Fredrik Lundh's page is also the location of the development version of the
 38xml.etree.ElementTree.
 39
 40.. _elementtree-functions:
 41
 42Functions
 43---------
 44
 45
 46.. function:: Comment([text])
 47
 48   Comment element factory.  This factory function creates a special element that
 49   will be serialized as an XML comment. The comment string can be either an 8-bit
 50   ASCII string or a Unicode string. *text* is a string containing the comment
 51   string. Returns an element instance representing a comment.
 52
 53
 54.. function:: dump(elem)
 55
 56   Writes an element tree or element structure to sys.stdout.  This function should
 57   be used for debugging only.
 58
 59   The exact output format is implementation dependent.  In this version, it's
 60   written as an ordinary XML file.
 61
 62   *elem* is an element tree or an individual element.
 63
 64
 65.. function:: Element(tag[, attrib][, **extra])
 66
 67   Element factory.  This function returns an object implementing the standard
 68   Element interface.  The exact class or type of that object is implementation
 69   dependent, but it will always be compatible with the _ElementInterface class in
 70   this module.
 71
 72   The element name, attribute names, and attribute values can be either 8-bit
 73   ASCII strings or Unicode strings. *tag* is the element name. *attrib* is an
 74   optional dictionary, containing element attributes. *extra* contains additional
 75   attributes, given as keyword arguments. Returns an element instance.
 76
 77
 78.. function:: fromstring(text)
 79
 80   Parses an XML section from a string constant.  Same as XML. *text* is a string
 81   containing XML data. Returns an Element instance.
 82
 83
 84.. function:: iselement(element)
 85
 86   Checks if an object appears to be a valid element object. *element* is an
 87   element instance. Returns a true value if this is an element object.
 88
 89
 90.. function:: iterparse(source[, events])
 91
 92   Parses an XML section into an element tree incrementally, and reports what's
 93   going on to the user. *source* is a filename or file object containing XML data.
 94   *events* is a list of events to report back.  If omitted, only "end" events are
 95   reported. Returns an :term:`iterator` providing ``(event, elem)`` pairs.
 96
 97   .. note::
 98
 99      :func:`iterparse` only guarantees that it has seen the ">"
100      character of a starting tag when it emits a "start" event, so the
101      attributes are defined, but the contents of the text and tail attributes
102      are undefined at that point.  The same applies to the element children;
103      they may or may not be present.
104
105      If you need a fully populated element, look for "end" events instead.
106
107
108.. function:: parse(source[, parser])
109
110   Parses an XML section into an element tree. *source* is a filename or file
111   object containing XML data. *parser* is an optional parser instance.  If not
112   given, the standard XMLTreeBuilder parser is used. Returns an ElementTree
113   instance.
114
115
116.. function:: ProcessingInstruction(target[, text])
117
118   PI element factory.  This factory function creates a special element that will
119   be serialized as an XML processing instruction. *target* is a string containing
120   the PI target. *text* is a string containing the PI contents, if given. Returns
121   an element instance, representing a processing instruction.
122
123
124.. function:: SubElement(parent, tag[, attrib[,  **extra]])
125
126   Subelement factory.  This function creates an element instance, and appends it
127   to an existing element.
128
129   The element name, attribute names, and attribute values can be either 8-bit
130   ASCII strings or Unicode strings. *parent* is the parent element. *tag* is the
131   subelement name. *attrib* is an optional dictionary, containing element
132   attributes. *extra* contains additional attributes, given as keyword arguments.
133   Returns an element instance.
134
135
136.. function:: tostring(element[, encoding])
137
138   Generates a string representation of an XML element, including all subelements.
139   *element* is an Element instance. *encoding* is the output encoding (default is
140   US-ASCII). Returns an encoded string containing the XML data.
141
142
143.. function:: XML(text)
144
145   Parses an XML section from a string constant.  This function can be used to
146   embed "XML literals" in Python code. *text* is a string containing XML data.
147   Returns an Element instance.
148
149
150.. function:: XMLID(text)
151
152   Parses an XML section from a string constant, and also returns a dictionary
153   which maps from element id:s to elements. *text* is a string containing XML
154   data. Returns a tuple containing an Element instance and a dictionary.
155
156
157.. _elementtree-element-interface:
158
159The Element Interface
160---------------------
161
162Element objects returned by Element or SubElement have the  following methods
163and attributes.
164
165
166.. attribute:: Element.tag
167
168   A string identifying what kind of data this element represents (the element
169   type, in other words).
170
171
172.. attribute:: Element.text
173
174   The *text* attribute can be used to hold additional data associated with the
175   element. As the name implies this attribute is usually a string but may be any
176   application-specific object. If the element is created from an XML file the
177   attribute will contain any text found between the element tags.
178
179
180.. attribute:: Element.tail
181
182   The *tail* attribute can be used to hold additional data associated with the
183   element. This attribute is usually a string but may be any application-specific
184   object. If the element is created from an XML file the attribute will contain
185   any text found after the element's end tag and before the next tag.
186
187
188.. attribute:: Element.attrib
189
190   A dictionary containing the element's attributes. Note that while the *attrib*
191   value is always a real mutable Python dictionary, an ElementTree implementation
192   may choose to use another internal representation, and create the dictionary
193   only if someone asks for it. To take advantage of such implementations, use the
194   dictionary methods below whenever possible.
195
196The following dictionary-like methods work on the element attributes.
197
198
199.. method:: Element.clear()
200
201   Resets an element.  This function removes all subelements, clears all
202   attributes, and sets the text and tail attributes to None.
203
204
205.. method:: Element.get(key[, default=None])
206
207   Gets the element attribute named *key*.
208
209   Returns the attribute value, or *default* if the attribute was not found.
210
211
212.. method:: Element.items()
213
214   Returns the element attributes as a sequence of (name, value) pairs. The
215   attributes are returned in an arbitrary order.
216
217
218.. method:: Element.keys()
219
220   Returns the elements attribute names as a list. The names are returned in an
221   arbitrary order.
222
223
224.. method:: Element.set(key, value)
225
226   Set the attribute *key* on the element to *value*.
227
228The following methods work on the element's children (subelements).
229
230
231.. method:: Element.append(subelement)
232
233   Adds the element *subelement* to the end of this elements internal list of
234   subelements.
235
236
237.. method:: Element.find(match)
238
239   Finds the first subelement matching *match*.  *match* may be a tag name or path.
240   Returns an element instance or ``None``.
241
242
243.. method:: Element.findall(match)
244
245   Finds all subelements matching *match*.  *match* may be a tag name or path.
246   Returns an iterable yielding all matching elements in document order.
247
248
249.. method:: Element.findtext(condition[, default=None])
250
251   Finds text for the first subelement matching *condition*.  *condition* may be a
252   tag name or path. Returns the text content of the first matching element, or
253   *default* if no element was found.  Note that if the matching element has no
254   text content an empty string is returned.
255
256
257.. method:: Element.getchildren()
258
259   Returns all subelements.  The elements are returned in document order.
260
261
262.. method:: Element.getiterator([tag=None])
263
264   Creates a tree iterator with the current element as the root.   The iterator
265   iterates over this element and all elements below it  that match the given tag.
266   If tag is ``None`` or ``'*'`` then all elements are iterated over. Returns an
267   iterable that provides element objects in document (depth first) order.
268
269
270.. method:: Element.insert(index, element)
271
272   Inserts a subelement at the given position in this element.
273
274
275.. method:: Element.makeelement(tag, attrib)
276
277   Creates a new element object of the same type as this element. Do not call this
278   method, use the SubElement factory function instead.
279
280
281.. method:: Element.remove(subelement)
282
283   Removes *subelement* from the element.   Unlike the findXYZ methods this method
284   compares elements based on  the instance identity, not on tag value or contents.
285
286Element objects also support the following sequence type methods for working
287with subelements: :meth:`__delitem__`, :meth:`__getitem__`, :meth:`__setitem__`,
288:meth:`__len__`.
289
290Caution: Because Element objects do not define a :meth:`__nonzero__` method,
291elements with no subelements will test as ``False``. ::
292
293   element = root.find('foo')
294
295   if not element: # careful!
296       print "element not found, or element has no subelements"
297
298   if element is None:
299       print "element not found"
300
301
302.. _elementtree-elementtree-objects:
303
304ElementTree Objects
305-------------------
306
307
308.. class:: ElementTree([element,] [file])
309
310   ElementTree wrapper class.  This class represents an entire element hierarchy,
311   and adds some extra support for serialization to and from standard XML.
312
313   *element* is the root element. The tree is initialized with the contents of the
314   XML *file* if given.
315
316
317   .. method:: _setroot(element)
318
319      Replaces the root element for this tree.  This discards the current
320      contents of the tree, and replaces it with the given element.  Use with
321      care. *element* is an element instance.
322
323
324   .. method:: find(path)
325
326      Finds the first toplevel element with given tag. Same as
327      getroot().find(path).  *path* is the element to look for. Returns the
328      first matching element, or ``None`` if no element was found.
329
330
331   .. method:: findall(path)
332
333      Finds all toplevel elements with the given tag. Same as
334      getroot().findall(path).  *path* is the element to look for. Returns a
335      list or :term:`iterator` containing all matching elements, in document
336      order.
337
338
339   .. method:: findtext(path[, default])
340
341      Finds the element text for the first toplevel element with given tag.
342      Same as getroot().findtext(path). *path* is the toplevel element to look
343      for. *default* is the value to return if the element was not
344      found. Returns the text content of the first matching element, or the
345      default value no element was found.  Note that if the element has is
346      found, but has no text content, this method returns an empty string.
347
348
349   .. method:: getiterator([tag])
350
351      Creates and returns a tree iterator for the root element.  The iterator
352      loops over all elements in this tree, in section order. *tag* is the tag
353      to look for (default is to return all elements)
354
355
356   .. method:: getroot()
357
358      Returns the root element for this tree.
359
360
361   .. method:: parse(source[, parser])
362
363      Loads an external XML section into this element tree. *source* is a file
364      name or file object. *parser* is an optional parser instance.  If not
365      given, the standard XMLTreeBuilder parser is used. Returns the section
366      root element.
367
368
369   .. method:: write(file[, encoding])
370
371      Writes the element tree to a file, as XML. *file* is a file name, or a
372      file object opened for writing. *encoding* [1]_ is the output encoding
373      (default is US-ASCII).
374
375This is the XML file that is going to be manipulated::
376
377    <html>
378        <head>
379            <title>Example page</title>
380        </head>
381        <body>
382            <p>Moved to <a href="http://example.org/">example.org</a>
383            or <a href="http://example.com/">example.com</a>.</p>
384        </body>
385    </html>
386
387Example of changing the attribute "target" of every link in first paragraph::
388
389    >>> from xml.etree.ElementTree import ElementTree
390    >>> tree = ElementTree()
391    >>> tree.parse("index.xhtml")
392    <Element html at b7d3f1ec>
393    >>> p = tree.find("body/p")     # Finds first occurrence of tag p in body
394    >>> p
395    <Element p at 8416e0c>
396    >>> links = p.getiterator("a")  # Returns list of all links
397    >>> links
398    [<Element a at b7d4f9ec>, <Element a at b7d4fb0c>]
399    >>> for i in links:             # Iterates through all found links
400    ...     i.attrib["target"] = "blank"
401    >>> tree.write("output.xhtml")
402
403.. _elementtree-qname-objects:
404
405QName Objects
406-------------
407
408
409.. class:: QName(text_or_uri[, tag])
410
411   QName wrapper.  This can be used to wrap a QName attribute value, in order to
412   get proper namespace handling on output. *text_or_uri* is a string containing
413   the QName value, in the form {uri}local, or, if the tag argument is given, the
414   URI part of a QName. If *tag* is given, the first argument is interpreted as an
415   URI, and this argument is interpreted as a local name. :class:`QName` instances
416   are opaque.
417
418
419.. _elementtree-treebuilder-objects:
420
421TreeBuilder Objects
422-------------------
423
424
425.. class:: TreeBuilder([element_factory])
426
427   Generic element structure builder.  This builder converts a sequence of start,
428   data, and end method calls to a well-formed element structure. You can use this
429   class to build an element structure using a custom XML parser, or a parser for
430   some other XML-like format. The *element_factory* is called to create new
431   Element instances when given.
432
433
434   .. method:: close()
435
436      Flushes the parser buffers, and returns the toplevel document
437      element. Returns an Element instance.
438
439
440   .. method:: data(data)
441
442      Adds text to the current element. *data* is a string.  This should be
443      either an 8-bit string containing ASCII text, or a Unicode string.
444
445
446   .. method:: end(tag)
447
448      Closes the current element. *tag* is the element name. Returns the closed
449      element.
450
451
452   .. method:: start(tag, attrs)
453
454      Opens a new element. *tag* is the element name. *attrs* is a dictionary
455      containing element attributes. Returns the opened element.
456
457
458.. _elementtree-xmltreebuilder-objects:
459
460XMLTreeBuilder Objects
461----------------------
462
463
464.. class:: XMLTreeBuilder([html,] [target])
465
466   Element structure builder for XML source data, based on the expat parser. *html*
467   are predefined HTML entities.  This flag is not supported by the current
468   implementation. *target* is the target object.  If omitted, the builder uses an
469   instance of the standard TreeBuilder class.
470
471
472   .. method:: close()
473
474      Finishes feeding data to the parser. Returns an element structure.
475
476
477   .. method:: doctype(name, pubid, system)
478
479      Handles a doctype declaration. *name* is the doctype name. *pubid* is the
480      public identifier. *system* is the system identifier.
481
482
483   .. method:: feed(data)
484
485      Feeds data to the parser. *data* is encoded data.
486
487:meth:`XMLTreeBuilder.feed` calls *target*\'s :meth:`start` method
488for each opening tag, its :meth:`end` method for each closing tag,
489and data is processed by method :meth:`data`. :meth:`XMLTreeBuilder.close`
490calls *target*\'s method :meth:`close`.
491:class:`XMLTreeBuilder` can be used not only for building a tree structure.
492This is an example of counting the maximum depth of an XML file::
493
494    >>> from xml.etree.ElementTree import XMLTreeBuilder
495    >>> class MaxDepth:                     # The target object of the parser
496    ...     maxDepth = 0
497    ...     depth = 0
498    ...     def start(self, tag, attrib):   # Called for each opening tag.
499    ...         self.depth += 1
500    ...         if self.depth > self.maxDepth:
501    ...             self.maxDepth = self.depth
502    ...     def end(self, tag):             # Called for each closing tag.
503    ...         self.depth -= 1
504    ...     def data(self, data):
505    ...         pass            # We do not need to do anything with data.
506    ...     def close(self):    # Called when all data has been parsed.
507    ...         return self.maxDepth
508    ...
509    >>> target = MaxDepth()
510    >>> parser = XMLTreeBuilder(target=target)
511    >>> exampleXml = """
512    ... <a>
513    ...   <b>
514    ...   </b>
515    ...   <b>
516    ...     <c>
517    ...       <d>
518    ...       </d>
519    ...     </c>
520    ...   </b>
521    ... </a>"""
522    >>> parser.feed(exampleXml)
523    >>> parser.close()
524    4
525
526
527.. rubric:: Footnotes
528
529.. [#] The encoding string included in XML output should conform to the
530   appropriate standards. For example, "UTF-8" is valid, but "UTF8" is
531   not. See http://www.w3.org/TR/2006/REC-xml11-20060816/#NT-EncodingDecl
532   and http://www.iana.org/assignments/character-sets.
533