Warning
Thepyexpat module is not secure against maliciouslyconstructed data. If you need to parse untrusted or unauthenticated data seeXML vulnerabilities.
Thexml.parsers.expat module is a Python interface to the Expatnon-validating XML parser. The module provides a single extension type,xmlparser, that represents the current state of an XML parser. Afteranxmlparser object has been created, various attributes of the objectcan be set to handler functions. When an XML document is then fed to theparser, the handler functions are called for the character data and markup inthe XML document.
This module uses thepyexpat module to provide access to the Expatparser. Direct use of thepyexpat module is deprecated.
This module provides one exception and one type object:
The exception raised when Expat reports an error. See sectionExpatError Exceptions for more information on interpreting Expat errors.
Alias forExpatError.
The type of the return values from theParserCreate() function.
Thexml.parsers.expat module contains two functions:
Returns an explanatory string for a given error numbererrno.
Creates and returns a newxmlparser object.encoding, if specified,must be a string naming the encoding used by the XML data. Expat doesn’tsupport as many encodings as Python does, and its repertoire of encodings can’tbe extended; it supports UTF-8, UTF-16, ISO-8859-1 (Latin1), and ASCII. Ifencoding[1] is given it will override the implicit or explicit encoding of thedocument.
Expat can optionally do XML namespace processing for you, enabled by providing avalue fornamespace_separator. The value must be a one-character string; aValueError will be raised if the string has an illegal length (Noneis considered the same as omission). When namespace processing is enabled,element type names and attribute names that belong to a namespace will beexpanded. The element name passed to the element handlersStartElementHandler andEndElementHandler will be theconcatenation of the namespace URI, the namespace separator character, and thelocal part of the name. If the namespace separator is a zero byte (chr(0))then the namespace URI and the local part will be concatenated without anyseparator.
For example, ifnamespace_separator is set to a space character ('') andthe following document is parsed:
<?xml version="1.0"?><root xmlns = "http://default-namespace.org/" xmlns:py = "http://www.python.org/ns/"> <py:elem1 /> <elem2 xmlns="" /></root>
StartElementHandler will receive the following strings for eachelement:
http://default-namespace.org/roothttp://www.python.org/ns/elem1elem2
See also
xmlparser objects have the following methods:
Parses the contents of the stringdata, calling the appropriate handlerfunctions to process the parsed data.isfinal must be true on the final callto this method.data can be the empty string at any time.
Parse XML data reading from the objectfile.file only needs to providetheread(nbytes) method, returning the empty string when there’s no moredata.
Sets the base to be used for resolving relative URIs in system identifiers indeclarations. Resolving relative identifiers is left to the application: thisvalue will be passed through as thebase argument to theExternalEntityRefHandler(),NotationDeclHandler(), andUnparsedEntityDeclHandler() functions.
Returns a string containing the base set by a previous call toSetBase(),orNone ifSetBase() hasn’t been called.
Returns the input data that generated the current event as a string. The data isin the encoding of the entity which contains the text. When called while anevent handler is not active, the return value isNone.
Create a “child” parser which can be used to parse an external parsed entityreferred to by content parsed by the parent parser. Thecontext parametershould be the string passed to theExternalEntityRefHandler() handlerfunction, described below. The child parser is created with theordered_attributes andspecified_attributes set to the values ofthis parser.
Control parsing of parameter entities (including the external DTD subset).Possibleflag values areXML_PARAM_ENTITY_PARSING_NEVER,XML_PARAM_ENTITY_PARSING_UNLESS_STANDALONE andXML_PARAM_ENTITY_PARSING_ALWAYS. Return true if setting the flagwas successful.
Calling this with a true value forflag (the default) will cause Expat to calltheExternalEntityRefHandler withNone for all arguments toallow an alternate DTD to be loaded. If the document does not contain adocument type declaration, theExternalEntityRefHandler will still becalled, but theStartDoctypeDeclHandler andEndDoctypeDeclHandler will not be called.
Passing a false value forflag will cancel a previous call that passed a truevalue, but otherwise has no effect.
This method can only be called before theParse() orParseFile()methods are called; calling it after either of those have been called causesExpatError to be raised with thecode attribute set toerrors.codes[errors.XML_ERROR_CANT_CHANGE_FEATURE_ONCE_PARSING].
xmlparser objects have the following attributes:
The size of the buffer used whenbuffer_text is true.A new buffer size can be set by assigning a new integer valueto this attribute.When the size is changed, the buffer will be flushed.
Setting this to true causes thexmlparser object to buffer textualcontent returned by Expat to avoid multiple calls to theCharacterDataHandler() callback whenever possible. This can improveperformance substantially since Expat normally breaks character data into chunksat every line ending. This attribute is false by default, and may be changed atany time.
Ifbuffer_text is enabled, the number of bytes stored in the buffer.These bytes represent UTF-8 encoded text. This attribute has no meaningfulinterpretation whenbuffer_text is false.
Setting this attribute to a non-zero integer causes the attributes to bereported as a list rather than a dictionary. The attributes are presented inthe order found in the document text. For each attribute, two list entries arepresented: the attribute name and the attribute value. (Older versions of thismodule also used this format.) By default, this attribute is false; it may bechanged at any time.
If set to a non-zero integer, the parser will report only those attributes whichwere specified in the document instance and not those which were derived fromattribute declarations. Applications which set this need to be especiallycareful to use what additional information is available from the declarations asneeded to comply with the standards for the behavior of XML processors. Bydefault, this attribute is false; it may be changed at any time.
The following attributes contain values relating to the most recent errorencountered by anxmlparser object, and will only have correct valuesonce a call toParse() orParseFile() has raised axml.parsers.expat.ExpatError exception.
Byte index at which an error occurred.
Numeric code specifying the problem. This value can be passed to theErrorString() function, or compared to one of the constants defined in theerrors object.
Column number at which an error occurred.
Line number at which an error occurred.
The following attributes contain values relating to the current parse locationin anxmlparser object. During a callback reporting a parse event theyindicate the location of the first of the sequence of characters that generatedthe event. When called outside of a callback, the position indicated will bejust past the last parse event (regardless of whether there was an associatedcallback).
Current byte index in the parser input.
Current column number in the parser input.
Current line number in the parser input.
Here is the list of handlers that can be set. To set a handler on anxmlparser objecto, useo.handlername=func.handlername mustbe taken from the following list, andfunc must be a callable object acceptingthe correct number of arguments. The arguments are all strings, unlessotherwise stated.
Called when the XML declaration is parsed. The XML declaration is the(optional) declaration of the applicable version of the XML recommendation, theencoding of the document text, and an optional “standalone” declaration.version andencoding will be strings, andstandalone will be1 if thedocument is declared standalone,0 if it is declared not to be standalone,or-1 if the standalone clause was omitted. This is only available withExpat version 1.95.0 or newer.
Called when Expat begins parsing the document type declaration (<!DOCTYPE...). ThedoctypeName is provided exactly as presented. ThesystemId andpublicId parameters give the system and public identifiers if specified, orNone if omitted.has_internal_subset will be true if the documentcontains and internal document declaration subset. This requires Expat version1.2 or newer.
Called when Expat is done parsing the document type declaration. This requiresExpat version 1.2 or newer.
Called once for each element type declaration.name is the name of theelement type, andmodel is a representation of the content model.
Called for each declared attribute for an element type. If an attribute listdeclaration declares three attributes, this handler is called three times, oncefor each attribute.elname is the name of the element to which thedeclaration applies andattname is the name of the attribute declared. Theattribute type is a string passed astype; the possible values are'CDATA','ID','IDREF', ...default gives the default value forthe attribute used when the attribute is not specified by the document instance,orNone if there is no default value (#IMPLIED values). If theattribute is required to be given in the document instance,required will betrue. This requires Expat version 1.95.0 or newer.
Called for the start of every element.name is a string containing theelement name, andattributes is the element attributes. Ifordered_attributes is true, this is a list (seeordered_attributes for a full description). Otherwise it’s adictionary mapping names to values.
Called for the end of every element.
Called for every processing instruction.
Called for character data. This will be called for normal character data, CDATAmarked content, and ignorable whitespace. Applications which must distinguishthese cases can use theStartCdataSectionHandler,EndCdataSectionHandler, andElementDeclHandler callbacks tocollect the required information.
Called for unparsed (NDATA) entity declarations. This is only present forversion 1.2 of the Expat library; for more recent versions, useEntityDeclHandler instead. (The underlying function in the Expatlibrary has been declared obsolete.)
Called for all entity declarations. For parameter and internal entities,value will be a string giving the declared contents of the entity; this willbeNone for external entities. ThenotationName parameter will beNone for parsed entities, and the name of the notation for unparsedentities.is_parameter_entity will be true if the entity is a parameter entityor false for general entities (most applications only need to be concerned withgeneral entities). This is only available starting with version 1.95.0 of theExpat library.
Called for notation declarations.notationName,base, andsystemId, andpublicId are strings if given. If the public identifier is omitted,publicId will beNone.
Called when an element contains a namespace declaration. Namespace declarationsare processed before theStartElementHandler is called for the elementon which declarations are placed.
Called when the closing tag is reached for an element that contained anamespace declaration. This is called once for each namespace declaration onthe element in the reverse of the order for which theStartNamespaceDeclHandler was called to indicate the start of eachnamespace declaration’s scope. Calls to this handler are made after thecorrespondingEndElementHandler for the end of the element.
Called for comments.data is the text of the comment, excluding the leading'<!--' and trailing'-->'.
Called at the start of a CDATA section. This andEndCdataSectionHandlerare needed to be able to identify the syntactical start and end for CDATAsections.
Called at the end of a CDATA section.
Called for any characters in the XML document for which no applicable handlerhas been specified. This means characters that are part of a construct whichcould be reported, but for which no handler has been supplied.
This is the same as theDefaultHandler(), but doesn’t inhibit expansionof internal entities. The entity reference will not be passed to the defaulthandler.
Called if the XML document hasn’t been declared as being a standalone document.This happens when there is an external subset or a reference to a parameterentity, but the XML declaration does not set standalone toyes in an XMLdeclaration. If this handler returns0, then the parser will raise anXML_ERROR_NOT_STANDALONE error. If this handler is not set, noexception is raised by the parser for this condition.
Called for references to external entities.base is the current base, as setby a previous call toSetBase(). The public and system identifiers,systemId andpublicId, are strings if given; if the public identifier is notgiven,publicId will beNone. Thecontext value is opaque and shouldonly be used as described below.
For external entities to be parsed, this handler must be implemented. It isresponsible for creating the sub-parser usingExternalEntityParserCreate(context), initializing it with the appropriatecallbacks, and parsing the entity. This handler should return an integer; if itreturns0, the parser will raise anXML_ERROR_EXTERNAL_ENTITY_HANDLING error, otherwise parsing willcontinue.
If this handler is not provided, external entities are reported by theDefaultHandler callback, if provided.
ExpatError exceptions have a number of interesting attributes:
Expat’s internal error number for the specific error. Theerrors.messages dictionary mapsthese error numbers to Expat’s error messages. For example:
fromxml.parsers.expatimportParserCreate,ExpatError,errorsp=ParserCreate()try:p.Parse(some_xml_document)exceptExpatErroraserr:print("Error:",errors.messages[err.code])
Theerrors module also provides error messageconstants and a dictionarycodes mappingthese messages back to the error codes, see below.
Line number on which the error was detected. The first line is numbered1.
Character offset into the line where the error occurred. The first column isnumbered0.
The following program defines three handlers that just print out theirarguments.
importxml.parsers.expat# 3 handler functionsdefstart_element(name,attrs):print('Start element:',name,attrs)defend_element(name):print('End element:',name)defchar_data(data):print('Character data:',repr(data))p=xml.parsers.expat.ParserCreate()p.StartElementHandler=start_elementp.EndElementHandler=end_elementp.CharacterDataHandler=char_datap.Parse("""<?xml version="1.0"?><parent id="top"><child1 name="paul">Text goes here</child1><child2 name="fred">More text</child2></parent>""",1)
The output from this program is:
Startelement:parent{'id':'top'}Startelement:child1{'name':'paul'}Characterdata:'Text goes here'Endelement:child1Characterdata:'\n'Startelement:child2{'name':'fred'}Characterdata:'More text'Endelement:child2Characterdata:'\n'Endelement:parent
Content modules are described using nested tuples. Each tuple contains fourvalues: the type, the quantifier, the name, and a tuple of children. Childrenare simply additional content module descriptions.
The values of the first two fields are constants defined in thexml.parsers.expat.model module. These constants can be collected in twogroups: the model type group and the quantifier group.
The constants in the model type group are:
The element named by the model name was declared to have a content model ofANY.
The named element allows a choice from a number of options; this is used forcontent models such as(A|B|C).
Elements which are declared to beEMPTY have this model type.
Models which represent a series of models which follow one after the other areindicated with this model type. This is used for models such as(A,B,C).
The constants in the quantifier group are:
No modifier is given, so it can appear exactly once, as forA.
The model is optional: it can appear once or not at all, as forA?.
The model must occur one or more times (likeA+).
The model must occur zero or more times, as forA*.
The following constants are provided in thexml.parsers.expat.errorsmodule. These constants are useful in interpreting some of the attributes oftheExpatError exception objects raised when an error has occurred.Since for backwards compatibility reasons, the constants’ value is the errormessage and not the numeric errorcode, you do this by comparing itscode attribute witherrors.codes[errors.XML_ERROR_CONSTANT_NAME].
Theerrors module has the following attributes:
A dictionary mapping numeric error codes to their string descriptions.
New in version 3.2.
A dictionary mapping string descriptions to their error codes.
New in version 3.2.
An entity reference in an attribute value referred to an external entity insteadof an internal entity.
A character reference referred to a character which is illegal in XML (forexample, character0, or ‘�‘).
An entity reference referred to an entity which was declared with a notation, socannot be parsed.
An attribute was used more than once in a start tag.
Raised when an input byte could not properly be assigned to a character; forexample, a NUL byte (value0) in a UTF-8 input stream.
Something other than whitespace occurred after the document element.
An XML declaration was found somewhere other than the start of the input data.
The document contains no elements (XML requires all documents to contain exactlyone top-level element)..
Expat was not able to allocate memory internally.
A parameter entity reference was found where it was not allowed.
An incomplete character was found in the input.
An entity reference contained another reference to the same entity; possibly viaa different name, and possibly indirectly.
Some unspecified syntax error was encountered.
An end tag did not match the innermost open start tag.
Some token (such as a start tag) was not closed before the end of the stream orthe next token was encountered.
A reference was made to a entity which was not defined.
The document encoding is not supported by Expat.
A CDATA marked section was not closed.
The parser determined that the document was not “standalone” though it declareditself to be in the XML declaration, and theNotStandaloneHandler wasset and returned0.
An operation was requested that requires DTD support to be compiled in, butExpat was configured without DTD support. This should never be reported by astandard build of thexml.parsers.expat module.
A behavioral change was requested after parsing started that can only be changedbefore parsing has started. This is (currently) only raised byUseForeignDTD().
An undeclared prefix was found when namespace processing was enabled.
The document attempted to remove the namespace declaration associated with aprefix.
A parameter entity contained incomplete markup.
The document contained no document element at all.
There was an error parsing a text declaration in an external entity.
Characters were found in the public id that are not allowed.
The requested operation was made on a suspended parser, but isn’t allowed. Thisincludes attempts to provide additional input or to stop the parser.
An attempt to resume the parser was made when the parser had not been suspended.
This should not be reported to Python applications.
The requested operation was made on a parser which was finished parsing input,but isn’t allowed. This includes attempts to provide additional input or tostop the parser.
Footnotes
| [1] | The encoding string included in XML output should conform to theappropriate standards. For example, “UTF-8” is valid, but “UTF8” isnot. Seehttp://www.w3.org/TR/2006/REC-xml11-20060816/#NT-EncodingDeclandhttp://www.iana.org/assignments/character-sets. |
20.12.xml.sax.xmlreader — Interface for XML parsers
21. Internet Protocols and Support
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