19.3. xml.parsers.expat — Fast XML parsing using Expat

The xml.parsers.expat module is a Python interface to the Expat non-validating XML parser. The module provides a single extension type, xmlparser, that represents the current state of an XML parser. After an xmlparser object has been created, various attributes of the object can be set to handler functions. When an XML document is then fed to the parser, the handler functions are called for the character data and markup in the XML document.

This module uses the pyexpat module to provide access to the Expat parser. Direct use of the pyexpat module is deprecated.

This module provides one exception and one type object:

exception xml.parsers.expat.ExpatError

The exception raised when Expat reports an error. See section ExpatError Exceptions for more information on interpreting Expat errors.

exception xml.parsers.expat.error

Alias for ExpatError.

xml.parsers.expat.XMLParserType

The type of the return values from the ParserCreate() function.

The xml.parsers.expat module contains two functions:

xml.parsers.expat.ErrorString(errno)

Returns an explanatory string for a given error number errno.

xml.parsers.expat.ParserCreate([encoding[, namespace_separator]])

Creates and returns a new xmlparser object. encoding, if specified, must be a string naming the encoding used by the XML data. Expat doesn’t support as many encodings as Python does, and its repertoire of encodings can’t be extended; it supports UTF-8, UTF-16, ISO-8859-1 (Latin1), and ASCII. If encoding [1] is given it will override the implicit or explicit encoding of the document.

Expat can optionally do XML namespace processing for you, enabled by providing a value for namespace_separator. The value must be a one-character string; a ValueError will be raised if the string has an illegal length (None is considered the same as omission). When namespace processing is enabled, element type names and attribute names that belong to a namespace will be expanded. The element name passed to the element handlers StartElementHandler and EndElementHandler will be the concatenation of the namespace URI, the namespace separator character, and the local 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 any separator.

For example, if namespace_separator is set to a space character (' ') and the 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 each element:

http://default-namespace.org/ root
http://www.python.org/ns/ elem1
elem2

See also

The Expat XML Parser

Home page of the Expat project.

19.3.1. XMLParser Objects

xmlparser objects have the following methods:

xmlparser.Parse(data[, isfinal])

Parses the contents of the string data, calling the appropriate handler functions to process the parsed data. isfinal must be true on the final call to this method. data can be the empty string at any time.

xmlparser.ParseFile(file)

Parse XML data reading from the object file. file only needs to provide the read(nbytes) method, returning the empty string when there’s no more data.

xmlparser.SetBase(base)

Sets the base to be used for resolving relative URIs in system identifiers in declarations. Resolving relative identifiers is left to the application: this value will be passed through as the base argument to the ExternalEntityRefHandler(), NotationDeclHandler(), and UnparsedEntityDeclHandler() functions.

xmlparser.GetBase()

Returns a string containing the base set by a previous call to SetBase(), or None if SetBase() hasn’t been called.

xmlparser.GetInputContext()

Returns the input data that generated the current event as a string. The data is in the encoding of the entity which contains the text. When called while an event handler is not active, the return value is None.

xmlparser.ExternalEntityParserCreate(context[, encoding])

Create a “child” parser which can be used to parse an external parsed entity referred to by content parsed by the parent parser. The context parameter should be the string passed to the ExternalEntityRefHandler() handler function, described below. The child parser is created with the ordered_attributes and specified_attributes set to the values of this parser.

xmlparser.UseForeignDTD([flag])

Calling this with a true value for flag (the default) will cause Expat to call the ExternalEntityRefHandler with None for all arguments to allow an alternate DTD to be loaded. If the document does not contain a document type declaration, the ExternalEntityRefHandler will still be called, but the StartDoctypeDeclHandler and EndDoctypeDeclHandler will not be called.

Passing a false value for flag will cancel a previous call that passed a true value, but otherwise has no effect.

This method can only be called before the Parse() or ParseFile() methods are called; calling it after either of those have been called causes ExpatError to be raised with the code attribute set to errors.XML_ERROR_CANT_CHANGE_FEATURE_ONCE_PARSING.

xmlparser objects have the following attributes:

xmlparser.buffer_size

The size of the buffer used when buffer_text is true. A new buffer size can be set by assigning a new integer value to this attribute. When the size is changed, the buffer will be flushed.

xmlparser.buffer_text

Setting this to true causes the xmlparser object to buffer textual content returned by Expat to avoid multiple calls to the CharacterDataHandler() callback whenever possible. This can improve performance substantially since Expat normally breaks character data into chunks at every line ending. This attribute is false by default, and may be changed at any time.

xmlparser.buffer_used

If buffer_text is enabled, the number of bytes stored in the buffer. These bytes represent UTF-8 encoded text. This attribute has no meaningful interpretation when buffer_text is false.

xmlparser.ordered_attributes

Setting this attribute to a non-zero integer causes the attributes to be reported as a list rather than a dictionary. The attributes are presented in the order found in the document text. For each attribute, two list entries are presented: the attribute name and the attribute value. (Older versions of this module also used this format.) By default, this attribute is false; it may be changed at any time.

xmlparser.specified_attributes

If set to a non-zero integer, the parser will report only those attributes which were specified in the document instance and not those which were derived from attribute declarations. Applications which set this need to be especially careful to use what additional information is available from the declarations as needed to comply with the standards for the behavior of XML processors. By default, this attribute is false; it may be changed at any time.

The following attributes contain values relating to the most recent error encountered by an xmlparser object, and will only have correct values once a call to Parse() or ParseFile() has raised a xml.parsers.expat.ExpatError exception.

xmlparser.ErrorByteIndex

Byte index at which an error occurred.

xmlparser.ErrorCode

Numeric code specifying the problem. This value can be passed to the ErrorString() function, or compared to one of the constants defined in the errors object.

xmlparser.ErrorColumnNumber

Column number at which an error occurred.

xmlparser.ErrorLineNumber

Line number at which an error occurred.

The following attributes contain values relating to the current parse location in an xmlparser object. During a callback reporting a parse event they indicate the location of the first of the sequence of characters that generated the event. When called outside of a callback, the position indicated will be just past the last parse event (regardless of whether there was an associated callback).

xmlparser.CurrentByteIndex

Current byte index in the parser input.

xmlparser.CurrentColumnNumber

Current column number in the parser input.

xmlparser.CurrentLineNumber

Current line number in the parser input.

Here is the list of handlers that can be set. To set a handler on an xmlparser object o, use o.handlername = func. handlername must be taken from the following list, and func must be a callable object accepting the correct number of arguments. The arguments are all strings, unless otherwise stated.

xmlparser.XmlDeclHandler(version, encoding, standalone)

Called when the XML declaration is parsed. The XML declaration is the (optional) declaration of the applicable version of the XML recommendation, the encoding of the document text, and an optional “standalone” declaration. version and encoding will be strings, and standalone will be 1 if the document is declared standalone, 0 if it is declared not to be standalone, or -1 if the standalone clause was omitted. This is only available with Expat version 1.95.0 or newer.

xmlparser.StartDoctypeDeclHandler(doctypeName, systemId, publicId, has_internal_subset)

Called when Expat begins parsing the document type declaration (<!DOCTYPE ...). The doctypeName is provided exactly as presented. The systemId and publicId parameters give the system and public identifiers if specified, or None if omitted. has_internal_subset will be true if the document contains and internal document declaration subset. This requires Expat version 1.2 or newer.

xmlparser.EndDoctypeDeclHandler()

Called when Expat is done parsing the document type declaration. This requires Expat version 1.2 or newer.

xmlparser.ElementDeclHandler(name, model)

Called once for each element type declaration. name is the name of the element type, and model is a representation of the content model.

xmlparser.AttlistDeclHandler(elname, attname, type, default, required)

Called for each declared attribute for an element type. If an attribute list declaration declares three attributes, this handler is called three times, once for each attribute. elname is the name of the element to which the declaration applies and attname is the name of the attribute declared. The attribute type is a string passed as type; the possible values are 'CDATA', 'ID', 'IDREF', ... default gives the default value for the attribute used when the attribute is not specified by the document instance, or None if there is no default value (#IMPLIED values). If the attribute is required to be given in the document instance, required will be true. This requires Expat version 1.95.0 or newer.

xmlparser.StartElementHandler(name, attributes)

Called for the start of every element. name is a string containing the element name, and attributes is a dictionary mapping attribute names to their values.

xmlparser.EndElementHandler(name)

Called for the end of every element.

xmlparser.ProcessingInstructionHandler(target, data)

Called for every processing instruction.

xmlparser.CharacterDataHandler(data)

Called for character data. This will be called for normal character data, CDATA marked content, and ignorable whitespace. Applications which must distinguish these cases can use the StartCdataSectionHandler, EndCdataSectionHandler, and ElementDeclHandler callbacks to collect the required information.

xmlparser.UnparsedEntityDeclHandler(entityName, base, systemId, publicId, notationName)

Called for unparsed (NDATA) entity declarations. This is only present for version 1.2 of the Expat library; for more recent versions, use EntityDeclHandler instead. (The underlying function in the Expat library has been declared obsolete.)

xmlparser.EntityDeclHandler(entityName, is_parameter_entity, value, base, systemId, publicId, notationName)

Called for all entity declarations. For parameter and internal entities, value will be a string giving the declared contents of the entity; this will be None for external entities. The notationName parameter will be None for parsed entities, and the name of the notation for unparsed entities. is_parameter_entity will be true if the entity is a parameter entity or false for general entities (most applications only need to be concerned with general entities). This is only available starting with version 1.95.0 of the Expat library.

xmlparser.NotationDeclHandler(notationName, base, systemId, publicId)

Called for notation declarations. notationName, base, and systemId, and publicId are strings if given. If the public identifier is omitted, publicId will be None.

xmlparser.StartNamespaceDeclHandler(prefix, uri)

Called when an element contains a namespace declaration. Namespace declarations are processed before the StartElementHandler is called for the element on which declarations are placed.

xmlparser.EndNamespaceDeclHandler(prefix)

Called when the closing tag is reached for an element that contained a namespace declaration. This is called once for each namespace declaration on the element in the reverse of the order for which the StartNamespaceDeclHandler was called to indicate the start of each namespace declaration’s scope. Calls to this handler are made after the corresponding EndElementHandler for the end of the element.

xmlparser.CommentHandler(data)

Called for comments. data is the text of the comment, excluding the leading ‘<!--‘ and trailing ‘-->‘.

xmlparser.StartCdataSectionHandler()

Called at the start of a CDATA section. This and EndCdataSectionHandler are needed to be able to identify the syntactical start and end for CDATA sections.

xmlparser.EndCdataSectionHandler()

Called at the end of a CDATA section.

xmlparser.DefaultHandler(data)

Called for any characters in the XML document for which no applicable handler has been specified. This means characters that are part of a construct which could be reported, but for which no handler has been supplied.

xmlparser.DefaultHandlerExpand(data)

This is the same as the DefaultHandler(), but doesn’t inhibit expansion of internal entities. The entity reference will not be passed to the default handler.

xmlparser.NotStandaloneHandler()

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 parameter entity, but the XML declaration does not set standalone to yes in an XML declaration. If this handler returns 0, then the parser will raise an XML_ERROR_NOT_STANDALONE error. If this handler is not set, no exception is raised by the parser for this condition.

xmlparser.ExternalEntityRefHandler(context, base, systemId, publicId)

Called for references to external entities. base is the current base, as set by a previous call to SetBase(). The public and system identifiers, systemId and publicId, are strings if given; if the public identifier is not given, publicId will be None. The context value is opaque and should only be used as described below.

For external entities to be parsed, this handler must be implemented. It is responsible for creating the sub-parser using ExternalEntityParserCreate(context), initializing it with the appropriate callbacks, and parsing the entity. This handler should return an integer; if it returns 0, the parser will raise an XML_ERROR_EXTERNAL_ENTITY_HANDLING error, otherwise parsing will continue.

If this handler is not provided, external entities are reported by the DefaultHandler callback, if provided.

19.3.2. ExpatError Exceptions

ExpatError exceptions have a number of interesting attributes:

ExpatError.code

Expat’s internal error number for the specific error. This will match one of the constants defined in the errors object from this module.

ExpatError.lineno

Line number on which the error was detected. The first line is numbered 1.

ExpatError.offset

Character offset into the line where the error occurred. The first column is numbered 0.

19.3.3. Example

The following program defines three handlers that just print out their arguments.

import xml.parsers.expat

# 3 handler functions
def start_element(name, attrs):
    print('Start element:', name, attrs)
def end_element(name):
    print('End element:', name)
def char_data(data):
    print('Character data:', repr(data))

p = xml.parsers.expat.ParserCreate()

p.StartElementHandler = start_element
p.EndElementHandler = end_element
p.CharacterDataHandler = char_data

p.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:

Start element: parent {'id': 'top'}
Start element: child1 {'name': 'paul'}
Character data: 'Text goes here'
End element: child1
Character data: '\n'
Start element: child2 {'name': 'fred'}
Character data: 'More text'
End element: child2
Character data: '\n'
End element: parent

19.3.4. Content Model Descriptions

Content modules are described using nested tuples. Each tuple contains four values: the type, the quantifier, the name, and a tuple of children. Children are simply additional content module descriptions.

The values of the first two fields are constants defined in the model object of the xml.parsers.expat module. These constants can be collected in two groups: the model type group and the quantifier group.

The constants in the model type group are:

xml.parsers.expat.XML_CTYPE_ANY

The element named by the model name was declared to have a content model of ANY.

xml.parsers.expat.XML_CTYPE_CHOICE

The named element allows a choice from a number of options; this is used for content models such as (A | B | C).

xml.parsers.expat.XML_CTYPE_EMPTY

Elements which are declared to be EMPTY have this model type.

xml.parsers.expat.XML_CTYPE_MIXED

xml.parsers.expat.XML_CTYPE_NAME

xml.parsers.expat.XML_CTYPE_SEQ

Models which represent a series of models which follow one after the other are indicated with this model type. This is used for models such as (A, B, C).

The constants in the quantifier group are:

xml.parsers.expat.XML_CQUANT_NONE

No modifier is given, so it can appear exactly once, as for A.

xml.parsers.expat.XML_CQUANT_OPT

The model is optional: it can appear once or not at all, as for A?.

xml.parsers.expat.XML_CQUANT_PLUS

The model must occur one or more times (like A+).

xml.parsers.expat.XML_CQUANT_REP

The model must occur zero or more times, as for A*.

19.3.5. Expat error constants

The following constants are provided in the errors object of the xml.parsers.expat module. These constants are useful in interpreting some of the attributes of the ExpatError exception objects raised when an error has occurred.

The errors object has the following attributes:

xml.parsers.expat.XML_ERROR_ASYNC_ENTITY

xml.parsers.expat.XML_ERROR_ATTRIBUTE_EXTERNAL_ENTITY_REF

An entity reference in an attribute value referred to an external entity instead of an internal entity.

xml.parsers.expat.XML_ERROR_BAD_CHAR_REF

A character reference referred to a character which is illegal in XML (for example, character 0, or ‘&#0;‘).

xml.parsers.expat.XML_ERROR_BINARY_ENTITY_REF

An entity reference referred to an entity which was declared with a notation, so cannot be parsed.

xml.parsers.expat.XML_ERROR_DUPLICATE_ATTRIBUTE

An attribute was used more than once in a start tag.

xml.parsers.expat.XML_ERROR_INCORRECT_ENCODING

xml.parsers.expat.XML_ERROR_INVALID_TOKEN

Raised when an input byte could not properly be assigned to a character; for example, a NUL byte (value 0) in a UTF-8 input stream.

xml.parsers.expat.XML_ERROR_JUNK_AFTER_DOC_ELEMENT

Something other than whitespace occurred after the document element.

xml.parsers.expat.XML_ERROR_MISPLACED_XML_PI

An XML declaration was found somewhere other than the start of the input data.

xml.parsers.expat.XML_ERROR_NO_ELEMENTS

The document contains no elements (XML requires all documents to contain exactly one top-level element)..

xml.parsers.expat.XML_ERROR_NO_MEMORY

Expat was not able to allocate memory internally.

xml.parsers.expat.XML_ERROR_PARAM_ENTITY_REF

A parameter entity reference was found where it was not allowed.

xml.parsers.expat.XML_ERROR_PARTIAL_CHAR

An incomplete character was found in the input.

xml.parsers.expat.XML_ERROR_RECURSIVE_ENTITY_REF

An entity reference contained another reference to the same entity; possibly via a different name, and possibly indirectly.

xml.parsers.expat.XML_ERROR_SYNTAX

Some unspecified syntax error was encountered.

xml.parsers.expat.XML_ERROR_TAG_MISMATCH

An end tag did not match the innermost open start tag.

xml.parsers.expat.XML_ERROR_UNCLOSED_TOKEN

Some token (such as a start tag) was not closed before the end of the stream or the next token was encountered.

xml.parsers.expat.XML_ERROR_UNDEFINED_ENTITY

A reference was made to a entity which was not defined.

xml.parsers.expat.XML_ERROR_UNKNOWN_ENCODING

The document encoding is not supported by Expat.

xml.parsers.expat.XML_ERROR_UNCLOSED_CDATA_SECTION

A CDATA marked section was not closed.

xml.parsers.expat.XML_ERROR_EXTERNAL_ENTITY_HANDLING

xml.parsers.expat.XML_ERROR_NOT_STANDALONE

The parser determined that the document was not “standalone” though it declared itself to be in the XML declaration, and the NotStandaloneHandler was set and returned 0.

xml.parsers.expat.XML_ERROR_UNEXPECTED_STATE

xml.parsers.expat.XML_ERROR_ENTITY_DECLARED_IN_PE

xml.parsers.expat.XML_ERROR_FEATURE_REQUIRES_XML_DTD

An operation was requested that requires DTD support to be compiled in, but Expat was configured without DTD support. This should never be reported by a standard build of the xml.parsers.expat module.

xml.parsers.expat.XML_ERROR_CANT_CHANGE_FEATURE_ONCE_PARSING

A behavioral change was requested after parsing started that can only be changed before parsing has started. This is (currently) only raised by UseForeignDTD().

xml.parsers.expat.XML_ERROR_UNBOUND_PREFIX

An undeclared prefix was found when namespace processing was enabled.

xml.parsers.expat.XML_ERROR_UNDECLARING_PREFIX

The document attempted to remove the namespace declaration associated with a prefix.

xml.parsers.expat.XML_ERROR_INCOMPLETE_PE

A parameter entity contained incomplete markup.

xml.parsers.expat.XML_ERROR_XML_DECL

The document contained no document element at all.

xml.parsers.expat.XML_ERROR_TEXT_DECL

There was an error parsing a text declaration in an external entity.

xml.parsers.expat.XML_ERROR_PUBLICID

Characters were found in the public id that are not allowed.

xml.parsers.expat.XML_ERROR_SUSPENDED

The requested operation was made on a suspended parser, but isn’t allowed. This includes attempts to provide additional input or to stop the parser.

xml.parsers.expat.XML_ERROR_NOT_SUSPENDED

An attempt to resume the parser was made when the parser had not been suspended.

xml.parsers.expat.XML_ERROR_ABORTED

This should not be reported to Python applications.

xml.parsers.expat.XML_ERROR_FINISHED

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 to stop the parser.

xml.parsers.expat.XML_ERROR_SUSPEND_PE

Footnotes

[1]	The encoding string included in XML output should conform to the appropriate standards. For example, “UTF-8” is valid, but “UTF8” is not. See http://www.w3.org/TR/2006/REC-xml11-20060816/#NT-EncodingDecl and http://www.iana.org/assignments/character-sets .