`xml.parsers.expat` — Fast XML parsing using Expat¶

Note

If you need to parse untrusted or unauthenticated data, see XML security.

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=None, namespace_separator=None)¶

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

Due to limitations in the Expat library used by pyexpat, the xmlparser instance returned can only be used to parse a single XML document. Call ParserCreate for each document to provide unique parser instances.

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; it allows the parsing of a single file in fragments, not the submission of multiple files. 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.SetParamEntityParsing(flag)¶: Control parsing of parameter entities (including the external DTD subset). Possible flag values are XML_PARAM_ENTITY_PARSING_NEVER, XML_PARAM_ENTITY_PARSING_UNLESS_STANDALONE and XML_PARAM_ENTITY_PARSING_ALWAYS. Return true if setting the flag was successful.

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.codes[errors.XML_ERROR_CANT_CHANGE_FEATURE_ONCE_PARSING].

xmlparser.SetReparseDeferralEnabled(enabled)¶

Warning

Calling SetReparseDeferralEnabled(False) has security implications, as detailed below; please make sure to understand these consequences prior to using the SetReparseDeferralEnabled method.

Expat 2.6.0 introduced a security mechanism called “reparse deferral” where instead of causing denial of service through quadratic runtime from reparsing large tokens, reparsing of unfinished tokens is now delayed by default until a sufficient amount of input is reached. Due to this delay, registered handlers may — depending of the sizing of input chunks pushed to Expat — no longer be called right after pushing new input to the parser. Where immediate feedback and taking over responsibility of protecting against denial of service from large tokens are both wanted, calling SetReparseDeferralEnabled(False) disables reparse deferral for the current Expat parser instance, temporarily or altogether. Calling SetReparseDeferralEnabled(True) allows re-enabling reparse deferral.

Note that SetReparseDeferralEnabled() has been backported to some prior releases of CPython as a security fix. Check for availability of SetReparseDeferralEnabled() using hasattr() if used in code running across a variety of Python versions.

Added in version 3.13.

xmlparser.GetReparseDeferralEnabled()¶: Returns whether reparse deferral is currently enabled for the given Expat parser instance.

Added in version 3.13.

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. Note that when it is false, data that does not contain newlines may be chunked too.

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 an 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 the element attributes. If ordered_attributes is true, this is a list (see ordered_attributes for a full description). Otherwise it’s a dictionary mapping names to 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. Note that the character data may be chunked even if it is short and so you may receive more than one call to CharacterDataHandler(). Set the buffer_text instance attribute to True to avoid that.

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

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

XMLParser Objects¶

`xml.parsers.expat` — Fast XML parsing using Expat¶