| Python 3.6.5 Documentation > "xml.parsers.expat" — Fast XML parsing using Expat
"xml.parsers.expat" — Fast XML parsing using Expat
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Warning: The "pyexpat" module is not secure against maliciously
constructed data. If you need to parse untrusted or unauthenticated
data see XML vulnerabilities.
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.
See also:
The Expat XML Parser
Home page of the Expat project.
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" 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 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.
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.
ExpatError Exceptions
=====================
"ExpatError" exceptions have a number of interesting attributes:
ExpatError.code
Expat’s internal error number for the specific error. The
"errors.messages" dictionary maps these error numbers to Expat’s
error messages. For example:
from xml.parsers.expat import ParserCreate, ExpatError, errors
p = ParserCreate()
try:
p.Parse(some_xml_document)
except ExpatError as err:
print("Error:", errors.messages[err.code])
The "errors" module also provides error message constants and a
dictionary "codes" mapping these messages back to the error codes,
see below.
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".
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
Content Model Descriptions
==========================
Content models 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 model descriptions.
The values of the first two fields are constants defined in the
"xml.parsers.expat.model" 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.model.XML_CTYPE_ANY
The element named by the model name was declared to have a content
model of "ANY".
xml.parsers.expat.model.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.model.XML_CTYPE_EMPTY
Elements which are declared to be "EMPTY" have this model type.
xml.parsers.expat.model.XML_CTYPE_MIXED
xml.parsers.expat.model.XML_CTYPE_NAME
xml.parsers.expat.model.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.model.XML_CQUANT_NONE
No modifier is given, so it can appear exactly once, as for "A".
xml.parsers.expat.model.XML_CQUANT_OPT
The model is optional: it can appear once or not at all, as for
"A?".
xml.parsers.expat.model.XML_CQUANT_PLUS
The model must occur one or more times (like "A+").
xml.parsers.expat.model.XML_CQUANT_REP
The model must occur zero or more times, as for "A*".
Expat error constants
=====================
The following constants are provided in the "xml.parsers.expat.errors"
module. These constants are useful in interpreting some of the
attributes of the "ExpatError" exception objects raised when an error
has occurred. Since for backwards compatibility reasons, the
constants’ value is the error *message* and not the numeric error
*code*, you do this by comparing its "code" attribute with
"errors.codes[errors.XML_ERROR_*CONSTANT_NAME*]".
The "errors" module has the following attributes:
xml.parsers.expat.errors.codes
A dictionary mapping numeric error codes to their string
descriptions.
New in version 3.2.
xml.parsers.expat.errors.messages
A dictionary mapping string descriptions to their error codes.
New in version 3.2.
xml.parsers.expat.errors.XML_ERROR_ASYNC_ENTITY
xml.parsers.expat.errors.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.errors.XML_ERROR_BAD_CHAR_REF
A character reference referred to a character which is illegal in
XML (for example, character "0", or ‘"�"’).
xml.parsers.expat.errors.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.errors.XML_ERROR_DUPLICATE_ATTRIBUTE
An attribute was used more than once in a start tag.
xml.parsers.expat.errors.XML_ERROR_INCORRECT_ENCODING
xml.parsers.expat.errors.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.errors.XML_ERROR_JUNK_AFTER_DOC_ELEMENT
Something other than whitespace occurred after the document
element.
xml.parsers.expat.errors.XML_ERROR_MISPLACED_XML_PI
An XML declaration was found somewhere other than the start of the
input data.
xml.parsers.expat.errors.XML_ERROR_NO_ELEMENTS
The document contains no elements (XML requires all documents to
contain exactly one top-level element)..
xml.parsers.expat.errors.XML_ERROR_NO_MEMORY
Expat was not able to allocate memory internally.
xml.parsers.expat.errors.XML_ERROR_PARAM_ENTITY_REF
A parameter entity reference was found where it was not allowed.
xml.parsers.expat.errors.XML_ERROR_PARTIAL_CHAR
An incomplete character was found in the input.
xml.parsers.expat.errors.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.errors.XML_ERROR_SYNTAX
Some unspecified syntax error was encountered.
xml.parsers.expat.errors.XML_ERROR_TAG_MISMATCH
An end tag did not match the innermost open start tag.
xml.parsers.expat.errors.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.errors.XML_ERROR_UNDEFINED_ENTITY
A reference was made to an entity which was not defined.
xml.parsers.expat.errors.XML_ERROR_UNKNOWN_ENCODING
The document encoding is not supported by Expat.
xml.parsers.expat.errors.XML_ERROR_UNCLOSED_CDATA_SECTION
A CDATA marked section was not closed.
xml.parsers.expat.errors.XML_ERROR_EXTERNAL_ENTITY_HANDLING
xml.parsers.expat.errors.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.errors.XML_ERROR_UNEXPECTED_STATE
xml.parsers.expat.errors.XML_ERROR_ENTITY_DECLARED_IN_PE
xml.parsers.expat.errors.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.errors.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.errors.XML_ERROR_UNBOUND_PREFIX
An undeclared prefix was found when namespace processing was
enabled.
xml.parsers.expat.errors.XML_ERROR_UNDECLARING_PREFIX
The document attempted to remove the namespace declaration
associated with a prefix.
xml.parsers.expat.errors.XML_ERROR_INCOMPLETE_PE
A parameter entity contained incomplete markup.
xml.parsers.expat.errors.XML_ERROR_XML_DECL
The document contained no document element at all.
xml.parsers.expat.errors.XML_ERROR_TEXT_DECL
There was an error parsing a text declaration in an external
entity.
xml.parsers.expat.errors.XML_ERROR_PUBLICID
Characters were found in the public id that are not allowed.
xml.parsers.expat.errors.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.errors.XML_ERROR_NOT_SUSPENDED
An attempt to resume the parser was made when the parser had not
been suspended.
xml.parsers.expat.errors.XML_ERROR_ABORTED
This should not be reported to Python applications.
xml.parsers.expat.errors.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.errors.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 https://www.w3.org/TR/2006/REC-
xml11-20060816/#NT- EncodingDecl and
https://www.iana.org/assignments/character-sets /character-
sets.xhtml.
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