class QXmlStreamReader

The QXmlStreamReader class provides a fast parser for reading well-formed XML via a simple streaming API. More…

Synopsis

Methods

Note

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Detailed Description

Warning

This section contains snippets that were automatically translated from C++ to Python and may contain errors.

QXmlStreamReader provides a simple streaming API to parse well-formed XML. It is an alternative to first loading the complete XML into a DOM tree (see QDomDocument). QXmlStreamReader reads data either from a QIODevice (see setDevice() ), or from a raw QByteArray (see addData() ).

Qt provides QXmlStreamWriter for writing XML.

The basic concept of a stream reader is to report an XML document as a stream of tokens, similar to SAX. The main difference between QXmlStreamReader and SAX is how these XML tokens are reported. With SAX, the application must provide handlers (callback functions) that receive so-called XML events from the parser at the parser’s convenience. With QXmlStreamReader , the application code itself drives the loop and pulls tokens from the reader, one after another, as it needs them. This is done by calling readNext() , where the reader reads from the input stream until it completes the next token, at which point it returns the tokenType() . A set of convenient functions including isStartElement() and text() can then be used to examine the token to obtain information about what has been read. The big advantage of this pulling approach is the possibility to build recursive descent parsers with it, meaning you can split your XML parsing code easily into different methods or classes. This makes it easy to keep track of the application’s own state when parsing XML.

A typical loop with QXmlStreamReader looks like this:

xml = QXmlStreamReader()
...
while not xml.atEnd():
      xml.readNext()
      ... // do processing

if xml.hasError():
      ... // do error handling

QXmlStreamReader is a well-formed XML 1.0 parser that does not include external parsed entities. As long as no error occurs, the application code can thus be assured, that

  • the data provided by the stream reader satisfies the W3C’s criteria for well-formed XML,

  • tokens are provided in a valid order.

Unless QXmlStreamReader raises an error, it guarantees the following:

  • All tags are nested and closed properly.

  • References to internal entities have been replaced with the correct replacement text.

  • Attributes have been normalized or added according to the internal subset of the DTD .

  • Tokens of type StartDocument happen before all others, aside from comments and processing instructions.

  • At most one DOCTYPE element (a token of type DTD ) is present.

  • If present, the DOCTYPE appears before all other elements, aside from StartDocument , comments and processing instructions.

In particular, once any token of type StartElement , EndElement , Characters , EntityReference or EndDocument is seen, no tokens of type StartDocument or DTD will be seen. If one is present in the input stream, out of order, an error is raised.

Note

The token types Comment and ProcessingInstruction may appear anywhere in the stream.

If an error occurs while parsing, atEnd() and hasError() return true, and error() returns the error that occurred. The functions errorString() , lineNumber() , columnNumber() , and characterOffset() are for constructing an appropriate error or warning message. To simplify application code, QXmlStreamReader contains a raiseError() mechanism that lets you raise custom errors that trigger the same error handling described.

The QXmlStream Bookmarks Example illustrates how to use the recursive descent technique to read an XML bookmark file (XBEL) with a stream reader.

Namespaces

QXmlStream understands and resolves XML namespaces. E.g. in case of a StartElement , namespaceUri() returns the namespace the element is in, and name() returns the element’s local name. The combination of namespaceUri and name uniquely identifies an element. If a namespace prefix was not declared in the XML entities parsed by the reader, the namespaceUri is empty.

If you parse XML data that does not utilize namespaces according to the XML specification or doesn’t use namespaces at all, you can use the element’s qualifiedName() instead. A qualified name is the element’s prefix() followed by colon followed by the element’s local name() - exactly like the element appears in the raw XML data. Since the mapping namespaceUri to prefix is neither unique nor universal, qualifiedName() should be avoided for namespace-compliant XML data.

In order to parse standalone documents that do use undeclared namespace prefixes, you can turn off namespace processing completely with the namespaceProcessing property.

Incremental Parsing

QXmlStreamReader is an incremental parser. It can handle the case where the document can’t be parsed all at once because it arrives in chunks (e.g. from multiple files, or over a network connection). When the reader runs out of data before the complete document has been parsed, it reports a PrematureEndOfDocumentError . When more data arrives, either because of a call to addData() or because more data is available through the network device() , the reader recovers from the PrematureEndOfDocumentError error and continues parsing the new data with the next call to readNext() .

For example, if your application reads data from the network using a network access manager, you would issue a network request to the manager and receive a network reply in return. Since a QNetworkReply is a QIODevice , you connect its readyRead() signal to a custom slot, e.g. slotReadyRead() in the code snippet shown in the discussion for QNetworkAccessManager. In this slot, you read all available data with readAll() and pass it to the XML stream reader using addData() . Then you call your custom parsing function that reads the XML events from the reader.

Performance and Memory Consumption

QXmlStreamReader is memory-conservative by design, since it doesn’t store the entire XML document tree in memory, but only the current token at the time it is reported. In addition, QXmlStreamReader avoids the many small string allocations that it normally takes to map an XML document to a convenient and Qt-ish API. It does this by reporting all string data as QStringView rather than real QString objects. Calling toString() on any of those objects returns an equivalent real QString object.

class TokenType

This enum specifies the type of token the reader just read.

Constant

Description

QXmlStreamReader.NoToken

The reader has not yet read anything.

QXmlStreamReader.Invalid

An error has occurred, reported in error() and errorString() .

QXmlStreamReader.StartDocument

The reader reports the XML version number in documentVersion() , and the encoding as specified in the XML document in documentEncoding() . If the document is declared standalone, isStandaloneDocument() returns true; otherwise it returns false.

QXmlStreamReader.EndDocument

The reader reports the end of the document.

QXmlStreamReader.StartElement

The reader reports the start of an element with namespaceUri() and name() . Empty elements are also reported as StartElement, followed directly by EndElement. The convenience function readElementText() can be called to concatenate all content until the corresponding EndElement. Attributes are reported in attributes() , namespace declarations in namespaceDeclarations() .

QXmlStreamReader.EndElement

The reader reports the end of an element with namespaceUri() and name() .

QXmlStreamReader.Characters

The reader reports characters in text() . If the characters are all white-space, isWhitespace() returns true. If the characters stem from a CDATA section, isCDATA() returns true.

QXmlStreamReader.Comment

The reader reports a comment in text() .

QXmlStreamReader.DTD

The reader reports a DTD in text() , notation declarations in notationDeclarations() , and entity declarations in entityDeclarations() . Details of the DTD declaration are reported in dtdName() , dtdPublicId() , and dtdSystemId() .

QXmlStreamReader.EntityReference

The reader reports an entity reference that could not be resolved. The name of the reference is reported in name() , the replacement text in text() .

QXmlStreamReader.ProcessingInstruction

The reader reports a processing instruction in processingInstructionTarget() and processingInstructionData() .
class ReadElementTextBehaviour

This enum specifies the different behaviours of readElementText() .

Constant

Description

QXmlStreamReader.ErrorOnUnexpectedElement

Raise an UnexpectedElementError and return what was read so far when a child element is encountered.

QXmlStreamReader.IncludeChildElements

Recursively include the text from child elements.

QXmlStreamReader.SkipChildElements

Skip child elements.

Added in version 4.6.

class Error

This enum specifies different error cases

Constant

Description

QXmlStreamReader.NoError

No error has occurred.

QXmlStreamReader.CustomError

A custom error has been raised with raiseError()

QXmlStreamReader.NotWellFormedError

The parser internally raised an error due to the read XML not being well-formed.

QXmlStreamReader.PrematureEndOfDocumentError

The input stream ended before a well-formed XML document was parsed. Recovery from this error is possible if more XML arrives in the stream, either by calling addData() or by waiting for it to arrive on the device() .

QXmlStreamReader.UnexpectedElementError

The parser encountered an element or token that was different to those it expected.
__init__()

Constructs a stream reader.

__init__(data)
Parameters:

data – str

Creates a new stream reader that reads from data.

Note

In Qt versions prior to 6.5, this constructor was overloaded for QString and const char*.

See also

addData() clear() setDevice()

__init__(device)
Parameters:

device – QIODevice

Creates a new stream reader that reads from device.

See also

setDevice() clear()

addData(data)
Parameters:

data – str

Adds more data for the reader to read. This function does nothing if the reader has a device() .

Note

In Qt versions prior to 6.5, this function was overloaded for QString and const char*.

See also

readNext() clear()

addExtraNamespaceDeclaration(extraNamespaceDeclaraction)
Parameters:

extraNamespaceDeclaraction – QXmlStreamNamespaceDeclaration

Adds an extraNamespaceDeclaration. The declaration will be valid for children of the current element, or - should the function be called before any elements are read - for the entire XML document.

addExtraNamespaceDeclarations(extraNamespaceDeclaractions)
Parameters:

extraNamespaceDeclaractions – .list of QXmlStreamNamespaceDeclaration

Adds a vector of declarations specified by extraNamespaceDeclarations.

atEnd()
Return type:

bool

Returns true if the reader has read until the end of the XML document, or if an error() has occurred and reading has been aborted. Otherwise, it returns false.

When atEnd() and hasError() return true and error() returns PrematureEndOfDocumentError , it means the XML has been well-formed so far, but a complete XML document has not been parsed. The next chunk of XML can be added with addData() , if the XML is being read from a QByteArray , or by waiting for more data to arrive if the XML is being read from a QIODevice . Either way, atEnd() will return false once more data is available.

attributes()
Return type:

QXmlStreamAttributes

Returns the attributes of a StartElement .

characterOffset()
Return type:

int

Returns the current character offset, starting with 0.

clear()

Removes any device() or data from the reader and resets its internal state to the initial state.

See also

addData()

columnNumber()
Return type:

int

Returns the current column number, starting with 0.

device()
Return type:

QIODevice

Returns the current device associated with the QXmlStreamReader , or None if no device has been assigned.

See also

setDevice()

documentEncoding()
Return type:

str

If the tokenType() is StartDocument , this function returns the encoding string as specified in the XML declaration. Otherwise an empty string is returned.

documentVersion()
Return type:

str

If the tokenType() is StartDocument , this function returns the version string as specified in the XML declaration. Otherwise an empty string is returned.

dtdName()
Return type:

str

If the tokenType() is DTD , this function returns the DTD’s name. Otherwise an empty string is returned.

dtdPublicId()
Return type:

str

If the tokenType() is DTD , this function returns the DTD’s public identifier. Otherwise an empty string is returned.

dtdSystemId()
Return type:

str

If the tokenType() is DTD , this function returns the DTD’s system identifier. Otherwise an empty string is returned.

entityDeclarations()
Return type:

.list of QXmlStreamEntityDeclaration

If the tokenType() is DTD , this function returns the DTD’s unparsed (external) entity declarations. Otherwise an empty vector is returned.

The QXmlStreamEntityDeclarations class is defined to be a QList of QXmlStreamEntityDeclaration .

entityExpansionLimit()
Return type:

int

Returns the maximum amount of characters a single entity is allowed to expand into. If a single entity expands past the given limit, the document is not considered well formed.

entityResolver()
Return type:

QXmlStreamEntityResolver

Returns the entity resolver, or None if there is no entity resolver.

error()
Return type:

Error

Returns the type of the current error, or NoError if no error occurred.

errorString()
Return type:

str

Returns the error message that was set with raiseError() .

hasError()
Return type:

bool

Returns true if an error has occurred, otherwise false.

hasStandaloneDeclaration()
Return type:

bool

Returns true if this document has an explicit standalone declaration (can be ‘yes’ or ‘no’); otherwise returns false;

If no XML declaration has been parsed, this function returns false.

isCDATA()
Return type:

bool

Returns true if the reader reports characters that stem from a CDATA section; otherwise returns false.

isCharacters()
Return type:

bool

Returns true if tokenType() equals Characters ; otherwise returns false.

isComment()
Return type:

bool

Returns true if tokenType() equals Comment ; otherwise returns false.

isDTD()
Return type:

bool

Returns true if tokenType() equals DTD ; otherwise returns false.

isEndDocument()
Return type:

bool

Returns true if tokenType() equals EndDocument ; otherwise returns false.

isEndElement()
Return type:

bool

Returns true if tokenType() equals EndElement ; otherwise returns false.

isEntityReference()
Return type:

bool

Returns true if tokenType() equals EntityReference ; otherwise returns false.

isProcessingInstruction()
Return type:

bool

Returns true if tokenType() equals ProcessingInstruction ; otherwise returns false.

isStandaloneDocument()
Return type:

bool

Returns true if this document has been declared standalone in the XML declaration; otherwise returns false.

If no XML declaration has been parsed, this function returns false.

isStartDocument()
Return type:

bool

Returns true if tokenType() equals StartDocument ; otherwise returns false.

isStartElement()
Return type:

bool

Returns true if tokenType() equals StartElement ; otherwise returns false.

isWhitespace()
Return type:

bool

Returns true if the reader reports characters that only consist of white-space; otherwise returns false.

lineNumber()
Return type:

int

Returns the current line number, starting with 1.

name()
Return type:

str

Returns the local name of a StartElement , EndElement , or an EntityReference .

namespaceDeclarations()
Return type:

.list of QXmlStreamNamespaceDeclaration

If the tokenType() is StartElement , this function returns the element’s namespace declarations. Otherwise an empty vector is returned.

The QXmlStreamNamespaceDeclarations class is defined to be a QList of QXmlStreamNamespaceDeclaration .

namespaceProcessing()
Return type:

bool

See also

setNamespaceProcessing()

namespaceUri()
Return type:

str

Returns the namespaceUri of a StartElement or EndElement .

notationDeclarations()
Return type:

.list of QXmlStreamNotationDeclaration

If the tokenType() is DTD , this function returns the DTD’s notation declarations. Otherwise an empty vector is returned.

The QXmlStreamNotationDeclarations class is defined to be a QList of QXmlStreamNotationDeclaration .

prefix()
Return type:

str

Returns the prefix of a StartElement or EndElement .

processingInstructionData()
Return type:

str

Returns the data of a ProcessingInstruction .

processingInstructionTarget()
Return type:

str

Returns the target of a ProcessingInstruction .

qualifiedName()
Return type:

str

Returns the qualified name of a StartElement or EndElement ;

A qualified name is the raw name of an element in the XML data. It consists of the namespace prefix, followed by colon, followed by the element’s local name. Since the namespace prefix is not unique (the same prefix can point to different namespaces and different prefixes can point to the same namespace), you shouldn’t use qualifiedName(), but the resolved namespaceUri() and the attribute’s local name() .

raiseError([message=""])
Parameters:

message – str

Raises a custom error with an optional error message.

readElementText([behaviour=QXmlStreamReader.ReadElementTextBehaviour.ErrorOnUnexpectedElement])
Parameters:

behaviour – ReadElementTextBehaviour

Return type:

str

Convenience function to be called in case a StartElement was read. Reads until the corresponding EndElement and returns all text in-between. In case of no error, the current token (see tokenType() ) after having called this function is EndElement .

The function concatenates text() when it reads either Characters or EntityReference tokens, but skips ProcessingInstruction and Comment . If the current token is not StartElement , an empty string is returned.

The behaviour defines what happens in case anything else is read before reaching EndElement . The function can include the text from child elements (useful for example for HTML), ignore child elements, or raise an UnexpectedElementError and return what was read so far (default).

readNext()
Return type:

TokenType

Reads the next token and returns its type.

With one exception, once an error() is reported by readNext(), further reading of the XML stream is not possible. Then atEnd() returns true, hasError() returns true, and this function returns Invalid .

The exception is when error() returns PrematureEndOfDocumentError . This error is reported when the end of an otherwise well-formed chunk of XML is reached, but the chunk doesn’t represent a complete XML document. In that case, parsing can be resumed by calling addData() to add the next chunk of XML, when the stream is being read from a QByteArray , or by waiting for more data to arrive when the stream is being read from a device() .

readNextStartElement()
Return type:

bool

Reads until the next start element within the current element. Returns true when a start element was reached. When the end element was reached, or when an error occurred, false is returned.

The current element is the element matching the most recently parsed start element of which a matching end element has not yet been reached. When the parser has reached the end element, the current element becomes the parent element.

This is a convenience function for when you’re only concerned with parsing XML elements. The QXmlStream Bookmarks Example makes extensive use of this function.

See also

readNext()

setDevice(device)
Parameters:

device – QIODevice

Sets the current device to device. Setting the device resets the stream to its initial state.

See also

device() clear()

setEntityExpansionLimit(limit)
Parameters:

limit – int

Sets the maximum amount of characters a single entity is allowed to expand into to limit. If a single entity expands past the given limit, the document is not considered well formed.

The limit is there to prevent DoS attacks when loading unknown XML documents where recursive entity expansion could otherwise exhaust all available memory.

The default value for this property is 4096 characters.

setEntityResolver(resolver)
Parameters:

resolver – QXmlStreamEntityResolver

Makes resolver the new entityResolver() .

The stream reader does not take ownership of the resolver. It’s the callers responsibility to ensure that the resolver is valid during the entire life-time of the stream reader object, or until another resolver or None is set.

See also

entityResolver()

setNamespaceProcessing(arg__1)
Parameters:

arg__1 – bool

See also

namespaceProcessing()

skipCurrentElement()

Reads until the end of the current element, skipping any child nodes. This function is useful for skipping unknown elements.

The current element is the element matching the most recently parsed start element of which a matching end element has not yet been reached. When the parser has reached the end element, the current element becomes the parent element.

text()
Return type:

str

Returns the text of Characters , Comment , DTD , or EntityReference .

tokenString()
Return type:

str

Returns the reader’s current token as string.

See also

tokenType()

tokenType()
Return type:

TokenType

Returns the type of the current token.

The current token can also be queried with the convenience functions isStartDocument() , isEndDocument() , isStartElement() , isEndElement() , isCharacters() , isComment() , isDTD() , isEntityReference() , and isProcessingInstruction() .

See also

tokenString()