QDataStream — PyQt Documentation v5.15.7 (original) (raw)

PyQt5.QtCore.QDataStream

Description¶

The QDataStream class provides serialization of binary data to a QIODevice.

A data stream is a binary stream of encoded information which is 100% independent of the host computer’s operating system, CPU or byte order. For example, a data stream that is written by a PC under Windows can be read by a Sun SPARC running Solaris.

You can also use a data stream to read/write raw unencoded binary data. If you want a “parsing” input stream, see QTextStream.

The QDataStream class implements the serialization of C++’s basic data types, like char, short, int, char \*, etc. Serialization of more complex data is accomplished by breaking up the data into primitive units.

A data stream cooperates closely with a QIODevice. A QIODevice represents an input/output medium one can read data from and write data to. The QFile class is an example of an I/O device.

Example (write binary data to a stream):

QFile file("file.dat");

file.open(QIODevice::WriteOnly);

QDataStream out(&file); // we will serialize the data into the file

out << QString("the answer is"); // serialize a string

out << (qint32)42; // serialize an integer

Example (read binary data from a stream):

QFile file("file.dat");

file.open(QIODevice::ReadOnly);

QDataStream in(&file); // read the data serialized from the file

QString str;

qint32 a;

in >> str >> a; // extract "the answer is" and 42

Each item written to the stream is written in a predefined binary format that varies depending on the item’s type. Supported Qt types include QBrush, QColor, QDateTime, QFont, QPixmap, QString, QVariant and many others. For the complete list of all Qt types supporting data streaming see Serializing Qt Data Types.

For integers it is best to always cast to a Qt integer type for writing, and to read back into the same Qt integer type. This ensures that you get integers of the size you want and insulates you from compiler and platform differences.

To take one example, a char \* string is written as a 32-bit integer equal to the length of the string including the ‘\0’ byte, followed by all the characters of the string including the ‘\0’ byte. When reading a char \* string, 4 bytes are read to create the 32-bit length value, then that many characters for the char \* string including the ‘\0’ terminator are read.

The initial I/O device is usually set in the constructor, but can be changed with setDevice(). If you’ve reached the end of the data (or if there is no I/O device set) atEnd() will return true.

Versioning¶

QDataStream’s binary format has evolved since Qt 1.0, and is likely to continue evolving to reflect changes done in Qt. When inputting or outputting complex types, it’s very important to make sure that the same version of the stream (version()) is used for reading and writing. If you need both forward and backward compatibility, you can hardcode the version number in the application:

stream.setVersion(QDataStream::Qt_4_0);

If you are producing a new binary data format, such as a file format for documents created by your application, you could use a QDataStream to write the data in a portable format. Typically, you would write a brief header containing a magic string and a version number to give yourself room for future expansion. For example:

QFile file("file.xxx");

file.open(QIODevice::WriteOnly);

QDataStream out(&file);

// Write a header with a "magic number" and a version

out << (quint32)0xA0B0C0D0;

out << (qint32)123;

out.setVersion(QDataStream::Qt_4_0);

// Write the data

out << lots_of_interesting_data;

Then read it in with:

QFile file("file.xxx");

file.open(QIODevice::ReadOnly);

QDataStream in(&file);

// Read and check the header

quint32 magic;

in >> magic;

if (magic != 0xA0B0C0D0)

return XXX_BAD_FILE_FORMAT;

// Read the version

qint32 version;

in >> version;

if (version < 100)

return XXX_BAD_FILE_TOO_OLD;

if (version > 123)

return XXX_BAD_FILE_TOO_NEW;

if (version <= 110)

in.setVersion(QDataStream::Qt_3_2);

else

in.setVersion(QDataStream::Qt_4_0);

// Read the data

in >> lots_of_interesting_data;

if (version >= 120)

in >> data_new_in_XXX_version_1_2;

in >> other_interesting_data;

You can select which byte order to use when serializing data. The default setting is big endian (MSB first). Changing it to little endian breaks the portability (unless the reader also changes to little endian). We recommend keeping this setting unless you have special requirements.

Reading and Writing Raw Binary Data¶

You may wish to read/write your own raw binary data to/from the data stream directly. Data may be read from the stream into a preallocated char \* using readRawData(). Similarly data can be written to the stream using writeRawData(). Note that any encoding/decoding of the data must be done by you.

A similar pair of functions is readBytes() and writeBytes(). These differ from their raw counterparts as follows: readBytes() reads a quint32 which is taken to be the length of the data to be read, then that number of bytes is read into the preallocated char \*; writeBytes() writes a quint32 containing the length of the data, followed by the data. Note that any encoding/decoding of the data (apart from the length quint32) must be done by you.

Reading and Writing Qt Collection Classes¶

The Qt container classes can also be serialized to a QDataStream. These include QList, QLinkedList, QVector, QSet, QHash, and QMap. The stream operators are declared as non-members of the classes.

Reading and Writing Other Qt Classes¶

In addition to the overloaded stream operators documented here, any Qt classes that you might want to serialize to a QDataStream will have appropriate stream operators declared as non-member of the class:

QDataStream &operator<<(QDataStream &, const QXxx &);

QDataStream &operator>>(QDataStream &, QXxx &);

For example, here are the stream operators declared as non-members of the QImage class:

QDataStream & operator<< (QDataStream& stream, const QImage& image);

QDataStream & operator>> (QDataStream& stream, QImage& image);

To see if your favorite Qt class has similar stream operators defined, check the Related Non-Members section of the class’s documentation page.

Using Read Transactions¶

When a data stream operates on an asynchronous device, the chunks of data can arrive at arbitrary points in time. The QDataStream class implements a transaction mechanism that provides the ability to read the data atomically with a series of stream operators. As an example, you can handle incomplete reads from a socket by using a transaction in a slot connected to the readyRead() signal:

in.startTransaction();

QString str;

qint32 a;

in >> str >> a; // try to read packet atomically

if (!in.commitTransaction())

return; // wait for more data

If no full packet is received, this code restores the stream to the initial position, after which you need to wait for more data to arrive.

Enums¶

ByteOrder

The byte order used for reading/writing the data.

FloatingPointPrecision

The precision of floating point numbers used for reading/writing the data. This will only have an effect if the version of the data stream is Qt_4_6 or higher.

Warning: The floating point precision must be set to the same value on the object that writes and the object that reads the data stream.

Status

This enum describes the current status of the data stream.

Version

This enum provides symbolic synonyms for the data serialization format version numbers.

See also

setVersion(), version().

Methods¶

__init__()

Constructs a data stream that has no I/O device.

__init__(QIODevice)

Constructs a data stream that uses the I/O device d.

See also

setDevice(), device().

__init__(QByteArray)

Constructs a read-only data stream that operates on byte array a. Use QDataStream(QByteArray*, int) if you want to write to a byte array.

Since QByteArray is not a QIODevice subclass, internally a QBuffer is created to wrap the byte array.

__init__(QByteArray, Union[OpenMode, OpenModeFlag])

TODO

abortTransaction()

TODO

atEnd() → bool

TODO

byteOrder() → ByteOrder

commitTransaction() → bool

TODO

device() → QIODevice

floatingPointPrecision() → FloatingPointPrecision

Returns the floating point precision of the data stream.

See also

FloatingPointPrecision, setFloatingPointPrecision().

__lshift__(QBitArray) → QDataStream

TODO

__lshift__(QByteArray) → QDataStream

TODO

__lshift__(QDate) → QDataStream

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__lshift__(QTime) → QDataStream

TODO

__lshift__(QDateTime) → QDataStream

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__lshift__(QEasingCurve) → QDataStream

TODO

__lshift__(QJsonDocument) → QDataStream

TODO

__lshift__(QJsonValue) → QDataStream

TODO

__lshift__(QLine) → QDataStream

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__lshift__(QLineF) → QDataStream

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__lshift__(QLocale) → QDataStream

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__lshift__(QMargins) → QDataStream

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__lshift__(QMarginsF) → QDataStream

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__lshift__(QPoint) → QDataStream

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__lshift__(QPointF) → QDataStream

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__lshift__(QRect) → QDataStream

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__lshift__(QRectF) → QDataStream

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__lshift__(QRegExp) → QDataStream

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__lshift__(QRegularExpression) → QDataStream

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__lshift__(QSize) → QDataStream

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__lshift__(QSizeF) → QDataStream

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__lshift__(QTimeZone) → QDataStream

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__lshift__(QUrl) → QDataStream

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__lshift__(QUuid) → QDataStream

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__lshift__(QVariant) → QDataStream

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__lshift__(Type) → QDataStream

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__lshift__(QVersionNumber) → QDataStream

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readBool() → bool

TODO

readBytes() → bytes

TODO

readDouble() → float

TODO

readFloat() → float

TODO

readInt() → int

TODO

readInt16() → int

TODO

readInt32() → int

TODO

readInt64() → int

TODO

readInt8() → int

TODO

readQString() → str

TODO

readQStringList() → List[str]

TODO

readQVariant() → Any

TODO

readQVariantHash() → Dict[str, Any]

TODO

readQVariantList() → List[Any]

TODO

readQVariantMap() → Dict[str, Any]

TODO

readRawData(int) → bytes

TODO

readString() → bytes

TODO

readUInt16() → int

TODO

readUInt32() → int

TODO

readUInt64() → int

TODO

readUInt8() → int

TODO

resetStatus()

Resets the status of the data stream.

See also

Status, status(), setStatus().

rollbackTransaction()

TODO

__rshift__(QBitArray) → QDataStream

TODO

__rshift__(QByteArray) → QDataStream

TODO

__rshift__(QDate) → QDataStream

TODO

__rshift__(QTime) → QDataStream

TODO

__rshift__(QDateTime) → QDataStream

TODO

__rshift__(QEasingCurve) → QDataStream

TODO

__rshift__(QJsonDocument) → QDataStream

TODO

__rshift__(QJsonValue) → QDataStream

TODO

__rshift__(QLine) → QDataStream

TODO

__rshift__(QLineF) → QDataStream

TODO

__rshift__(QLocale) → QDataStream

TODO

__rshift__(QMargins) → QDataStream

TODO

__rshift__(QMarginsF) → QDataStream

TODO

__rshift__(QPoint) → QDataStream

TODO

__rshift__(QPointF) → QDataStream

TODO

__rshift__(QRect) → QDataStream

TODO

__rshift__(QRectF) → QDataStream

TODO

__rshift__(QRegExp) → QDataStream

TODO

__rshift__(QRegularExpression) → QDataStream

TODO

__rshift__(QSize) → QDataStream

TODO

__rshift__(QSizeF) → QDataStream

TODO

__rshift__(QTimeZone) → QDataStream

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__rshift__(QUrl) → QDataStream

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__rshift__(QUuid) → QDataStream

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__rshift__(QVariant) → QDataStream

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__rshift__(Type) → QDataStream

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__rshift__(QVersionNumber) → QDataStream

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setByteOrder(ByteOrder)

Sets the serialization byte order to bo.

The bo parameter can be BigEndian or LittleEndian.

The default setting is big endian. We recommend leaving this setting unless you have special requirements.

setDevice(QIODevice)

void (QIODevice *d)

Sets the I/O device to d, which can be 0 to unset to current I/O device.

setFloatingPointPrecision(FloatingPointPrecision)

Sets the floating point precision of the data stream to precision. If the floating point precision is DoublePrecision and the version of the data stream is Qt_4_6 or higher, all floating point numbers will be written and read with 64-bit precision. If the floating point precision is SinglePrecision and the version is Qt_4_6 or higher, all floating point numbers will be written and read with 32-bit precision.

For versions prior to Qt_4_6, the precision of floating point numbers in the data stream depends on the stream operator called.

The default is DoublePrecision.

Note that this property does not affect the serialization or deserialization of qfloat16 instances.

Warning: This property must be set to the same value on the object that writes and the object that reads the data stream.

setStatus(Status)

Sets the status of the data stream to the status given.

Subsequent calls to are ignored until resetStatus() is called.

See also

Status, status(), resetStatus().

setVersion(int)

skipRawData(int) → int

Skips len bytes from the device. Returns the number of bytes actually skipped, or -1 on error.

This is equivalent to calling readRawData() on a buffer of length len and ignoring the buffer.

startTransaction()

TODO

status() → Status

Returns the status of the data stream.

See also

Status, setStatus(), resetStatus().

version() → int

writeBool(bool)

TODO

writeBytes(bytes) → QDataStream

Writes the length specifier len and the buffer s to the stream and returns a reference to the stream.

The len is serialized as a quint32, followed by len bytes from s. Note that the data is not encoded.

writeDouble(float)

TODO

writeFloat(float)

TODO

writeInt(int)

TODO

writeInt16(int)

TODO

writeInt32(int)

TODO

writeInt64(int)

TODO

writeInt8(int)

TODO

writeQString(str)

TODO

writeQStringList(Iterable[str])

TODO

writeQVariant(Any)

TODO

writeQVariantHash(Dict[str, Any])

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writeQVariantList(Iterable[Any])

TODO

writeQVariantMap(Dict[str, Any])

TODO

writeRawData(bytes) → int

Writes len bytes from s to the stream. Returns the number of bytes actually written, or -1 on error. The data is not encoded.

writeString(str)

TODO

writeUInt16(int)

TODO

writeUInt32(int)

TODO

writeUInt64(int)

TODO

writeUInt8(int)

TODO