QIODevice¶

PyQt5.QtCore.QIODevice

Inherits from QObject.

Inherited by QAbstractSocket, QBluetoothSocket, QBuffer, QFileDevice, QLocalSocket, QNetworkReply, QProcess, QSerialPort.

Description¶

The QIODevice class is the base interface class of all I/O devices in Qt.

QIODevice provides both a common implementation and an abstract interface for devices that support reading and writing of blocks of data, such as QFile, QBuffer and QTcpSocket. QIODevice is abstract and can not be instantiated, but it is common to use the interface it defines to provide device-independent I/O features. For example, Qt’s XML classes operate on a QIODevice pointer, allowing them to be used with various devices (such as files and buffers).

Before accessing the device, open() must be called to set the correct OpenMode (such as ReadOnly or ReadWrite). You can then write to the device with write() or putChar(), and read by calling either read(), readLine(), or readAll(). Call close() when you are done with the device.

QIODevice distinguishes between two types of devices: random-access devices and sequential devices.

Random-access devices support seeking to arbitrary positions using seek(). The current position in the file is available by calling pos(). QFile and QBuffer are examples of random-access devices.
Sequential devices don’t support seeking to arbitrary positions. The data must be read in one pass. The functions pos() and size() don’t work for sequential devices. QTcpSocket and QProcess are examples of sequential devices.

You can use isSequential() to determine the type of device.

QIODevice emits readyRead when new data is available for reading; for example, if new data has arrived on the network or if additional data is appended to a file that you are reading from. You can call bytesAvailable() to determine the number of bytes that are currently available for reading. It’s common to use bytesAvailable() together with the readyRead signal when programming with asynchronous devices such as QTcpSocket, where fragments of data can arrive at arbitrary points in time. QIODevice emits the bytesWritten signal every time a payload of data has been written to the device. Use bytesToWrite() to determine the current amount of data waiting to be written.

Certain subclasses of QIODevice, such as QTcpSocket and QProcess, are asynchronous. This means that I/O functions such as write() or read() always return immediately, while communication with the device itself may happen when control goes back to the event loop. QIODevice provides functions that allow you to force these operations to be performed immediately, while blocking the calling thread and without entering the event loop. This allows QIODevice subclasses to be used without an event loop, or in a separate thread:

waitForReadyRead() - This function suspends operation in the calling thread until new data is available for reading.
waitForBytesWritten() - This function suspends operation in the calling thread until one payload of data has been written to the device.
waitFor….() - Subclasses of QIODevice implement blocking functions for device-specific operations. For example, QProcess has a function called waitForStarted() which suspends operation in the calling thread until the process has started.

Calling these functions from the main, GUI thread, may cause your user interface to freeze. Example:

# QProcess gzip;
# gzip.start("gzip", QStringList() << "-c");
# if (!gzip.waitForStarted())
#     return false;

# gzip.write("uncompressed data");

# QByteArray compressed;
# while (gzip.waitForReadyRead())
#     compressed += gzip.readAll();

By subclassing QIODevice, you can provide the same interface to your own I/O devices. Subclasses of QIODevice are only required to implement the protected readData() and writeData() functions. QIODevice uses these functions to implement all its convenience functions, such as getChar(), readLine() and write(). QIODevice also handles access control for you, so you can safely assume that the device is opened in write mode if writeData() is called.

Some subclasses, such as QFile and QTcpSocket, are implemented using a memory buffer for intermediate storing of data. This reduces the number of required device accessing calls, which are often very slow. Buffering makes functions like getChar() and putChar() fast, as they can operate on the memory buffer instead of directly on the device itself. Certain I/O operations, however, don’t work well with a buffer. For example, if several users open the same device and read it character by character, they may end up reading the same data when they meant to read a separate chunk each. For this reason, QIODevice allows you to bypass any buffering by passing the Unbuffered flag to open(). When subclassing QIODevice, remember to bypass any buffer you may use when the device is open in Unbuffered mode.

Usually, the incoming data stream from an asynchronous device is fragmented, and chunks of data can arrive at arbitrary points in time. To handle incomplete reads of data structures, use the transaction mechanism implemented by QIODevice. See startTransaction() and related functions for more details.

Some sequential devices support communicating via multiple channels. These channels represent separate streams of data that have the property of independently sequenced delivery. Once the device is opened, you can determine the number of channels by calling the readChannelCount() and writeChannelCount() functions. To switch between channels, call setCurrentReadChannel() and setCurrentWriteChannel(), respectively. QIODevice also provides additional signals to handle asynchronous communication on a per-channel basis.

See also

QBuffer, QFile, QTcpSocket.

Classes¶

OpenMode

Enums¶

OpenModeFlag

This enum is used with open() to describe the mode in which a device is opened. It is also returned by openMode().

Certain flags, such as Unbuffered and Truncate, are meaningless when used with some subclasses. Some of these restrictions are implied by the type of device that is represented by a subclass. In other cases, the restriction may be due to the implementation, or may be imposed by the underlying platform; for example, QTcpSocket does not support Unbuffered mode, and limitations in the native API prevent QFile from supporting Unbuffered on Windows.

Member	Value	Description
Append	0x0004	The device is opened in append mode so that all data is written to the end of the file.
ExistingOnly	TODO	Fail if the file to be opened does not exist. This flag must be specified alongside , , or . Note that using this flag with alone is redundant, as already fails when the file does not exist. This flag currently only affects QFile. Other classes might use this flag in the future, but until then using this flag with any classes other than QFile may result in undefined behavior. (since Qt 5.11)
NewOnly	TODO	Fail if the file to be opened already exists. Create and open the file only if it does not exist. There is a guarantee from the operating system that you are the only one creating and opening the file. Note that this mode implies , and combining it with is allowed. This flag currently only affects QFile. Other classes might use this flag in the future, but until then using this flag with any classes other than QFile may result in undefined behavior. (since Qt 5.11)
NotOpen	0x0000	The device is not open.
ReadOnly	0x0001	The device is open for reading.
ReadWrite	ReadOnly \| WriteOnly	The device is open for reading and writing.
Text	0x0010	When reading, the end-of-line terminators are translated to ‘\n’. When writing, the end-of-line terminators are translated to the local encoding, for example ‘\r\n’ for Win32.
Truncate	0x0008	If possible, the device is truncated before it is opened. All earlier contents of the device are lost.
Unbuffered	0x0020	Any buffer in the device is bypassed.
WriteOnly	0x0002	The device is open for writing. Note that, for file-system subclasses (e.g. QFile), this mode implies Truncate unless combined with , Append or NewOnly.

Methods¶

__init__(): Constructs a QIODevice object.

__init__(QObject): Constructs a QIODevice object with the given parent.

atEnd() → bool

Returns true if the current read and write position is at the end of the device (i.e. there is no more data available for reading on the device); otherwise returns false.

For some devices, can return true even though there is more data to read. This special case only applies to devices that generate data in direct response to you calling read() (e.g., /dev or /proc files on Unix and macOS, or console input / stdin on all platforms).

bytesAvailable() → int

Returns the number of bytes that are available for reading. This function is commonly used with sequential devices to determine the number of bytes to allocate in a buffer before reading.

Subclasses that reimplement this function must call the base implementation in order to include the size of the buffer of QIODevice. Example:

# qint64 CustomDevice::bytesAvailable() const
# {
#     return buffer.size() + QIODevice::bytesAvailable();
# }

bytesToWrite() → int

For buffered devices, this function returns the number of bytes waiting to be written. For devices with no buffer, this function returns 0.

Subclasses that reimplement this function must call the base implementation in order to include the size of the buffer of QIODevice.

canReadLine() → bool

Returns true if a complete line of data can be read from the device; otherwise returns false.

Note that unbuffered devices, which have no way of determining what can be read, always return false.

This function is often called in conjunction with the readyRead signal.

Subclasses that reimplement this function must call the base implementation in order to include the contents of the QIODevice’s buffer. Example:

# bool CustomDevice::canReadLine() const
# {
#     return buffer.contains('\n') || QIODevice::canReadLine();
# }

See also

readyRead, readLine().

close(): First emits aboutToClose, then closes the device and sets its OpenMode to NotOpen. The error string is also reset.

See also

setOpenMode(), OpenMode.

commitTransaction(): TODO

currentReadChannel() → int: TODO

currentWriteChannel() → int: TODO

errorString() → str: Returns a human-readable description of the last device error that occurred.

See also

setErrorString().

getChar() → (bool, str): TODO

isOpen() → bool: Returns true if the device is open; otherwise returns false. A device is open if it can be read from and/or written to. By default, this function returns false if openMode() returns NotOpen.

See also

openMode(), OpenMode.

isReadable() → bool

Returns true if data can be read from the device; otherwise returns false. Use bytesAvailable() to determine how many bytes can be read.

This is a convenience function which checks if the OpenMode of the device contains the ReadOnly flag.

See also

openMode(), OpenMode.

isSequential() → bool

Returns true if this device is sequential; otherwise returns false.

Sequential devices, as opposed to a random-access devices, have no concept of a start, an end, a size, or a current position, and they do not support seeking. You can only read from the device when it reports that data is available. The most common example of a sequential device is a network socket. On Unix, special files such as /dev/zero and fifo pipes are sequential.

Regular files, on the other hand, do support random access. They have both a size and a current position, and they also support seeking backwards and forwards in the data stream. Regular files are non-sequential.

See also

bytesAvailable().

isTextModeEnabled() → bool: Returns true if the Text flag is enabled; otherwise returns false.

See also

setTextModeEnabled().

isTransactionStarted() → bool: TODO

isWritable() → bool

Returns true if data can be written to the device; otherwise returns false.

This is a convenience function which checks if the OpenMode of the device contains the WriteOnly flag.

See also

openMode(), OpenMode.

open(Union[OpenMode, OpenModeFlag]) → bool: Opens the device and sets its OpenMode to mode. Returns true if successful; otherwise returns false. This function should be called from any reimplementations of or other functions that open the device.

See also

openMode(), OpenMode.

openMode() → OpenMode: Returns the mode in which the device has been opened; i.e. ReadOnly or WriteOnly.

See also

setOpenMode(), OpenMode.

peek(int) → QByteArray

This is an overloaded function.

Peeks at most maxSize bytes from the device, returning the data peeked as a QByteArray.

Example:

# bool isExeFile(QFile *file)
# {
#     return file->peek(2) == "MZ";
# }

This function has no way of reporting errors; returning an empty QByteArray can mean either that no data was currently available for peeking, or that an error occurred.

See also

read().

pos() → int

For random-access devices, this function returns the position that data is written to or read from. For sequential devices or closed devices, where there is no concept of a “current position”, 0 is returned.

The current read/write position of the device is maintained internally by QIODevice, so reimplementing this function is not necessary. When subclassing QIODevice, use seek() to notify QIODevice about changes in the device position.

See also

isSequential(), seek().

putChar(str) → bool: TODO

read(int) → bytes

This is an overloaded function.

Reads at most maxSize bytes from the device, and returns the data read as a QByteArray.

This function has no way of reporting errors; returning an empty QByteArray can mean either that no data was currently available for reading, or that an error occurred.

readAll() → QByteArray

Reads all remaining data from the device, and returns it as a byte array.

This function has no way of reporting errors; returning an empty QByteArray can mean either that no data was currently available for reading, or that an error occurred.

readChannelCount() → int: TODO

readData(int) → bytes: TODO

readLine(maxlen: int = 0) → bytes

This is an overloaded function.

Reads a line from the device, but no more than maxSize characters, and returns the result as a byte array.

This function has no way of reporting errors; returning an empty QByteArray can mean either that no data was currently available for reading, or that an error occurred.

readLineData(int) → bytes

Reads up to maxSize characters into data and returns the number of characters read.

This function is called by readLine(), and provides its base implementation, using getChar(). Buffered devices can improve the performance of readLine() by reimplementing this function.

readLine() appends a ‘\0’ byte to data; does not need to do this.

If you reimplement this function, be careful to return the correct value: it should return the number of bytes read in this line, including the terminating newline, or 0 if there is no line to be read at this point. If an error occurs, it should return -1 if and only if no bytes were read. Reading past EOF is considered an error.

reset() → bool

Seeks to the start of input for random-access devices. Returns true on success; otherwise returns false (for example, if the device is not open).

Note that when using a QTextStream on a QFile, calling on the QFile will not have the expected result because QTextStream buffers the file. Use the seek() function instead.

See also

seek().

rollbackTransaction(): TODO

seek(int) → bool

For random-access devices, this function sets the current position to pos, returning true on success, or false if an error occurred. For sequential devices, the default behavior is to produce a warning and return false.

When subclassing QIODevice, you must call at the start of your function to ensure integrity with QIODevice’s built-in buffer.

See also

pos(), isSequential().

setCurrentReadChannel(int): TODO

setCurrentWriteChannel(int): TODO

setErrorString(str): Sets the human readable description of the last device error that occurred to str.

See also

errorString().

setOpenMode(Union[OpenMode, OpenModeFlag]): Sets the OpenMode of the device to openMode. Call this function to set the open mode if the flags change after the device has been opened.

See also

openMode(), OpenMode.

setTextModeEnabled(bool)

If enabled is true, this function sets the Text flag on the device; otherwise the Text flag is removed. This feature is useful for classes that provide custom end-of-line handling on a QIODevice.

The IO device should be opened before calling this function.

size() → int

For open random-access devices, this function returns the size of the device. For open sequential devices, bytesAvailable() is returned.

If the device is closed, the size returned will not reflect the actual size of the device.

See also

isSequential(), pos().

skip(int) → int: TODO

startTransaction(): TODO

ungetChar(str)

Puts the character c back into the device, and decrements the current position unless the position is 0. This function is usually called to “undo” a getChar() operation, such as when writing a backtracking parser.

If c was not previously read from the device, the behavior is undefined.

Note: This function is not available while a transaction is in progress.

waitForBytesWritten(int) → bool

For buffered devices, this function waits until a payload of buffered written data has been written to the device and the bytesWritten signal has been emitted, or until msecs milliseconds have passed. If msecs is -1, this function will not time out. For unbuffered devices, it returns immediately.

Returns true if a payload of data was written to the device; otherwise returns false (i.e. if the operation timed out, or if an error occurred).

This function can operate without an event loop. It is useful when writing non-GUI applications and when performing I/O operations in a non-GUI thread.

If called from within a slot connected to the bytesWritten signal, bytesWritten will not be reemitted.

Reimplement this function to provide a blocking API for a custom device. The default implementation does nothing, and returns false.

Warning: Calling this function from the main (GUI) thread might cause your user interface to freeze.

See also

waitForReadyRead().

waitForReadyRead(int) → bool

Blocks until new data is available for reading and the readyRead signal has been emitted, or until msecs milliseconds have passed. If msecs is -1, this function will not time out.

Returns true if new data is available for reading; otherwise returns false (if the operation timed out or if an error occurred).

This function can operate without an event loop. It is useful when writing non-GUI applications and when performing I/O operations in a non-GUI thread.

If called from within a slot connected to the readyRead signal, readyRead will not be reemitted.

Reimplement this function to provide a blocking API for a custom device. The default implementation does nothing, and returns false.

Warning: Calling this function from the main (GUI) thread might cause your user interface to freeze.

See also

waitForBytesWritten().

write(Union[QByteArray, bytes, bytearray]) → int: TODO

writeChannelCount() → int: TODO

writeData(bytes) → int: TODO

Signals¶

aboutToClose(): TODO

bytesWritten(int): TODO

channelBytesWritten(int, int): TODO

channelReadyRead(int): TODO

readChannelFinished(): TODO

readyRead(): TODO