QFile

The QFile class provides an interface for reading from and writing to files. More

Inheritance diagram of PySide2.QtCore.QFile

Inherited by: QTemporaryFile

Synopsis

Functions

Static functions

Detailed Description

QFile is an I/O device for reading and writing text and binary files and resources . A QFile may be used by itself or, more conveniently, with a QTextStream or QDataStream .

The file name is usually passed in the constructor, but it can be set at any time using setFileName() . QFile expects the file separator to be ‘/’ regardless of operating system. The use of other separators (e.g., ‘') is not supported.

You can check for a file’s existence using exists() , and remove a file using remove() . (More advanced file system related operations are provided by QFileInfo and QDir .)

The file is opened with open() , closed with close() , and flushed with flush() . Data is usually read and written using QDataStream or QTextStream , but you can also call the QIODevice -inherited functions read() , readLine() , readAll() , write() . QFile also inherits getChar() , putChar() , and ungetChar() , which work one character at a time.

The size of the file is returned by size() . You can get the current file position using pos() , or move to a new file position using seek() . If you’ve reached the end of the file, atEnd() returns true .

Reading Files Directly

The following example reads a text file line by line:

QFile file("in.txt");
if (!file.open(QIODevice::ReadOnly | QIODevice::Text))
    return;

while (!file.atEnd()) {
    QByteArray line = file.readLine();
    process_line(line);
}

The Text flag passed to open() tells Qt to convert Windows-style line terminators (“\r\n”) into C++-style terminators (“\n”). By default, QFile assumes binary, i.e. it doesn’t perform any conversion on the bytes stored in the file.

Using Streams to Read Files

The next example uses QTextStream to read a text file line by line:

QFile file("in.txt");
if (!file.open(QIODevice::ReadOnly | QIODevice::Text))
    return;

QTextStream in(&file);
while (!in.atEnd()) {
    QString line = in.readLine();
    process_line(line);
}

QTextStream takes care of converting the 8-bit data stored on disk into a 16-bit Unicode QString . By default, it assumes that the user system’s local 8-bit encoding is used (e.g., UTF-8 on most unix based operating systems; see codecForLocale() for details). This can be changed using setCodec() .

To write text, we can use operator<<(), which is overloaded to take a QTextStream on the left and various data types (including QString ) on the right:

QFile file("out.txt");
if (!file.open(QIODevice::WriteOnly | QIODevice::Text))
    return;

QTextStream out(&file);
out << "The magic number is: " << 49 << "\n";

QDataStream is similar, in that you can use operator<<() to write data and operator>>() to read it back. See the class documentation for details.

When you use QFile , QFileInfo , and QDir to access the file system with Qt, you can use Unicode file names. On Unix, these file names are converted to an 8-bit encoding. If you want to use standard C++ APIs (<cstdio> or <iostream> ) or platform-specific APIs to access files instead of QFile , you can use the encodeName() and decodeName() functions to convert between Unicode file names and 8-bit file names.

On Unix, there are some special system files (e.g. in /proc ) for which size() will always return 0, yet you may still be able to read more data from such a file; the data is generated in direct response to you calling read() . In this case, however, you cannot use atEnd() to determine if there is more data to read (since atEnd() will return true for a file that claims to have size 0). Instead, you should either call readAll() , or call read() or readLine() repeatedly until no more data can be read. The next example uses QTextStream to read /proc/modules line by line:

QFile file("/proc/modules");
if (!file.open(QIODevice::ReadOnly | QIODevice::Text))
    return;

QTextStream in(&file);
QString line = in.readLine();
while (!line.isNull()) {
    process_line(line);
    line = in.readLine();
}

Signals

Unlike other QIODevice implementations, such as QTcpSocket , QFile does not emit the aboutToClose() , bytesWritten() , or readyRead() signals. This implementation detail means that QFile is not suitable for reading and writing certain types of files, such as device files on Unix platforms.

Platform Specific Issues

File permissions are handled differently on Unix-like systems and Windows. In a non writable directory on Unix-like systems, files cannot be created. This is not always the case on Windows, where, for instance, the ‘My Documents’ directory usually is not writable, but it is still possible to create files in it.

Qt’s understanding of file permissions is limited, which affects especially the setPermissions() function. On Windows, Qt will set only the legacy read-only flag, and that only when none of the Write* flags are passed. Qt does not manipulate access control lists (ACLs), which makes this function mostly useless for NTFS volumes. It may still be of use for USB sticks that use VFAT file systems. POSIX ACLs are not manipulated, either.

class QFile

QFile(parent)

QFile(name)

QFile(name, parent)

param parent

QObject

param name

unicode

Constructs a QFile object.

Constructs a new file object with the given parent .

static PySide2.QtCore.QFile.copy(fileName, newName)
Parameters
  • fileName – unicode

  • newName – unicode

Return type

bool

This is an overloaded function.

Copies the file fileName to newName . Returns true if successful; otherwise returns false .

If a file with the name newName already exists, copy() returns false (i.e., QFile will not overwrite it).

See also

rename()

PySide2.QtCore.QFile.copy(newName)
Parameters

newName – unicode

Return type

bool

Copies the file currently specified by fileName() to a file called newName . Returns true if successful; otherwise returns false .

Note that if a file with the name newName already exists, returns false (i.e. QFile will not overwrite it).

The source file is closed before it is copied.

See also

setFileName()

static PySide2.QtCore.QFile.decodeName(localFileName)
Parameters

localFileNameQByteArray

Return type

unicode

static PySide2.QtCore.QFile.decodeName(localFileName)
Parameters

localFileName – str

Return type

unicode

This is an overloaded function.

Returns the Unicode version of the given localFileName . See encodeName() for details.

static PySide2.QtCore.QFile.encodeName(fileName)
Parameters

fileName – unicode

Return type

QByteArray

Converts fileName to the local 8-bit encoding determined by the user’s locale. This is sufficient for file names that the user chooses. File names hard-coded into the application should only use 7-bit ASCII filename characters.

See also

decodeName()

PySide2.QtCore.QFile.exists()
Return type

bool

This is an overloaded function.

Returns true if the file specified by fileName() exists; otherwise returns false .

See also

fileName() setFileName()

static PySide2.QtCore.QFile.exists(fileName)
Parameters

fileName – unicode

Return type

bool

Returns true if the file specified by fileName exists; otherwise returns false .

Note

If fileName is a symlink that points to a non-existing file, false is returned.

Parameters

newName – unicode

Return type

bool

Creates a link named linkName that points to the file currently specified by fileName() . What a link is depends on the underlying filesystem (be it a shortcut on Windows or a symbolic link on Unix). Returns true if successful; otherwise returns false .

This function will not overwrite an already existing entity in the file system; in this case, link() will return false and set error() to return RenameError .

Note

To create a valid link on Windows, linkName must have a .lnk file extension.

See also

setFileName()

static PySide2.QtCore.QFile.link(oldname, newName)
Parameters
  • oldname – unicode

  • newName – unicode

Return type

bool

This is an overloaded function.

Creates a link named linkName that points to the file fileName . What a link is depends on the underlying filesystem (be it a shortcut on Windows or a symbolic link on Unix). Returns true if successful; otherwise returns false .

See also

link()

PySide2.QtCore.QFile.open(fd, ioFlags[, handleFlags=QFileDevice.DontCloseHandle])
Parameters
  • fdint

  • ioFlagsOpenMode

  • handleFlagsFileHandleFlags

Return type

bool

This is an overloaded function.

Opens the existing file descriptor fd in the given mode . handleFlags may be used to specify additional options. Returns true if successful; otherwise returns false .

When a QFile is opened using this function, behaviour of close() is controlled by the AutoCloseHandle flag. If AutoCloseHandle is specified, and this function succeeds, then calling close() closes the adopted handle. Otherwise, close() does not actually close the file, but only flushes it.

The QFile that is opened using this function is automatically set to be in raw mode; this means that the file input/output functions are slow. If you run into performance issues, you should try to use one of the other open functions.

Warning

If fd is not a regular file, e.g, it is 0 (stdin ), 1 (stdout ), or 2 (stderr ), you may not be able to seek() . In those cases, size() returns 0 . See isSequential() for more information.

Warning

Since this function opens the file without specifying the file name, you cannot use this QFile with a QFileInfo .

See also

close()

static PySide2.QtCore.QFile.permissions(filename)
Parameters

filename – unicode

Return type

Permissions

This is an overloaded function.

Returns the complete OR-ed together combination of Permission for fileName .

Return type

unicode

Note

This function is deprecated.

Use symLinkTarget() instead.

static PySide2.QtCore.QFile.readLink(fileName)
Parameters

fileName – unicode

Return type

unicode

Note

This function is deprecated.

Use symLinkTarget() instead.

PySide2.QtCore.QFile.remove()
Return type

bool

Removes the file specified by fileName() . Returns true if successful; otherwise returns false .

The file is closed before it is removed.

See also

setFileName()

static PySide2.QtCore.QFile.remove(fileName)
Parameters

fileName – unicode

Return type

bool

This is an overloaded function.

Removes the file specified by the fileName given.

Returns true if successful; otherwise returns false .

See also

remove()

PySide2.QtCore.QFile.rename(newName)
Parameters

newName – unicode

Return type

bool

Renames the file currently specified by fileName() to newName . Returns true if successful; otherwise returns false .

If a file with the name newName already exists, returns false (i.e., QFile will not overwrite it).

The file is closed before it is renamed.

If the rename operation fails, Qt will attempt to copy this file’s contents to newName , and then remove this file, keeping only newName . If that copy operation fails or this file can’t be removed, the destination file newName is removed to restore the old state.

See also

setFileName()

static PySide2.QtCore.QFile.rename(oldName, newName)
Parameters
  • oldName – unicode

  • newName – unicode

Return type

bool

This is an overloaded function.

Renames the file oldName to newName . Returns true if successful; otherwise returns false .

If a file with the name newName already exists, rename() returns false (i.e., QFile will not overwrite it).

See also

rename()

static PySide2.QtCore.QFile.resize(filename, sz)
Parameters
  • filename – unicode

  • szqint64

Return type

bool

This is an overloaded function.

Sets fileName to size (in bytes) sz . Returns true if the resize succeeds; false otherwise. If sz is larger than fileName currently is the new bytes will be set to 0, if sz is smaller the file is simply truncated.

Warning

This function can fail if the file doesn’t exist.

See also

resize()

PySide2.QtCore.QFile.setFileName(name)
Parameters

name – unicode

Sets the name of the file. The name can have no path, a relative path, or an absolute path.

Do not call this function if the file has already been opened.

If the file name has no path or a relative path, the path used will be the application’s current directory path at the time of the open() ** call.

Example:

file = QFile()
QDir.setCurrent("/tmp")
file.setFileName("readme.txt")
QDir.setCurrent("/home")
file.open(QIODevice.ReadOnly)       # opens "/home/readme.txt" under Unix

Note that the directory separator “/” works for all operating systems supported by Qt.

See also

fileName() QFileInfo QDir

static PySide2.QtCore.QFile.setPermissions(filename, permissionSpec)
Parameters
  • filename – unicode

  • permissionSpecPermissions

Return type

bool

This is an overloaded function.

Sets the permissions for fileName file to permissions .

PySide2.QtCore.QFile.symLinkTarget()
Return type

unicode

This is an overloaded function.

Returns the absolute path of the file or directory a symlink (or shortcut on Windows) points to, or a an empty string if the object isn’t a symbolic link.

This name may not represent an existing file; it is only a string. exists() returns true if the symlink points to an existing file.

See also

fileName() setFileName()

static PySide2.QtCore.QFile.symLinkTarget(fileName)
Parameters

fileName – unicode

Return type

unicode

Returns the absolute path of the file or directory referred to by the symlink (or shortcut on Windows) specified by fileName , or returns an empty string if the fileName does not correspond to a symbolic link.

This name may not represent an existing file; it is only a string. exists() returns true if the symlink points to an existing file.