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

PyQt5.QtCore.QFile

Description¶

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

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 isWritable() 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.

Methods¶

__init__()

Constructs a QFile object.

__init__(str)

Constructs a new file object to represent the file with the given name.

__init__(QObject)

Constructs a new file object with the given parent.

__init__(str, QObject)

Constructs a new file object with the given parent to represent the file with the specified name.

copy(str) → 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.

@staticmethod
copy(str, str) → 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).

@staticmethod
decodeName(Union[QByteArray, bytes, bytearray]) → str

TODO

@staticmethod
decodeName(str) → str

TODO

@staticmethod
encodeName(str) → QByteArray

TODO

exists() → bool

This is an overloaded function.

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

QFile file;

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.

setPermissions(Union[Permissions, Permission]) → bool

TODO

@staticmethod
setPermissions(str, Union[Permissions, Permission]) → bool

TODO

size() → int

TODO

symLinkTarget() → str

TODO

@staticmethod
symLinkTarget(str) → str

TODO