Create file descriptor python

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Descriptors let objects customize attribute lookup, storage, and deletion. The second section shows a complete, practical descriptor example. If you already know the basics, start there. The third section provides a more technical tutorial that goes into the detailed mechanics of how descriptors work.

The last section has pure Python equivalents for built-in descriptors that are written in C. An interactive session shows the difference between normal attribute lookup and descriptor lookup:.

In the a. Calling that method returns Note that the value 10 is not stored in either the class dictionary or the instance dictionary.

Instead, the value 10 is computed on demand. For retrieving constants, normal attribute lookup would be better. An interactive session shows that the lookup is dynamic — it computes different, updated answers each time:.

The self parameter is size , an instance of DirectorySize. The obj parameter is either g or s , an instance of Directory.

The objtype parameter is the class Directory. A popular use for descriptors is managing access to instance data. The descriptor is assigned to a public attribute in the class dictionary while the actual data is stored as a private attribute in the instance dictionary. When the public attribute is accessed, the descriptor logs the lookup or update:. An interactive session shows that all access to the managed attribute age is logged, but that the regular attribute name is not logged:.

That means that each instance can only have one logged attribute and that its name is unchangeable. In this example, the Person class has two descriptor instances, name and age.

Here we call vars to look up the descriptor without triggering it:. This is only used in cases where a descriptor needs to know either the class where it was created or the name of class variable it was assigned to. This method, if present, is called even if the class is not a descriptor. Descriptors get invoked by the dot operator during attribute lookup. The main motivation for descriptors is to provide a hook allowing objects stored in class variables to control what happens during attribute lookup.

Traditionally, the calling class controls what happens during lookup. Descriptors invert that relationship and allow the data being looked-up to have a say in the matter.

Descriptors are used throughout the language. It is how functions turn into bound methods. Common tools like classmethod , staticmethod , property , and functools. In this example, we create a practical and powerful tool for locating notoriously hard to find data corruption bugs. A validator is a descriptor for managed attribute access. Prior to storing any data, it verifies that the new value meets various type and range restrictions. This Validator class is both an abstract base class and a managed attribute descriptor:.

Custom validators need to inherit from Validator and must supply a validate method to test various restrictions as needed. OneOf verifies that a value is one of a restricted set of options. Number verifies that a value is either an int or float. Optionally, it verifies that a value is between a given minimum or maximum. String verifies that a value is a str. Optionally, it validates a given minimum or maximum length.

To maintain backward compatibility, the argument order is somewhat odd; it is recommended to use keyword arguments for clarity. Return a file-like object that can be used as a temporary storage area. The file is created securely, using the same rules as mkstemp. It will be destroyed as soon as it is closed including an implicit close when the object is garbage collected.

Under Unix, the directory entry for the file is either not created at all or is removed immediately after the file is created. Other platforms do not support this; your code should not rely on a temporary file created using this function having or not having a visible name in the file system.

The resulting object can be used as a context manager see Examples. On completion of the context or destruction of the file object the temporary file will be removed from the filesystem. Binary mode is used so that it behaves consistently on all platforms without regard for the data that is stored. The dir , prefix and suffix parameters have the same meaning and defaults as with mkstemp. On other platforms, it is a file-like object whose file attribute is the underlying true file object.

The os. Raises an auditing event tempfile. Changed in version 3. This function operates exactly as TemporaryFile does, except that the file is guaranteed to have a visible name in the file system on Unix, the directory entry is not unlinked. That name can be retrieved from the name attribute of the returned file-like object. Whether the name can be used to open the file a second time, while the named temporary file is still open, varies across platforms it can be so used on Unix; it cannot on Windows NT or later.

If delete is true the default , the file is deleted as soon as it is closed. The returned object is always a file-like object whose file attribute is the underlying true file object. This file-like object can be used in a with statement, just like a normal file. The resulting file has one additional method, rollover , which causes the file to roll over to an on-disk file regardless of its size.

BytesIO or io. TextIOWrapper object depending on whether binary or text mode was specified or a true file object, depending on whether rollover has been called. This function securely creates a temporary directory using the same rules as mkdtemp. This PEP proposes to make all file descriptors created by Python non-inheritable by default to reduce the risk of these issues.

This PEP fixes also a race condition in multi-threaded applications on operating systems supporting atomic flags to create non-inheritable file descriptors.

We are aware of the code breakage this is likely to cause, and doing it anyway for the good of mankind. Details in the section "Backward Compatibility" below. Each operating system handles the inheritance of file descriptors differently. There is one exception: os. The reason is an implementation artifact: os. The issue proposes to change os. Using the spawnv function, all inheritable handles and all inheritable file descriptors are inherited in the child process.

So when at least one standard stream is replaced, all inheritable handles are inherited by the child process. Read Programmatically controlling which handles are inherited by new processes in Win32 Raymond Chen, Dec for more information. This option works if all other handles are non-inheritable. The Python issue "Add an atfork module" proposes to add such lock, it can be used to make handles non-inheritable without the race condition. Such lock only protects against a race condition between Python threads; C threads are not protected.

If the child program cannot be modified, an intermediate program can be used to steal handles from the parent process before spawning the final child program. The intermediate program has to pass the handle from the child process to the parent process.

The parent may have to close duplicated handles if all handles were not stolen, for example if the intermediate process fails. If the command line is used to pass the handle values, the command line must be modified when handles are duplicated, because their values are modified. This PEP does not include a solution to this problem because there is no perfect solution working on all Windows versions.

This point is deferred until use cases relying on handle or file descriptor inheritance on Windows are well known, so we can choose the best solution and carefully test its implementation. Ask Question. Asked 3 years, 4 months ago. Active 3 years, 4 months ago. Viewed times.

This is all on Linux and doesn't have to be portable in any way. You could use a pipe? Does the subprocess also insist on mmapping the file?

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