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# I/O Typing (`astropy.io.typing`)

`astropy.io` provides type annotations through the `astropy.io.typing` module. These type annotations allow users to specify the expected types of variables, function parameters, and return values when working with I/O. By using type annotations, developers can improve code readability, catch potential type-related errors early, and enable better code documentation and tooling support.

For example, the following function uses type annotations to specify that the `filename` parameter can be any type of path-like object (e.g. a string, byte-string, or pathlib.Path object).

    from astropy.io import fits
    from astropy.io.typing import PathLike

    def read_fits_file(filename: PathLike) -> fits.HDUList:
         return fits.open(filename)

The `astropy.io.typing` module also provides type aliases for file-like objects that support reading and writing. The following example uses the `ReadableFileLike` type alias to specify that the `fileobj` parameter can be any file-like object that supports reading. Using a `TypeVar`, the return type of the function is specified to be the same type as the file-like object can read.

    from typing import TypeVar
    from astropy.io.typing import ReadableFileLike

    R = TypeVar('R')  # type of object returned by fileobj.read()

    def read_from_file(fileobj: ReadableFileLike[R]) -> R:
         """Reads from a file-like object and returns the result."""
         return fileobj.read()

## Reference/API

### astropy.io.typing Module

Type annotations for `astropy.io`.

These are type annotations for I/O-related functions and classes. Some of the type objects can also be used as runtime-checkable `Protocol` objects.

#### Classes

`ReadableFileLike`(*args, **kwargs) | A file-like object that supports reading with a method `read`.
`WriteableFileLike`(*args, **kwargs) | A file-like object that supports writing with a method `write`.

#### Class Inheritance Diagram