# 14.9 `imghdr` -- Determine the type of an image

The `imghdr` module determines the type of image contained in a file or byte stream.

The `imghdr` module defines the following function:

`what`( | filename[, h])

  Tests the image data contained in the file named by filename, and returns a string describing the image type. If optional h is provided, the filename is ignored and h is assumed to contain the byte stream to test.

The following image types are recognized, as listed below with the return value from `what()`:

Value | Image format
--- | ---
`'rgb'` | SGI ImgLib Files
`'gif'` | GIF 87a and 89a Files
`'pbm'` | Portable Bitmap Files
`'pgm'` | Portable Graymap Files
`'ppm'` | Portable Pixmap Files
`'tiff'` | TIFF Files
`'rast'` | Sun Raster Files
`'xbm'` | X Bitmap Files
`'jpeg'` | JPEG data in JFIF format
`'bmp'` | BMP files
`'png'` | Portable Network Graphics

You can extend the list of file types `imghdr` can recognize by appending to this variable:

`tests`
  A list of functions performing the individual tests. Each function takes two arguments: the byte-stream and an open file-like object. When `what()` is called with a byte-stream, the file-like object will be `None`.

  The test function should return a string describing the image type if the test succeeded, or `None` if it failed.

Example:

    >>> import imghdr
    >>> imghdr.what('/tmp/bass.gif')
    'gif'