6. I/O Plugin Infrastructure

A plugin consists of two files, the source and the descriptor .ini. Let’s say we’d like to provide a plugin for imshow using matplotlib. We’ll call our plugin mpl:

skimage/io/_plugins/mpl.py
skimage/io/_plugins/mpl.ini

The name of the .py and .ini files must correspond. Inside the .ini file, we give the plugin meta-data:

[mpl] <-- name of the plugin, may be anything
description = Matplotlib image I/O plugin
provides = imshow <-- a comma-separated list, one or more of
                      imshow, imsave, imread, _app_show

The “provides”-line lists all the functions provided by the plugin. Since our plugin provides imshow, we have to define it inside mpl.py:

# This is mpl.py

import matplotlib.pyplot as plt

def imshow(img):
    plt.imshow(img)

Note that, by default, imshow is non-blocking, so a special function _app_show must be provided to block the GUI. We can modify our plugin to provide it as follows:

[mpl]
provides = imshow, _app_show

# This is mpl.py

import matplotlib.pyplot as plt

def imshow(img):
    plt.imshow(img)

def _app_show():
    plt.show()

Any plugin in the _plugins directory is automatically examined by skimage.io upon import. You may list all the plugins on your system:

>>> import skimage.io as io
>>> io.find_available_plugins()
{'gtk': ['imshow'],
 'matplotlib': ['imshow', 'imread', 'imread_collection'],
 'pil': ['imread', 'imsave', 'imread_collection'],
 'test': ['imsave', 'imshow', 'imread', 'imread_collection'],}

or only those already loaded:

>>> io.find_available_plugins(loaded=True)
{'matplotlib': ['imshow', 'imread', 'imread_collection'],
 'pil': ['imread', 'imsave', 'imread_collection']}

A plugin is loaded using the use_plugin command:

>>> import skimage.io as io
>>> io.use_plugin('pil') # Use all capabilities provided by PIL

or

>>> io.use_plugin('pil', 'imread') # Use only the imread capability of PIL

Note that, if more than one plugin provides certain functionality, the last plugin loaded is used.

To query a plugin’s capabilities, use plugin_info:

>>> io.plugin_info('pil')
>>>
{'description': 'Image reading via the Python Imaging Library',
 'provides': 'imread, imsave'}