In addition to the detailed answer from @gall, I’ve discovered that intersphinx can also be run as a module:

python -m sphinx.ext.intersphinx 'http://python-eve.org/objects.inv'

This outputs nicely formatted info. For reference: https://github.com/sphinx-doc/sphinx/blob/master/sphinx/ext/intersphinx.py#L390

The accepted answer no longer works in the new version (1.5.x) …

import requests
import posixpath
from sphinx.ext.intersphinx import read_inventory

uri = 'http://docs.python.org/2.7/'

r = requests.get(uri + 'objects.inv', stream=True)
r.raise_for_status()

inv = read_inventory(r.raw, uri, posixpath.join)

How to use OpenCV 2.4 (cv2) intersphinx

Inspired by @Gall’s answer, I wanted to compare the contents of the OpenCV & numpy inventory files. I couldn’t get sphinx.ext.intersphinx.fetch_inventory to work from ipython, but the following does work:

curl http://docs.opencv.org/2.4/objects.inv | tail -n +5 | zlib-flate -uncompress > cv2.inv
curl https://docs.scipy.org/doc/numpy/objects.inv | tail -n +5 | zlib-flate -uncompress > numpy.inv

numpy.inv has lines like this:

numpy.ndarray py:class 1 reference/generated/numpy.ndarray.html#$ -

whereas cv2.inv has lines like this:

cv2.imread ocv:pyfunction 1 modules/highgui/doc/reading_and_writing_images_and_video.html#$ -

So presumably you’d link to the OpenCV docs with :ocv:pyfunction:`cv2.imread` instead of :py:function:`cv2.imread`. Sphinx doesn’t like it though:

WARNING: Unknown interpreted text role “ocv:pyfunction”.

A bit of Googling revealed that the OpenCV project has its own “ocv” sphinx domain: https://github.com/opencv/opencv/blob/2.4/doc/ocv.py — presumably because they need to document C, C++ and Python APIs all at the same time.

To use it, save ocv.py next to your Sphinx conf.py, and modify your conf.py:

sys.path.insert(0, os.path.abspath('.'))
import ocv
extensions = [
    'ocv',
]
intersphinx_mapping = {
    'cv2': ('http://docs.opencv.org/2.4/', None),
}

In your rst files you need to say :ocv:pyfunc:`cv2.imread` (not :ocv:pyfunction:).

Sphinx prints some warnings (unparseable C++ definition: u'cv2.imread') but the generated html documentation actually looks ok with a link to http://docs.opencv.org/2.4/modules/highgui/doc/reading_and_writing_images_and_video.html#cv2.imread. You can edit ocv.py and remove the line that prints that warning.

An additional way to inspect the objects.inv file is with the sphobjinv module.

You can search local or even remote inventory files (with fuzzy matching). For instance with scipy:

$ sphobjinv suggest -t 90 -u https://docs.scipy.org/doc/scipy/reference/objects.inv "signal.convolve2d"

Remote inventory found.

:py:function:`scipy.signal.convolve2d`
:std:doc:`generated/scipy.signal.convolve2d`

Note that you may need to use :py:func: and not :py:function: (I’d be happy to know why).

Stubborn fool that I am, I used 2to3 and the Sphinx deprecated APIs chart to revive @david-röthlisberger’s ocv.py-based answer so it’ll work with Sphinx 2.3 on Python 3.5.

The fixed-up version is here:

https://gist.github.com/ssokolow/a230b27b7ea4a31f7fb40621e6461f9a

…and the quick version of what I did was:

1. Run 2to3 -w ocv.py && rm ocv.py.bak
2. Cycle back and forth between running Sphinx and renaming functions to their replacements in the chart. I believe these were the only changes I had to make on this step:
   1. Directive now has to be imported from docutils.parsers.rst
   2. Replace calls to l_(...) with calls to _(...) and remove the l_ import.
3. Replace calls to env.warn with calls to log.warn where log = sphinx.util.logging.getLogger(__name__).

Then, you just pair it with this intersphinx definition and you get something still new enough to be relevant for most use cases:

'cv2': ('https://docs.opencv.org/3.0-last-rst/', None)

For convenience, I made a small extension for aliasing intersphinx cross references. This is useful as sometimes the object inventory gets confused when an object from a submodule is imported from a package’s __init__.py.

See also https://github.com/sphinx-doc/sphinx/issues/5603

###
# Workaround of
# Intersphinx references to objects imported at package level can"t be mapped.
#
# See https://github.com/sphinx-doc/sphinx/issues/5603

intersphinx_aliases = {
    ("py:class", "click.core.Group"):
        ("py:class", "click.Group"),
    ("py:class", "click.core.Command"):
        ("py:class", "click.Command"),
}


def add_intersphinx_aliases_to_inv(app):
    from sphinx.ext.intersphinx import InventoryAdapter
    inventories = InventoryAdapter(app.builder.env)

    for alias, target in app.config.intersphinx_aliases.items():
        alias_domain, alias_name = alias
        target_domain, target_name = target
        try:
            found = inventories.main_inventory[target_domain][target_name]
            try:
                inventories.main_inventory[alias_domain][alias_name] = found
            except KeyError:
                print("could not add to inv")
                continue
        except KeyError:
            print("missed :(")
            continue


def setup(app):
    app.add_config_value("intersphinx_aliases", {}, "env")
    app.connect("builder-inited", add_intersphinx_aliases_to_inv)

To use this, I paste the above code in my conf.py and add aliases to the intersphinx_aliases dictionary.