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# `sphinx.ext.intersphinx` – Link to other projects’ documentation

New in version 0.5.

This extension can generate automatic links to the documentation of objects in other projects.

Usage is simple: whenever Sphinx encounters a cross-reference that has no matching target in the current documentation set, it looks for targets in the documentation sets configured in `intersphinx_mapping`. A reference like `:py:class:`zipfile.ZipFile`` can then link to the Python documentation for the ZipFile class, without you having to specify where it is located exactly.

When using the “new” format (see below), you can even force lookup in a foreign set by prefixing the link target appropriately. A link like `:ref:`comparison manual <python:comparisons>`` will then link to the label “comparisons” in the doc set “python”, if it exists.

Behind the scenes, this works as follows:

- Each Sphinx HTML build creates a file named `objects.inv` that contains a mapping from object names to URIs relative to the HTML set’s root.
- Projects using the Intersphinx extension can specify the location of such mapping files in the `intersphinx_mapping` config value. The mapping will then be used to resolve otherwise missing references to objects into links to the other documentation.
- By default, the mapping file is assumed to be at the same location as the rest of the documentation; however, the location of the mapping file can also be specified individually, e.g. if the docs should be buildable without Internet access.

To use intersphinx linking, add `'sphinx.ext.intersphinx'` to your `extensions` config value, and use these new config values to activate linking:

`intersphinx_mapping`

  This config value contains the locations and names of other projects that should be linked to in this documentation.

  Relative local paths for target locations are taken as relative to the base of the built documentation, while relative local paths for inventory locations are taken as relative to the source directory.

  When fetching remote inventory files, proxy settings will be read from the `$HTTP_PROXY` environment variable.

  Old format for this config value

  This is the format used before Sphinx 1.0. It is still recognized.

  A dictionary mapping URIs to either `None` or an URI. The keys are the base URI of the foreign Sphinx documentation sets and can be local paths or HTTP URIs. The values indicate where the inventory file can be found: they can be `None` (at the same location as the base URI) or another local or HTTP URI.

  New format for this config value

  New in version 1.0.

  A dictionary mapping unique identifiers to a tuple `(target, inventory)`. Each `target` is the base URI of a foreign Sphinx documentation set and can be a local path or an HTTP URI. The `inventory` indicates where the inventory file can be found: it can be `None` (at the same location as the base URI) or another local or HTTP URI.

  The unique identifier can be used to prefix cross-reference targets, so that it is clear which intersphinx set the target belongs to. A link like `:ref:`comparison manual <python:comparisons>`` will link to the label “comparisons” in the doc set “python”, if it exists.

  Example

  To add links to modules and objects in the Python standard library documentation, use:

      intersphinx_mapping = {'python': ('https://docs.python.org/3.4', None)}

  This will download the corresponding `objects.inv` file from the Internet and generate links to the pages under the given URI. The downloaded inventory is cached in the Sphinx environment, so it must be re-downloaded whenever you do a full rebuild.

  A second example, showing the meaning of a non-`None` value of the second tuple item:

      intersphinx_mapping = {'python': ('https://docs.python.org/3.4',
                                        'python-inv.txt')}

  This will read the inventory from `python-inv.txt` in the source directory, but still generate links to the pages under `https://docs.python.org/3.4`. It is up to you to update the inventory file as new objects are added to the Python documentation.

  Multiple target for the inventory

  New in version 1.3.

  Alternative files can be specified for each inventory. One can give a tuple for the second inventory tuple item as shown in the following example. This will read the inventory iterating through the (second) tuple items until the first successful fetch. The primary use case for this to specify mirror sites for server downtime of the primary inventory:

      intersphinx_mapping = {'python': ('https://docs.python.org/3.4',
                                        (None, 'python-inv.txt'))}

`intersphinx_cache_limit`

  The maximum number of days to cache remote inventories. The default is `5`, meaning five days. Set this to a negative value to cache inventories for unlimited time.