Is it possible to reuse hyperlink defined in another file in restructuredtext (or sphinx)

Suppose I have two files a.rst and b.rst in the same folder, and a.rst looks like this

.. _foo: http://stackoverflow.com

`foo`_ is a website

It seems using foo in b.rst is not allowed. Is there a way to define hyperlinks and use them in multiple files?

Followup

I used the extlinks extension as Steve Piercy suggested. Its implementation and docstring can be seen here on github.

In my case, I define wikipedia link in my conf.py

extlinks = {'wiki': ('https://en.wikipedia.org/wiki/%s', '')}

and in the .rst files, use them like

:wiki:`Einstein <Albert_Einstein>`

where Einstein will be displayed as a link to https://en.wikipedia.org/wiki/Albert_Einstein

Solution

There are at least four possible solutions.

1. repeat yourself

Put your complete reST in each file. You probably don't want that.

2. combined `rst_epilog` and substitution

This one is clever. Configure the rst_epilog value, in your conf.py along with a substition with the replace directive:

rst_epilog = """
.. |foo| replace:: foo
.. _foo: http://stackoverflow.com
"""

and reST:

|foo|_ is a website

yields:

<a class="reference external" href="http://stackoverflow.com">foo</a>

3. `extlinks`

For links to external websites where you want to have a base URL and append path segments or arguments, you can use extlinks in your conf.py:

extensions = [
...
    'sphinx.ext.extlinks',
...
]
...
extlinks = {'so': ('https://stackoverflow.com/%s', None)}

Then in your reST:

:so:`questions/49016433`

Yields:

<a class="reference external"
 href="https://stackoverflow.com/questions/49016433">
 https://stackoverflow.com/questions/49016433
</a>

4. `intersphinx`

For external websites that are documentation generated by Sphinx, then you can use intersphinx, in your conf.py:

extensions = [
...
    'sphinx.ext.intersphinx',
...
]
...
intersphinx_mapping = {
    'python': ('https://docs.python.org/3', None),
}