Can I browse Cython Docs within the notebook()?

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You can find Cython under the site-packages/ directory of the version of Python that is shipped with Sage. For example:

[mvngu@sage sage-4.5.3.alpha2]$ ls local/lib/python/site-packages/Cython
CodeWriter.py   Debugging.pyc  __init__.pyc  Shadow.pyc        TestUtils.py
CodeWriter.pyc  Distutils      Plex          StringIOTree.py   TestUtils.pyc
Compiler        Includes       Runtime       StringIOTree.pyc  Utils.py
Debugging.py    __init__.py    Shadow.py     Tests             Utils.pyc

Tucked in the recesses of the Sage documentation are the commands search_src, search_def, and search_doc. They take a string as argument and return all the files containing that string (well, one searches docstrings, one searches definitions, and the other searches all source code; you can probably guess which is which). So in addition to the other answers here, these commands can give you an idea of _which_ file to look at.

In a source distribution of Sage, you also get the source of all standard packages in Sage. Standard packages reside under SAGE_ROOT/spkg/standard. You should be able to uncompress each spkg using tar and bunzip2, then hunt around an uncompressed spkg for the documentation of that upstream package. For example, here is a way to read the documentation of Cython:

[mvngu@sage mvngu]$ ls
dot_sage  sage-4.5.3.alpha2
[mvngu@sage mvngu]$ cd sage-4.5.3.alpha2/spkg/standard/
[mvngu@sage standard]$ tar -jxf cython-0.12.1.spkg 
[mvngu@sage standard]$ cd cython-0.12.1/src/Doc/
[mvngu@sage Doc]$ ls
About.html            FAQ.html    overview.html  sharing.html
extension_types.html  index.html  primes.c       special_methods.html

Once uncompressed, you could also read the source of the upstream package.