Importing module functions from python packages
Question: I saw the command listed as below for `sklearn.tree.tree`

    
    
    from ._tree import Criterion, Splitter, Tree
    from . import _tree
    

Yet in the same tree folder I cannot find any file or class named `_tree`. Can
anyone tell me where exactly I can find this class?


Answer: The module you are looking for is written in Cython. The corresponding file is
called `_tree.pyx`. It can be found in `.../scikit-
learn/sklearn/tree/_tree.pyx`, if you have the scikit learn sources, e.g. in
form of the git repo, on your computer.

Cython is translated to C code, which can be found in `_tree.c`.

The compiled C code is what is imported in the lines you found, and the
corresponding file is called `_tree.so`. In a typical scikit-learn
installation, this may be the only file you find. It is not human-readable, so
if you are interested in the source, check it out
[here](https://github.com/scikit-learn/scikit-
learn/blob/master/sklearn/tree/_tree.pyx)

