**cython - 1/1** Blog machine_learning (5)

# cython - 1/1¶

## Nogil, numpy, cython¶

2019-03-25

I had to implement a custom criterion to optimize
a decision tree and I wanted to leverage scikit-learn
instead of rewriting my own. Version 0.21 of scikit-learn
introduced some changed in the API which make possible
to overload an existing criterion and replace some of the logic
by another one: _criterion.pyx.
The purpose was to show that a fast implementation requires
some tricks (see Custom DecisionTreeRegressor adapted to a linear regression) and
piecewise_tree_regression_criterion.pyx,
piecewise_tree_regression_criterion_fast.pyx
for the code. Other than that, every function to overlaod is marked as
nogil. Every function or method marked as *nogil* cannot
go through the GIL (see also PEP-0311),
which no python object can be created in that method.
In fact, no python can be called inside a Cython
method protected with *nogil*. The issue with that is that
any numpy method cannot be called.

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