Modules written in Cython usually comes with a
setup.py script that compiles Cython source codes into native shared libary. For whom not so familiar with Python’s packaging and distributing toolchains, such step is sometimes scary, and turns out to be a stumbling block for Cython freshmen. Moreover, the workflow, “run setup.py -> debug -> edit .pyx files -> run setup.py”, is also less convenient and troublesome for fast iterating projects.
pyximport is a handy tool from Cython official, provided to address the above problem. The module enables users to “directly import”
.pyx files, with no explicit
setup.py required. Let’s start from an example here. Say we have two files residing in the same directory:
# hl: begin
# hl: end
cpdef int sqr(int x):
return x * x
highlighted line registers some import hooks to let Python recognize
.pyx files. When the
.pyx files imported for the first time or modified later,
pyximport compiles or re-compiles them behind the scene automatically.
By default, the built shared libaries are placed at
~/.pyxbld/, which can be overrided with the
build_dir argument of
pyximport.install. One may also pass
inplace=True to place the libraries sibling to their source files. For example:
# hl: begin
# hl: end
will generate a
.pyd in Windows) file in the same directory.
But there’s a problem with inplace building – the extension will not be automatically rebuilt. If you modify the
.pyx file and re-run the program, the compilation process will not be triggered. The reason is that Python itself is able to recognize
.pyd modules, and their importers have higher priority than customized ones. Thus, if there’s an
.so and a
.pyx file with same module names reside in the same directory, the
.so file will be imported instead of
Therefore, if you want the compiled binaries be placed within your project directory, the best practice is to specify the
build_dir argument. By setting
build_dir to somewhere inside your project, compiled libraries will be kept within your project directory, but at the same time you can still enjoy the automatic rebuild service.