Little talk at Kiev-Py #10 Taras Lyapun

1. Cython
Close to the metal Python
Kyiv.py #10
Taras Lyapun

2. Python is a wonderful language that
allows us to solve problems quickly and
elegantly, but...

3. ...but nothing ideal in this world. As a
interpreted, dynamical language without
static typization, Python is slow.

4. How many times have you heard or said,
"We can always rewrite bottlenecks on
C"?

5. How can we do this?
- By hand using Python C API
- SWIG
- Boost.Python
- ctypes
- SIP, Py++, f2py, PyD, Interrogate, Robin, ...
- Cython

6. Cython
- Programming language based on Python
- The source code gets translated into
optimized C/C++ code and compiled as Python
extension modules
- Originally based on the well-known Pyrex
- This code is executed within the CPython
runtime environment, but at the speed of
compiled C and with the ability to call directly
into C libraries.

7. Install
pip install cython

8. Building with a distutils
from distutils.core import setup
from distutils.extension import Extension
from Cython.Distutils import build_ext
ext_modules = [Extension("hello", ["hello.pyx"])]
setup(
name = 'Hello world app',
cmdclass = {'build_ext': build_ext},
ext_modules = ext_modules
)
python setup.py build_ext --inplace

9. Building with pyximport
import pyximport
pyximport.install()

10. Building manually
cython path/to/your.pyx
gcc ...

11. Workflow
- Python code
- Profiling
- Unittests
- Cython code

12. Original

13. mv util.py util.pyx
x2 performance

14. cython -a util.pyx

15. Just add static typization
ncalls tottime percall cumtime percall filename:lineno(function)
8 0.010 0.001 0.010 0.001 {qrcode.util.lost_point}
x16 performance.
Easy achieve 100-1000x!

16. cython -a util.pyx

17. Overview
- Almost like Python (with few exceptions)
- top-level classes and functions
- loops, with, try-except/finally...
- lambda
- generators
- import
- Py3 support

18. Type declaration
- cdef - for static typization
cdef double dx, s
- cdef - as a C function
cdef double f(double x):
return sin(x**2)
- cdef class - extensions
cdef class MyType:
cdef int field
- cpdef - C function + Python wrapper

19. Classes
cdef class MyClass(SomeClass):
- "builtin" extension type
- single inheritance, only from other extension
types
- fixed, typed fields
- C-only access by default, or readonly/public
- Python + C methods

20. Pure python mode
@cython.ccall
@cython.returns(cython.double)
@cython.locals(x=cython.double, y=cython.
double)
def float_mult(x, y):
return x * y

21. Pure python mode
# test.py
def float_mult(x, y):
return x * y
# test.pxd
cpdef double float_mult(double x, double y)
(but no access to C functions)

22. Exceptions
cdef double f(double x) except -1:
cdef double f(double x) except? -1:
cdef int spam() except *:
cdef int spam() except +:

23. Exceptions
File "run.py", line 9, in <module>
"""
File "qrcode/main.py", line 7, in make
return qr.make_image()
File "qrcode/main.py", line 169, in make_image
self.make()
File "qrcode/main.py", line 53, in make
self.best_fit(start=self.version)
File "qrcode/main.py", line 108, in best_fit
self.error_correction, self.data_list)
File "util.pyx", line 401, in qrcode.util.create_data (qrcode/util.c:7844)
File "util.pyx", line 418, in qrcode.util.create_data (qrcode/util.c:7513)
qrcode.util.DataOverflowError: Code length overflow. Data size (916) > size
available (128)

24. Profiling
# cython: profile=True
@cython.profile(False)
cython -X profile=True ...

25. Also
- debugging with GDB
- Easy wrap C/C++ libraries
- Numpy, cmath, etc
- with nogil - release GIL
- cython.parallel - for native parallelism (OpenMP)
- and a lot of other cool hackers things

26. Thanks!
http://cython.org/