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Cython vs SWIG, Fight! by Mark Kohler

PyCon 2013
March 16, 2013
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Cython vs SWIG, Fight! by Mark Kohler

PyCon 2013

March 16, 2013
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Transcript

  1. Pre-fight • import statement • libadder • passing ints •

    passing structs • C strings • memory management • generalizations
  2. Code first and ask questions later 1.C code 2.SWIG code

    3.SWIG demonstration 4.Cython code 5.Cython demonstration
  3. We are Here • import statement • libadder • passing

    ints • passing structs • C strings • memory management • generalizations
  4. import this >>> import socket >>> import datetime >>> import

    time • What is socket.__file__? • What is datetime.__file__? • What is time.__file__?
  5. import datetime >>> datetime.__file__ '/usr/lib/python2.7/lib- dynload/datetime.so' >>> $ file /usr/lib/python2.7/lib-

    dynload/datetime.so ELF 64-bit LSB shared object, x86-64, dynamically linked $
  6. import time >>> time.__file__ Traceback (most recent call last): File

    "<stdin>", line 1, in <module> AttributeError: 'module' object has no attribute '__file__' >>>
  7. We are Here • import statement • libadder • passing

    ints • passing structs • C strings • memory management • generalizations
  8. We are Here • import statement • libadder • passing

    ints • passing structs • C strings • memory management • generalizations
  9. SWIG build diagram 1.Start with: adder.h, libadder.so 2.SWIG adder.h +

    adder.i --> adder_wrap.c + adder.py 3.Compile adder.h + adder-wrap.c --> adder_wrap.o 4.Link libadder.so + adder_wrap.o --> _adder.so
  10. Cython build diagram 1.Cython adder.h + c_adder.pxd + cy_adder.pyx -->

    cy_adder.c 2.Compile adder.h + cy_adder.c --> cy_adder.o 3.Link lib_adder.so + cy_adder.o --> cy_adder.so
  11. Cython build review 1.Given: adder.h, libadder.so 2.Cython adder.h + c_adder.pxd

    + cy_adder.pyx --> cy_adder.c 3.Compile adder.h + cy_adder.c --> cy_adder.o 4.Link libadder.so + cy_adder.o --> cy_adder.so
  12. We are Here • import statement • libadder • passing

    ints • passing structs • C strings • memory management • generalizations
  13. adder.h: pair_add() typedef struct _PAIR { int x; int y;

    } PAIR; int pair_add(PAIR * ppair);
  14. adder.i: pair_add() typedef struct _PAIR { int x; int y;

    } PAIR; int pair_add(PAIR * ppair);
  15. demo of SWIG's pair_add() >>> import adder >>> my_pair =

    adder.PAIR() >>> type(my_pair) <class 'adder.PAIR'> >>> my_pair.x = 3 >>> my_pair.y = 4 >>> adder.pair_add(my_pair) 7 >>>
  16. We are Here • import statement • libadder • passing

    ints • passing structs • C strings • memory management • generalizations
  17. demo of SWIG's get_version() >>> import adder >>> adder.get_version() 'v1.0'

    >>> adder.get_version().__class__ <type 'str'> >>>
  18. demo of Cython's get_version() >>> import cy_adder >>> cy_adder.get_version() 'v1.0'

    >>> cy_adder.get_version().__class__ <type 'str'> >>>
  19. Cython and C Strings "C strings are slow and cumbersome"

    "...avoid using C strings where possible" "...more likely to introduce bugs"
  20. SWIG and C Strings "The problems (and perils) of using

    char * are well-known. However, SWIG is not in the business of enforcing morality." SWIG documentation, Section 8.3 C String Handling
  21. We are Here • import statement • libadder • passing

    ints • passing structs • C strings • memory management • generalizations
  22. adder.c: sgreeting() static char hello[] = "Hello, "; int sgreeting(char

    * name, char * outp, int buflen) { if (outp && buflen) { if (buflen < (strlen(hello) + strlen(name) + 1)) { outp[0] = 0; return 0; } strcpy(outp, hello); strcat(outp, name); } return strlen(hello) + strlen(name); }
  23. cy_adder.pyx: sgreeting() def sgreeting(name): c_len = c_adder.sgreeting(name, <char * >

    0, 0) py_str = 'x' * (c_len + 1) cdef char * c_str = py_str c_adder.sgreeting(name, c_str, len(py_str)) return c_str
  24. We are Here • import statement • libadder • passing

    ints • passing structs • C strings • memory management • generalizations
  25. Cython Advantages • It's Python and it's C • explore

    performance trade-offs between C and Python