//- 💫 DOCS > API > CYTHON > ARCHITECTURE

include ../_includes/_mixins

+section("overview")
    +aside("What's Cython?")
        |  #[+a("http://cython.org/") Cython] is a language for writing
        |  C extensions for Python. Most Python code is also valid Cython, but
        |  you can add type declarations to get efficient memory-managed code
        |  just like C or C++.

    p
        |  This section documents spaCy's C-level data structures and
        |  interfaces, intended for use from Cython. Some of the attributes are
        |  primarily for internal use, and all C-level functions and methods are
        |  designed for speed over safety – if you make a mistake and access an
        |  array out-of-bounds, the program may crash abruptly.

    p
        |  With Cython there are four ways of declaring complex data types.
        |  Unfortunately we use all four in different places, as they all have
        |  different utility:

    +table(["Declaration", "Description", "Example"])
        +row
            +cell #[code class]
            +cell A normal Python class.
            +cell #[+api("language") #[code Language]]

        +row
            +cell #[code cdef class]
            +cell
                |  A Python extension type. Differs from a normal Python class
                |  in that its attributes can be defined on the underlying
                |  struct. Can have C-level objects as attributes (notably
                |  structs and pointers), and can have methods which have
                |  C-level objects as arguments or return types.
            +cell #[+api("cython-classes#lexeme") #[code Lexeme]]

        +row
            +cell #[code cdef struct]
            +cell
                |  A struct is just a collection of variables, sort of like a
                |  named tuple, except the memory is contiguous. Structs can't
                |  have methods, only attributes.
            +cell #[+api("cython-structs#lexemec") #[code LexemeC]]

        +row
            +cell #[code cdef cppclass]
            +cell
                |  A C++ class. Like a struct, this can be allocated on the
                |  stack, but can have methods, a constructor and a destructor.
                |  Differs from `cdef class` in that it can be created and
                |  destroyed without acquiring the Python global interpreter
                |  lock. This style is the most obscure.
            +cell #[+src(gh("spacy", "spacy/syntax/_state.pxd")) #[code StateC]]

    p
        |  The most important classes in spaCy are defined as #[code cdef class]
        |  objects. The underlying data for these objects is usually gathered
        |  into a struct, which is usually named #[code c]. For instance, the
        |  #[+api("cython-classses#lexeme") #[code Lexeme]] class holds a
        |  #[+api("cython-structs#lexemec") #[code LexemeC]] struct, at
        |  #[code Lexeme.c]. This lets you shed the Python container, and pass
        |  a pointer to the underlying data into C-level functions.

+section("conventions")
    +h(2, "conventions") Conventions

    p
        |  spaCy's core data structures are implemented as
        |  #[+a("http://cython.org/") Cython] #[code cdef] classes. Memory is
        |  managed through the #[+a(gh("cymem")) #[code cymem]]
        |  #[code cymem.Pool] class, which allows you
        |  to allocate memory which will be freed when the #[code Pool] object
        |  is garbage collected. This means you usually don't have to worry
        |  about freeing memory. You just have to decide which Python object
        |  owns the memory, and make it own the #[code Pool]. When that object
        |  goes out of scope, the memory will be freed. You do have to take
        |  care that no pointers outlive the object that owns them — but this
        |  is generally quite easy.

    p
        |  All Cython modules should have the #[code # cython: infer_types=True]
        |  compiler directive at the top of the file. This makes the code much
        |  cleaner, as it avoids the need for many type declarations. If
        |  possible, you should prefer to declare your functions #[code nogil],
        |  even if you don't especially care about multi-threading. The reason
        |  is that #[code nogil] functions help the Cython compiler reason about
        |  your code quite a lot — you're telling the compiler that no Python
        |  dynamics are possible. This lets many errors be raised, and ensures
        |  your function will run at C speed.


    p
        |  Cython gives you many choices of sequences: you could have a Python
        |  list, a numpy array, a memory view, a C++ vector, or a pointer.
        |  Pointers are preferred, because they are fastest, have the most
        |  explicit semantics, and let the compiler check your code more
        |  strictly. C++ vectors are also great — but you should only use them
        |  internally in functions. It's less friendly to accept a vector as an
        |  argument, because that asks the user to do much more work. Here's
        |  how to get a pointer from a numpy array, memory view or vector:

    +code.
        cdef void get_pointers(np.ndarray[int, mode='c'] numpy_array, vector[int] cpp_vector, int[::1] memory_view) nogil:
        pointer1 = <int*>numpy_array.data
        pointer2 = cpp_vector.data()
        pointer3 = &memory_view[0]

    p
        |  Both C arrays and C++ vectors reassure the compiler that no Python
        |  operations are possible on your variable. This is a big advantage:
        |  it lets the Cython compiler raise many more errors for you.

    p
        |  When getting a pointer from a numpy array or memoryview, take care
        |  that the data is actually stored in C-contiguous order — otherwise
        |  you'll get a pointer to nonsense. The type-declarations in the code
        |  above should generate runtime errors if buffers with incorrect
        |  memory layouts are passed in. To iterate over the array, the
        |  following style is preferred:

    +code.
        cdef int c_total(const int* int_array, int length) nogil:
            total = 0
            for item in int_array[:length]:
                total += item
            return total

    p
        |  If this is confusing, consider that the compiler couldn't deal with
        |  #[code for item in int_array:] — there's no length attached to a raw
        |  pointer, so how could we figure out where to stop? The length is
        |  provided in the slice notation as a solution to this. Note that we
        |  don't have to declare the type of #[code item] in the code above —
        |  the compiler can easily infer it. This gives us tidy code that looks
        |  quite like Python, but is exactly as fast as C — because we've made
        |  sure the compilation to C is trivial.

    p
        |  Your functions cannot be declared #[code nogil] if they need to
        |  create Python objects or call Python functions. This is perfectly
        |  okay — you shouldn't torture your code just to get #[code nogil]
        |  functions. However, if your function isn't #[code nogil], you should
        |  compile your module with #[code cython -a --cplus my_module.pyx] and
        |  open the resulting #[code my_module.html] file in a browser. This
        |  will let you see how Cython is compiling your code. Calls into the
        |  Python run-time will be in bright yellow. This lets you easily see
        |  whether Cython is able to correctly type your code, or whether there
        |  are unexpected problems.

    p
        |  Working in Cython is very rewarding once you're over the initial
        |  learning curve. As with C and C++, the first way you write something
        |  in Cython will often be the performance-optimal approach. In
        |  contrast, Python optimisation generally requires a lot of
        |  experimentation. Is it faster to have an #[code if item in my_dict]
        |  check, or to use #[code .get()]? What about
        |  #[code try]/#[code except]? Does this numpy operation create a copy?
        |  There's no way to guess the answers to these questions, and you'll
        |  usually be dissatisfied with your results — so there's no way to
        |  know when to stop this process. In the worst case, you'll make a
        |  mess that invites the next reader to try their luck too. This is
        |  like one of those
        |  #[+a("http://www.wemjournal.org/article/S1080-6032%2809%2970088-2/abstract") volcanic gas-traps],
        |  where the rescuers keep passing out from low oxygen, causing
        |  another rescuer to follow — only to succumb themselves. In short,
        |  just say no to optimizing your Python. If it's not fast enough the
        |  first time, just switch to Cython.

    +infobox("Resources")
        +list.o-no-block
            +item #[+a("http://docs.cython.org/en/latest/") Official Cython documentation] (cython.org)
            +item #[+a("https://explosion.ai/blog/writing-c-in-cython", true) Writing C in Cython] (explosion.ai)
            +item #[+a("https://explosion.ai/blog/multithreading-with-cython") Multi-threading spaCy’s parser and named entity recogniser] (explosion.ai)