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8.5 KiB
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135 lines
8.5 KiB
Markdown
---
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title: Cython Architecture
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next: /api/cython-structs
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menu:
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- ['Overview', 'overview']
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- ['Conventions', 'conventions']
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---
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## Overview {#overview hidden="true"}
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> #### What's Cython?
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>
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> [Cython](http://cython.org/) is a language for writing C extensions for
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> Python. Most Python code is also valid Cython, but you can add type
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> declarations to get efficient memory-managed code just like C or C++.
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This section documents spaCy's C-level data structures and interfaces, intended
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for use from Cython. Some of the attributes are primarily for internal use, and
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all C-level functions and methods are designed for speed over safety – if you
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make a mistake and access an array out-of-bounds, the program may crash
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abruptly.
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With Cython there are four ways of declaring complex data types. Unfortunately
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we use all four in different places, as they all have different utility:
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| Declaration | Description | Example |
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| --------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ---------------------------------------------------------------------- |
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| `class` | A normal Python class. | [`Language`](/api/language) |
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| `cdef class` | 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. | [`Lexeme`](/api/cython-classes#lexeme) |
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| `cdef struct` | 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. | [`LexemeC`](/api/cython-structs#lexemec) |
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| `cdef cppclass` | 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. | [`StateC`](%%GITHUB_SPACY/spacy/pipeline/_parser_internals/_state.pxd) |
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The most important classes in spaCy are defined as `cdef class` objects. The
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underlying data for these objects is usually gathered into a struct, which is
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usually named `c`. For instance, the [`Lexeme`](/api/cython-classses#lexeme)
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class holds a [`LexemeC`](/api/cython-structs#lexemec) struct, at `Lexeme.c`.
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This lets you shed the Python container, and pass a pointer to the underlying
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data into C-level functions.
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## Conventions {#conventions}
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spaCy's core data structures are implemented as [Cython](http://cython.org/)
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`cdef` classes. Memory is managed through the
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[`cymem`](https://github.com/explosion/cymem) `cymem.Pool` class, which allows
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you to allocate memory which will be freed when the `Pool` object is garbage
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collected. This means you usually don't have to worry about freeing memory. You
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just have to decide which Python object owns the memory, and make it own the
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`Pool`. When that object goes out of scope, the memory will be freed. You do
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have to take care that no pointers outlive the object that owns them — but this
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is generally quite easy.
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All Cython modules should have the `# cython: infer_types=True` compiler
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directive at the top of the file. This makes the code much cleaner, as it avoids
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the need for many type declarations. If possible, you should prefer to declare
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your functions `nogil`, even if you don't especially care about multi-threading.
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The reason is that `nogil` functions help the Cython compiler reason about your
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code quite a lot — you're telling the compiler that no Python dynamics are
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possible. This lets many errors be raised, and ensures your function will run at
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C speed.
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Cython gives you many choices of sequences: you could have a Python list, a
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numpy array, a memory view, a C++ vector, or a pointer. Pointers are preferred,
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because they are fastest, have the most explicit semantics, and let the compiler
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check your code more strictly. C++ vectors are also great — but you should only
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use them internally in functions. It's less friendly to accept a vector as an
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argument, because that asks the user to do much more work. Here's how to get a
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pointer from a numpy array, memory view or vector:
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```python
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cdef void get_pointers(np.ndarray[int, mode='c'] numpy_array, vector[int] cpp_vector, int[::1] memory_view) nogil:
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pointer1 = <int*>numpy_array.data
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pointer2 = cpp_vector.data()
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pointer3 = &memory_view[0]
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```
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Both C arrays and C++ vectors reassure the compiler that no Python operations
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are possible on your variable. This is a big advantage: it lets the Cython
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compiler raise many more errors for you.
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When getting a pointer from a numpy array or memoryview, take care that the data
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is actually stored in C-contiguous order — otherwise you'll get a pointer to
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nonsense. The type-declarations in the code above should generate runtime errors
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if buffers with incorrect memory layouts are passed in. To iterate over the
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array, the following style is preferred:
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```python
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cdef int c_total(const int* int_array, int length) nogil:
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total = 0
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for item in int_array[:length]:
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total += item
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return total
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```
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If this is confusing, consider that the compiler couldn't deal with
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`for item in int_array:` — there's no length attached to a raw pointer, so how
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could we figure out where to stop? The length is provided in the slice notation
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as a solution to this. Note that we don't have to declare the type of `item` in
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the code above — the compiler can easily infer it. This gives us tidy code that
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looks quite like Python, but is exactly as fast as C — because we've made sure
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the compilation to C is trivial.
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Your functions cannot be declared `nogil` if they need to create Python objects
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or call Python functions. This is perfectly okay — you shouldn't torture your
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code just to get `nogil` functions. However, if your function isn't `nogil`, you
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should compile your module with `cython -a --cplus my_module.pyx` and open the
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resulting `my_module.html` file in a browser. This will let you see how Cython
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is compiling your code. Calls into the Python run-time will be in bright yellow.
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This lets you easily see whether Cython is able to correctly type your code, or
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whether there are unexpected problems.
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Working in Cython is very rewarding once you're over the initial learning curve.
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As with C and C++, the first way you write something in Cython will often be the
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performance-optimal approach. In contrast, Python optimization generally
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requires a lot of experimentation. Is it faster to have an `if item in my_dict`
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check, or to use `.get()`? What about `try`/`except`? Does this numpy operation
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create a copy? There's no way to guess the answers to these questions, and
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you'll usually be dissatisfied with your results — so there's no way to know
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when to stop this process. In the worst case, you'll make a mess that invites
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the next reader to try their luck too. This is like one of those
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[volcanic gas-traps](http://www.wemjournal.org/article/S1080-6032%2809%2970088-2/abstract),
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where the rescuers keep passing out from low oxygen, causing another rescuer to
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follow — only to succumb themselves. In short, just say no to optimizing your
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Python. If it's not fast enough the first time, just switch to Cython.
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<Infobox title="Resources" emoji="📖">
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- [Official Cython documentation](http://docs.cython.org/en/latest/)
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(cython.org)
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- [Writing C in Cython](https://explosion.ai/blog/writing-c-in-cython)
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(explosion.ai)
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- [Multi-threading spaCy’s parser and named entity recognizer](https://explosion.ai/blog/multithreading-with-cython)
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(explosion.ai)
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</Infobox>
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