I was writing some Python code that uses [ctypes][] for interfacing with the [Windows ToolHelp API](https://docs.microsoft.com/en-us/windows/win32/api/_toolhelp/). Specifically, I had to [define a Python equivalent][structs] for the [PROCESSENTRY32](https://docs.microsoft.com/en-us/windows/win32/api/tlhelp32/ns-tlhelp32-processentry32) struct. Now, the [ctypes][] [struct definitions][structs] are a little annoying because they do not give you type hints. You can add the type hints _on top_ of the `_fields_` attribute but that looks a little silly because you _literally_ [write everything twice][wet]. In Python 3.7+ ((Starting in Python 3.7, dictionaries keep the insertion order.)), you can solve this using a metaclass, and one stigma of metaclasses is that they have very few applications, so I decided to blog about this one:
```python
class FieldsFromTypeHints(type(ctypes.Structure)):
def __new__(cls, name, bases, namespace):
from typing import get_type_hints
class AnnotationDummy:
__annotations__ = namespace.get('__annotations__', {})
annotations = get_type_hints(AnnotationDummy)
namespace['_fields_'] = list(annotations.items())
return type(ctypes.Structure).__new__(cls, name, bases, namespace)
```
and now you can write:
```python
class PROCESSENTRY32(ctypes.Structure, metaclass=FieldsFromTypeHints):
dwSize : ctypes.c_uint32
cntUsage : ctypes.c_uint32
th32ProcessID : ctypes.c_uint32
th32DefaultHeapID : ctypes.POINTER(ctypes.c_ulong)
th32ModuleID : ctypes.c_uint32
cntThreads : ctypes.c_uint32
th32ParentProcessID : ctypes.c_uint32
pcPriClassBase : ctypes.c_long
dwFlags : ctypes.c_uint32
szExeFile : ctypes.c_char * 260
```
The metaclass simply deduces the `_fields_` attribute of the new class for you before the class is _even created_. It might make sense to think of metaclasses as "class creation hooks", i.e. you get to modify what you have written before the class is actually being defined. The following bit is just to be compatible with [PEP-563](https://www.python.org/dev/peps/pep-0563/):
```python
from typing import get_type_hints
class AnnotationDummy:
__annotations__ = namespace.get('__annotations__', {})
annotations = get_type_hints(AnnotationDummy)
```
And then, the following is the actual magic line:
```python
namespace['_fields_'] = list(annotations.items())
```
This adds the attribute `_fields_` before the class is created.
[ctypes]: https://docs.python.org/3/library/ctypes.html
[structs]: https://docs.python.org/3/library/ctypes.html#structures-and-unions
[wet]: https://en.wikipedia.org/wiki/Don%27t_repeat_yourself
<a href="https://blag.nullteilerfrei.de/2021/06/20/prettier-struct-definitions-for-python-ctypes/#more-5700" class="more-link">Do you want to know what I was coding?</a>