[Python-Dev] The os module, unix and win32 (original) (raw)

Thomas Heller theller at python.net
Fri Jan 9 12:04:35 EST 2004

Guido van Rossum <guido at python.org> writes:

[Dave]

>> Another possibility: we'd get a lot more mileage out of simply >> adding ctypes to the standard install. That would solve the >> immediate popen problem, let us get rid of winreg entirely (and >> replace it with a pure Python version), and make future Win32 >> problems easier to solve. [Guido] > I don't know that a Python version of winreg using ctypes would be > preferable over a C version. Since there is a C version, there's no need to code it in Python again. > I'd expect it to be slower, less readable than the C version, and more > susceptible to the possibility of causing segfaults. Slower: yes, but I don't know how much. Less readable: no, more readable. Susceptible to segfaults: If written correctly, it should be bullet proof - exactly the same as a C version. But the version that was referenced here before isn't bulletproof, right? Wouldn't the bullet-proofing reduce the readability?

Here is a small part a Dave's code:

def CreateKey(baseKey, subKey): 'Creates/opens the given key and returns it' RCK = windll.advapi32.RegCreateKeyExA key = c_int(0) RCK(baseKey, subKey, 0, 0, REG_OPTION_NON_VOLATILE, KEY_ALL_ACCESS, 0, byref(key), 0) return key.value

He doesn't check that baseKey and subKey are strings, and he doesn't check the return value of the function call.

There are several ways to fix these problems. A simple one would be to write this instead:

def CreateKey(baseKey, subKey): 'Creates/opens the given key and returns it' RCK = windll.advapi32.RegCreateKeyExA key = c_int(0) result = RCK(str(baseKey), str(subKey), 0, 0, REG_OPTION_NON_VOLATILE, KEY_ALL_ACCESS, 0, byref(key), 0): raise WindowsError(result) return key.value

But ctypes supports a kind of 'function prototypes' (which do automatic argument type checking and/or conversion= as well as automatic result checking:

def CheckNonNull(errcode): # if errcode is nonzero, raise a WindowsError if errcode: raise WindowsError(errcode)

RCK = windll.advapi32.RegCreateKeyExA

specify the argument types

RCK.argtypes = (HKEY, LPCTSTR, DWORD, REGSAM, LPSECURITY_ATTRIBUTES, PMKEY, LPDWROD)

RegCreateKeyExA returns 0 on success, or a windows error code

RCK.restype = CheckNonNull

def CreateKey(baseKey, subKey): 'Creates/opens the given key and returns it' key = c_int(0) RCK(baseKey, subKey, 0, 0, REG_OPTION_NON_VOLATILE, KEY_ALL_ACCESS, 0, byref(key), 0) return key.value

> That doesn't mean I don't think ctypes is a good idea -- just that I > don't think applying it to winreg would be useful.

Cool. Should this be a Windows only version, or cross-platform? I don't know enough about ctypes and its user community to answer that (I doubt I'd have much direct need for it myself). But in general I'm biased towards cross-platform tools.

I had reports that it works on Solaris, Linux, MacOS, BSD. Maybe more systems.

The problem is that the non-windows version uses libffi, which is difficult to find and install - it seems to be maintained now as part of gcc, although the license is more BSD like.

I would like to get rid of libffi - but the only multi-architecture alternative I know of is Bruno Haible's ffcall (which is GPL).

There may be other options (including writing assembly code). Sam Rushing's calldll did this, AFAIK.

And, remember: you can write bulletproof code with ctypes, but you can also easily crash Python. Or other weird things.

Thomas

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