[issue1962] ctypes feature request: Automatic type conversion of input arguments to C functions

[Amaury Forgeot d'Arc]

Changes by Amaury Forgeot d'Arc amaur...@gmail.com:


--
resolution:  - works for me
status: open - closed

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[issue1962] ctypes feature request: Automatic type conversion of input arguments to C functions

[Amaury Forgeot d'Arc]

Amaury Forgeot d'Arc amaur...@gmail.com added the comment:

I don't think this should happen by default.
but what the user wants is already possible, by using the from_param() method.  
For example, the AutoStrParam type converts everything to a string (and a 
char*):

from ctypes import *

class AutoStrParam(c_char_p):
@classmethod
def from_param(cls, value):
return str(value)

strlen = cdll.LoadLibrary('msvcrt').strlen
strlen.argtypes = [AutoStrParam]

print strlen(None) # None  -  4
print strlen(type) # type 'type' - 13

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[issue1962] ctypes feature request: Automatic type conversion of input arguments to C functions

[Mark Lawrence]

Mark Lawrence breamore...@yahoo.co.uk added the comment:

Please ensure tha I'm taken off of the email list as I've been banned from 
contributing to Python by Raymond Hettinger


From: Amaury Forgeot d'Arc rep...@bugs.python.org
To: breamore...@yahoo.co.uk
Sent: Tue, 21 September, 2010 18:56:51
Subject: [issue1962] ctypes feature request: Automatic type conversion of input 
arguments to C functions

Amaury Forgeot d'Arc amaur...@gmail.com added the comment:

I don't think this should happen by default.
but what the user wants is already possible, by using the from_param() method.  
For example, the AutoStrParam type converts everything to a string (and a 
char*):

from ctypes import *

class AutoStrParam(c_char_p):
@classmethod
def from_param(cls, value):
return str(value)

strlen = cdll.LoadLibrary('msvcrt').strlen
strlen.argtypes = [AutoStrParam]

print strlen(None) # None  -  4
print strlen(type) # type 'type' - 13

--
nosy: +amaury.forgeotdarc

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Added file: http://bugs.python.org/file18958/unnamed

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___htmlheadstyle type=text/css!-- DIV {margin:0px;} 
--/style/headbodydiv style=font-family:times new roman,new 
york,times,serif;font-size:12ptdivPlease ensure tha I'm taken off of the 
email list as I've been banned from contributing to Python by Raymond 
Hettingerbr/divdiv style=font-family: times new roman,new 
york,times,serif; font-size: 12pt;brdiv style=font-family: 
arial,helvetica,sans-serif; font-size: 13px;font face=Tahoma size=2hr 
size=1bspan style=font-weight: bold;From:/span/b Amaury Forgeot 
d'Arc lt;rep...@bugs.python.orggt;brbspan style=font-weight: 
bold;To:/span/b breamore...@yahoo.co.ukbrbspan style=font-weight: 
bold;Sent:/span/b Tue, 21 September, 2010 18:56:51brbspan 
style=font-weight: bold;Subject:/span/b [issue1962] ctypes feature 
request: Automatic type conversion of input arguments to C 
functionsbr/fontbrbrAmaury Forgeot d'Arc lt;a
 ymailto=mailto:amaur...@gmail.com; 
href=mailto:amaur...@gmail.com;amaur...@gmail.com/agt; added the 
comment:brbrI don't think this should happen by default.brbut what the 
user wants is already possible, by using the from_param() method.nbsp; For 
example, the AutoStrParam type converts everything to a string (and a 
char*):brbrfrom ctypes import *brbrclass 
AutoStrParam(c_char_p):brnbsp; nbsp; @classmethodbrnbsp; nbsp; def 
from_param(cls, value):brnbsp; nbsp; nbsp; nbsp; return 
str(value)brbrstrlen = cdll.LoadLibrary('msvcrt').strlenbrstrlen.argtypes 
= [AutoStrParam]brbrprint strlen(None)nbsp; nbsp;  # Nonenbsp; nbsp; 
nbsp; nbsp; nbsp; -gt;nbsp; 4brprint strlen(type)nbsp; nbsp;  # 
lt;type 'type'gt; -gt; 13brbr--brnosy: 
+amaury.forgeotdarcbrbr___brPython 
tracker lt;a ymailto=mailto:rep...@bugs.python.org;
 href=mailto:rep...@bugs.python.org;rep...@bugs.python.org/agt;brlt;a 
href=http://bugs.python.org/issue1962; 
target=_blankhttp://bugs.python.org/issue1962/agt;br___br/div/div
/divbr



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[issue1962] ctypes feature request: Automatic type conversion of input arguments to C functions

[Amaury Forgeot d'Arc]

Changes by Amaury Forgeot d'Arc amaur...@gmail.com:


Removed file: http://bugs.python.org/file18958/unnamed

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[issue1962] ctypes feature request: Automatic type conversion of input arguments to C functions

[Amaury Forgeot d'Arc]

Changes by Amaury Forgeot d'Arc amaur...@gmail.com:


--
nosy:  -BreamoreBoy

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[issue1962] ctypes feature request: Automatic type conversion of input arguments to C functions

[Amaury Forgeot d'Arc]

Changes by Amaury Forgeot d'Arc amaur...@gmail.com:


--
Removed message: http://bugs.python.org/msg117111

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[issue1962] ctypes feature request: Automatic type conversion of input arguments to C functions

[Mark Lawrence]

Mark Lawrence breamore...@yahoo.co.uk added the comment:

Is this ever likely to happen, given that there's been 2.75 years since the 
request without a response?

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[issue1962] ctypes feature request: Automatic type conversion of input arguments to C functions

[Terry J. Reedy]

Changes by Terry J. Reedy tjre...@udel.edu:


--
versions: +Python 3.2 -Python 2.6

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[issue1962] ctypes feature request: Automatic type conversion of input arguments to C functions

[mattbaas]

New submission from mattbaas:

This is rather a feature request instead of a bug report. 
Below is the mail I posted to the ctypes-users mailing list. In short: 
When ctypes checks input argument types using the argtypes attribute, 
it would be useful if it would try to convert the input value 
automatically if it isn't already an appropriate ctypes type (but a 
compatible Python type).

Here is the full mail with some examples:

I'm wrapping a couple of C functions from a DLL and I'm using the 
argtypes attribute to declare the types of the input arguments. I can 
call the functions just fine, but I was wondering why I have to provide 
the exact ctypes type as input when using more complex types such as 
arrays or callbacks (whereas Python floats are automatically converted).

Here is an example code snippet (I was using Python 2.5 on WinXP). The 
library ri.dll contains the functions RiColor() which takes an array of 
3 floats as input and a function RiErrorHandler() which takes a pointer 
to a function as input:

   # Create the required types...
   RtColor = 3*c_float
   RtErrorHandler = CFUNCTYPE(None, c_int, c_int, c_char_p)

   # Load the library and declare the input arguments...
   ri = cdll.LoadLibrary(ri.dll)
   ri.RiColor.argtypes = [RtColor]
   ri.RiErrorHandler.argtypes = [RtErrorHandler]

Now I can call the color function like this:

   ri.RiColor(RtColor(1,0,0))

But sometimes it would be more convenient to work with other types like 
tuples, lists or, in this case, a special vector type (that may come 
from another module but that behaves like a list of 3 floats).
But when I try to pass in just a Python tuple or list I get the 
following errors:

   ri.RiColor((1,0,0))

   -- ctypes.ArgumentError: argument 1: type 'exceptions.TypeError': 
Don't know how to convert parameter 1

   ri.RiColor([1,0,0])

   -- ctypes.ArgumentError: argument 1: type 'exceptions.TypeError': 
expected c_float_Array_3 instance instead of list

Similarly with the error handler function. I have to wrap a Python 
function with the RtErrorHandler type, otherwise ctypes won't accept it:

   def errHandler(code, severity, message):
   pass

   ri.RiErrorHandler(RtErrorHandler(errHandler)) # works
   ri.RiErrorHandler(errHandler) # produces a TypeError


So whenever an input type doesn't match what was specified in argtypes, 
couldn't ctypes try to convert the value before issuing an error? (it 
works with floats, so why not with other types as well?)
The conversion could just be done by passing the value to the 
constructor of the required type. In the color example this also means 
that array types should also accept sequences as input (i.e. anything 
that supports iteration and has the right number of elements).

I think this modification to ctypes would make the wrapped functions 
more flexible without having to write additional wrapper functions in 
Python.

--
components: Extension Modules
messages: 61815
nosy: mattbaas
severity: normal
status: open
title: ctypes feature request: Automatic type conversion of input arguments to 
C functions
type: behavior
versions: Python 2.5

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