Bugs item #1484580, was opened at 2006-05-09 10:59
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Category: Documentation
Group: Python 2.5
Status: Open
Resolution: None
Priority: 5
Submitted By: Nicko van Someren (nicko)
Assigned to: Nobody/Anonymous (nobody)
Summary: ctypes documenation obscure and incomplete
Initial Comment:
The documentation for ctypes in Python 2.5, drawn largely from the
original ctypes tutorial, misses a number of important points which
users will want to know. In particular the documentation surrounding
casting of types is currently non-existant and the documentation
regarding the relationship between pointer(), byref() and array objects
is at best limited.
In order to fix this I would suggest the following changes:
1) A new sub-sub-sub-section 13.14.1.x should be added to
document the "cast()" function provided by ctypes. The cast function
takes two parameters, a ctypes object of some sort and a ctypes type
class, and it returns the former cast to the later. e.g.:
class Foo(Structure):
_fields_ = [("x", c_int), ("y", c_int)]
class Bar(Structure):
_fields_ = [("count", c_int),("values", c_void_p)]
f = Foo(1,2)
b = Bar(1, cast(pointer(f), c_void_p))
I'm sure that there are many other subtleties to the cast() function
which I don't know about, because they are not currently documented!
2) Nowhere is is explicitly stated that, where a pointer to a type is
expected, a ctypes array object can be assigned instead. While this is
arguably intuitive to any C programmer it could with being
documented. Otherwise people will write:
someFun.argtypes = [POINTER(Foo)]
tenFoos = (Foo * 10)()
someFun(cast(byref(tenFoos), POINTER(Foo)))
when the last line is better written:
someFun(tenFoos)
Adding some words on this subject to section 13.14.1.13 (Arrays) or
13.14.1.14 (Pointers) would be helpful.
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