Python CTypes translation of (pv != NULL)

[Gabriel Genellina]

At Wednesday 27/9/2006 13:35, p.lavarre at ieee.org wrote:

> > > http://starship.python.net/crew/theller/ctypes/tutorial.html
> > ...
> > direct discussion only of how to construct null pointers,
> > no discussion of how to test for them ...
> > ...
> > could be read to mean try ... == ... is ... False ... None ...
>
>CTypes nulls fetched from a struct feel more like None than do CTypes
>nulls fetched from elsewhere, astonishing me the Python newbie, e.g.:

No, the difference comes from the kind of pointer type, char* gets 
converted to string or None. See the table on section 14.14.1.4 
Fundamental data types

>$ python nulls.py
>True True True None None
>True True True None None
>True False False None c_char_p(None)
>$
>$ cat nulls.py
>from ctypes import *
>class struct_aa(Structure):
>     _fields_ = [("chars", c_char_p)]
>pv = None
>print not pv, pv == None, pv is None, cast(pv, c_void_p).value, pv
>pv = struct_aa().chars
>print not pv, pv == None, pv is None, cast(pv, c_void_p).value, pv
>pv = cast(c_void_p(0), c_char_p)
>print not pv, pv == None, pv is None, cast(pv, c_void_p).value, pv
>$

c_char_p is *not* a pointer in Python, since Python does not have pointers.
Either the original C pointer was NULL: then the Python value is None
Or the original C pointer was not NULL: then the Python value is a string


>Possibly relevant, though grabbed at random, is:
>
>/// PEP 8 -- Style Guide for Python Code
>/// http://www.python.org/dev/peps/pep-0008/
>
>... beware of writing "if x" when you really mean "if x is not None" --
>e.g. when testing whether a variable or argument that defaults to None
>was set to some other value.  The other value might have a type (such
>as a container) that could be false in a boolean context!

This is exactly the case: None is *not* a pointer (neither any other 
kind of ctypes object), it's an object -a singleton, i.e., there 
exist exactly a single one instance of None).
If you want to check if some kind of pointer is NULL or not, do *not* 
check if it is None or not, check its Boolean value, again:
if ptr: xxx
if not ptr: xxx



Gabriel Genellina
Softlab SRL 


	
	
		
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