Python CTypes translation of (pv != NULL)

Thomas Heller theller at python.net
Wed Sep 27 20:22:47 CEST 2006


p.lavarre at ieee.org schrieb:
>> It says:
>>
>> NULL pointers have a False boolean value:
>>  >>> null_ptr = POINTER(c_int)()
>>  >>> print bool(null_ptr)
>> False
> 
> Yes.
> 
>> That means that NULL pointers are considered False in a boolean
>> expression (and you could assume that non-NULL pointers are True, as
>> any other object in general),
> 
> I see this now that you show the clueless newbie me, yes thank you.
> Except now by showing me here we have provoked the authority Thomas
> Heller to say:
> 
>> > > Generally pointer instances have a False boolean value, so
>> > > 'if pv: ....'
>> > > should work.  Except for c_void_p, c_char_p and c_wchar_p instances.
> 
> That English I do not understand.  "Except" how?

Actually I was wrong - there is no exception.  To summarize:

All ctypes NULL pointers have a False boolean value.
So, this C-code:

    if (pv) { /* or 'if (pv != NULL)' */
        return *pv; /* whatever */
    } else {
        /* handle NULL pointer */
    }

translates to this Python code:

    if pv:
        return pv[0] # or whatever
    else:
        # pv is a NULL pointer

Works for instances of c_char_p, c_void_p, c_wchar_p,
and instances of POINTER(some_ctype).

If you want to make the 'if pv:' line more verbose, you could as well
write 'if bool(pv):' or 'if bool(pv) == False', but why would
you want to do this?

Thomas




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