depythonifying 'char', got 'str' for pyobjc

Question

0

Story would be: I was using a hardware which can be automatic controlled by a objc framework, it was already used by many colleagues so I can see it as a "fixed" library. But I would like to use it via Python, so with pyobjc I can already connect to this device, but failed to send data into it. The objc command in header is like this

(BOOL) executeabcCommand:(NSString*)commandabc

            withArgs:(uint32_t)args
             withData:(uint8_t*)data
          writeLength:(NSUInteger)writeLength
           readLength:(NSUInteger)readLength
  timeoutMilliseconds:(NSUInteger)timeoutMilliseconds
                error:(NSError **) error;

and from my python code, data is an argument which can contain 256bytes of data such as 0x00, 0x01, 0xFF. My python code looks like this:

senddata=Device.alloc().initWithCommunicationInterface_(tcpInterface)
            command = 'ABCw'
            args= 0x00
            writelength = 0x100
            readlength = 0x100
            data = '\x50\x40'
            timeout = 500
success, error = senddata.executeabcCommand_withArgs_withData_writeLength_readLength_timeoutMilliseconds_error_(command, args, data, writelength, readlength, timeout, None)

Whatever I sent into it, it always showing that.

ValueError: depythonifying 'char', got 'str'

I tired to dig in a little bit, but failed to find anything about convert string or list to char with pyobjc

Answer 1

Objective-C follows the rules that apply to C.
So in objc as well as C when we look at uint8_t*, it is in fact the very same as char* in memory. string differs from this only in that sense that it is agreed that the last character ends in \0 to indicate that the char* block that we call string has its cap. So char* blocks end with \0 because, well its a string.

What do we do in C to find out the length of a character block? We iterate the whole block until we find \0. Usually with a while loop, and break the loop when you find it, your counter inside the loop tells you your length if you did not give it somehow anyway.

It is up to you to interpret the data in the desired format. Which is why sometime it is easier to cast from void* or to take indeed a char* block which is then cast to and declared as uint8_t data inside the function which makes use if it. Thats the nice part of C to be able to define that as you wish, use that force that was given to you.

So to make your life easier, you could define a length parameter like so -withData:(uint8_t*)data andLength:(uint64_t)len; to avoid parsing the character stream again, as you know already it is/or should be 256 characters long. The only thing you want to avoid at all cost in C is reading attempts at indices that are out of bound throwing an BAD_ACCESS exception.

But this basic information should enable you to find a way to declare your char* block containing uint8_t data addressed with the very first pointer (*) which also contains the first uint8_t character of the block as str with a specific length or up to the first appearance of \0.

Sidenote: objective-c @"someNSString" == pythons u"pythonstring"

PS: in your question is not clear who throw that error msg. Python? Because it could not interpret the data when receiving? Pyobjc? Because it is python syntax hell when you mix with objc? The objc runtime? Because it follows the strict rules of C as well?

Answer 2

Python has always been very forgiving about shoe-horning one type into another, but python3 uses Unicode strings by default, which need to be converted into binary strings before plugging into pyobjc methods.