Ivan,
Thanks for this explanation!
The pointer to the difference between class-level variable and
instance-level variables was the missing link. The magic behind this two
variables with identical names but different content wasn't obvious.
Setting the instance-level variable options did the job.
Best regards,
Helge
------------------------------------------------------------------------
*From:* Ivan Wick [mailto:[email protected]]
*Sent:* Monday, November 1, 2021, 8:42 PM
*To:* Helge Kruse
*Cc:* [email protected]
*Subject:* [sigrok-devel] Accessing protocol decoder options
The "options" in the Decoder class is a class-level variable, shared
by all instances of the Decoder class (like static in C++ or Java). It
is meant to be an abstract description of the options, it does not
store the concrete values of the options for any one instance.
When you instantiate a Decoder object, it must take the tuple of
class-level options (really, a description of options) and create an
instance-level dictionary for the concrete options of that instance.
In Python, you must use "self" to refer to the instance. So you would
need code like:
class Decoder:
# class options description
options = ({'id': 'baudrate', 'desc':'Baud rate', 'default':115200},)
def __init__(self):
self.options = {} # new instance options
for opt_spec in Decoder.options:
opt_id = opt_spec['id']
opt_default = opt_spec['default']
self.options[opt_id] = opt_default
def start(self):
print('baudrate: %d' % self.options['baudrate'])
if __name__ == '__main__':
d = Decoder()
# expect baudrate to be default 115200 unless changed
# d.options['baudrate'] = 9600
d.start()
For reference, this happens in function srd_inst_option_set of
instance.c around line 142
https://sigrok.org/gitweb/?p=libsigrokdecode.git;a=blob;f=instance.c;hb=02aa01ad5f05f2730309200abda0ac75d3721e1d#l133
<https://sigrok.org/gitweb/?p=libsigrokdecode.git;a=blob;f=instance.c;hb=02aa01ad5f05f2730309200abda0ac75d3721e1d#l133>
On Mon, Nov 1, 2021 at 9:47 AM Helge Kruse <[email protected]
<mailto:[email protected]>> wrote:
Hello,
I am not an experienced Python programmer. So I’d like to ask some
detail to the syntax I found in sigrok protocol decoders.
I wonder about the syntax or probably syntactic sugar in the
libsigrokdecode library. The sigrok protocol decoder API
(https://www.sigrok.org/wiki/Protocol_decoder_API
<https://www.sigrok.org/wiki/Protocol_decoder_API>) includes the
attribute “options” as a tuple consisting of dictionaries. The
decoder code itself reads the current options value with a “string
index” like in the UART decoder:
self.bw <http://self.bw> = (self.options['data_bits'] + 7) // 8
I want to run my decoder in a plain Python environment to make
some tests. Instead of deriving from “srd.Decoder” I can write the
decoder class like this:
class Decoder:
options = ( {'id': 'baudrate', 'desc': 'Baud rate', 'default':
115200}, )
def __init__(self):
pass
def start(self):
if options['baudrate'] == 9600:
pass
# test the class
if __name__ == "__main__":
d = Decoder()
d.start()
If I instantiate the class I get an exception for the first line
in the start method: “tuple indices must be integer or slices, not
str”.
What is the magic behind the libsigrok implementation, that allows
to use a string to access a specific options value? Is there some
Python magic that helps to make the start method running in a
plain Python environment?
Best regards,
Helge
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