Message Based write_ascii_values Incorrectly Formats Commands

[ryan-summers]

Utilizing Message-based resources to write ascii values contains an error in creating the message. A space needs to be inserted between the message and the values blocks to form the message properly.

pyvisa/pyvisa/resources/messagebased.py

Line 257 in 7f26022

message = message.encode(enco) + block.encode(enco)

Example:

instance.write_ascii_values('RES', [4], converter='d', termination='') 

Returns 4 bytes transmitted because the message that was sent was RES4 instead of the properly formatted RES 4, which is 5 bytes in length when the space is included.

[MatthieuDartiailh]

This may very well be instrument dependent. Some instruments expect for example MSG:VALUE. For such cases adding a mandatory space would be a big issue. If you want to submit a documentation update to clarify that the values are directly appended to the message, I would probably be fine with it.

[ryan-summers]

I think another proposal would be to change the signature to allow for a 'joining' string.
E.g.

write_ascii_values(msg, values, converter='f', termination=None, prefix='')

where prefix would specify the string placed between the message and value blocks. This would permit MSG:VALUE as well as MSG VALUE. What do you think?

Also, I was assuming the layer underneath VISA was SCPI (in which case the values are spaced), so thanks for bringing up other scenarios when spaces aren't required. :)

[MatthieuDartiailh]

No sadly not all instruments use SCPI (or respect the standard at the letter). We could have a joining string but we would have to keep the current behavior for backward compatibility. If you want to go this route please add the same option to write_binary_values.

[MatthieuDartiailh]

The takeaway here is that write_ascii/binary_values append the values right after the specified header without inserting any extra character.

Closing for now, feel free to reopen if necessary.