Python struct.pack() for individual elements in a list?

python list struct udp pack
user1636547 picture user1636547 · May 4, 2013 · Viewed 15.4k times · Source

I would like to pack all the data in a list into a single buffer to send over a UDP socket. The list is relatively long, so indexing each element in the list is tedious. This is what I have so far:

NumElements = len(data)
buf = struct.pack('d'*NumElements,data[0],data[1],data[2],data[3],data[4])

But I would like to do something more pythonic that doesn't require I change the call if I added more elements to the list... something like:

NumElements = len(data)
buf = struct.pack('d'*NumElements,data)  # Returns error

Is there a good way of doing this??

Answer

abarnert picture abarnert · May 4, 2013

Yes, you can use the *args calling syntax.

Instead of this:

buf = struct.pack('d'*NumElements,data)  # Returns error

… do this:

buf = struct.pack('d'*NumElements, *data) # Works

See Unpacking Argument Lists in the tutorial. (But really, read all of section 4.7, not just 4.7.4, or you won't know what "The reverse situation…" is referring to…) Briefly:

… when the arguments are already in a list or tuple but need to be unpacked for a function call requiring separate positional arguments… write the function call with the *-operator to unpack the arguments out of a list or tuple…

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