Question

I was reading though a library of python code, and I’m stumped by this statement:

struct.pack( "<ii%ds"%len(value), ParameterTypes.String, len(value), value.encode("UTF8") )

I understand everything but%d, and I’m not sure why the length of value is being packed in twice.

As I understand it, the structure will have little endian encoding (<) and will contain two integers (ii) followed by %d, followed by a string (s).

What is the significance of %d?

Answer 1

It is an ordinary string format which is being used to create the struct format

Try reading it to begin with as an ordinary string (forget struct for the moment) …

"<ii%ds" % len(value)

If, for example, the length of the value iterable is 4 then the string will be, <ii4s. This is then passed to struct.pack ready to pack two integers followed by a string of length four bytes from the value iterable