Why does a grouped struct.pack write wrong data?

Question

I just spent ~30 minutes debugging and double checking Python & C# code, to find out that my struct.pack was writing the wrong data. When I separated this into separate calls, it works fine.

This is what I had before

file.write(struct.pack("fffHf", kf_time / frame_divisor, kf_in_tangent, kf_out_tangent, kf_interpolation_type, kf_value))

This is what I have now

file.write(struct.pack("f", kf_time / frame_divisor))
file.write(struct.pack("f", kf_in_tangent))
file.write(struct.pack("f", kf_out_tangent))
file.write(struct.pack("H", kf_interpolation_type))
file.write(struct.pack("f", kf_value))

Why does the first variation not write the data that I expected? What is so different than writing these separately?

(File is opened in binary mode, platform is 64 bit Windows, Python 3.5)

• CIsForCookies: can you provide a MCVE? stackoverflow.com/help/mcve
• Sven Marnach: What platform are you on? Is this Python 2 or 3? Did you open the file in binary mode?
• aaro4130: 64 bit Windows. Python 3.5. File is opened in Binary mode.

Answer 1

Presumably because, as the struct documentation clearly states:

Note By default, the result of packing a given C struct includes pad bytes in order to maintain proper alignment for the C types involved; similarly, alignment is taken into account when unpacking. This behavior is chosen so that the bytes of a packed struct correspond exactly to the layout in memory of the corresponding C struct. To handle platform-independent data formats or omit implicit pad bytes, use standard size and alignment instead of native size and alignment: see Byte Order, Size, and Alignment for details.

• Sven Marnach: Yep, that's the reason. You should add a =, < or > at the beginning of the format string, depending on the endianness you want.
• aaro4130: Here I was thinking the "</!/>/=" only meant endianness, and had no effect other than that. Thanks!