python error on struct.unpack

Question

I am new to python and I am trying to use unpack like this:

data = f.read(4)
AAA=len(data)
BBB=struct.calcsize(cformat)
print AAA
print BBB
value = struct.unpack(cformat, data)
return value[0]

This runs fine as long as AAA == BBB but sometimes, f.read only reads 3 bytes and then I get an error. The actual value in the file that I am trying to read is 26. It reads all of the values from 1-221 except for 26 where it errors because f.read(size) only reads three bytes

• Robᵩ: You have described what happens and why. Do you have a question?

Answer 1

Assuming the question is "How should I read a 26 without an error?"

First check the arguments to the open() that produces f. Under Windows, unless you open a file in binary mode (f = open(filename, "rb")), Python assumes that the file is a text file. Windows treats byte value 26 (Ctrl+Z) in a text file as an end-of-file marker, a quirk that it inherited from CP/M.

Answer 2

You have opened a binary file in text mode, and you are using an operating system where the distinction matters. Try adding b to the mode parameter when you open the file:

f = open("my_input_file.bin", "rb")