How to read a file in Python written out by C++

Question

I have one program written in C++ that outputs the data from several different types of arrays. For simplicity, I'm using ints and just writing them out one at a time to figure this out.

I need to be able to read the file in on Python, but clearly am missing something. I'm having trouble translating the concepts from C++ over to Python.

This is the C++ I have that's working - it writes out two numbers to a file and then reads that file back in (yes, I have to use the ostream.write() and istream.read() functions - that's how at the base level the library I'm using does it and I can't change it).

int main(int argc, char **argv) {
  std::ofstream fout;
  std::ifstream fin;

  int outval1 = 1234;
  int outval2 = 5678;

  fout.open("out.txt");
  fout.write(reinterpret_cast<const char*>(&outval1), sizeof(int));
  fout.write(reinterpret_cast<const char*>(&outval2), sizeof(int));
  fout.close();
  
  int inval;
  fin.open("out.txt");

  while (fin.read(reinterpret_cast<char*>(&inval), sizeof(int))) {
    std::cout << inval << std::endl;
  }

  fin.close();

  return 0;
}

This is what I have on the Python side, but I know it's not correct. I don't think I should need to read in as binary but that's the only way it's working so far

with open("out.txt", "rb") as f:
    while (byte := f.read(1)):
        print(byte)

If you want to write the file out as text, maybe use itoa() or even just fprintf? Then you can read it in as text on the Python side. — Samwise
– Samwise, Commented Nov 5, 2022 at 23:14
Those reinterpret_cast and write and read files look very suspect to me. You're lying and saying that &outval1 is a pointer to a const char and saying to write the first sizeof(int) many const chars from the "array" being pointed to. Which I suppose has the effect of writing out the binary representation of the int. But... really, why are you doing it that way to begin with? — Nathan Pierson
– Nathan Pierson, Commented Nov 5, 2022 at 23:15
You should open the files in binary mode in this code. It's a must to avoid corruption if you plan to run your program on Windows and just good practice to show intent on other platforms where it doesn't matter as much. — Retired Ninja
– Retired Ninja, Commented Nov 5, 2022 at 23:15
@NathanPierson that's just how the library is written. It's meant to take in a several different types of values from an array like structure and write them out. I suppose it was written like this to keep it flexible. I couldn't say — Dave
– Dave, Commented Nov 5, 2022 at 23:16

Steven · Accepted Answer · 2022-11-06 02:25:46Z

3

In the simple case you have provided, it is easy to write the Python code to read out 1234 and 5678 (assuming sizeof(int) is 4 bytes) by using int.from_bytes. And you should open the file in binary mode.

import sys

with open("out.txt", "rb") as f:
    while (byte := f.read(4)):
        print(int.from_bytes(byte, sys.byteorder))

To deal with floats, you may want to try struct.unpack:

import struct


byte = f.read(4)
print(struct.unpack("f", byte)[0])

edited Nov 6, 2022 at 2:25

answered Nov 5, 2022 at 23:37

Steven

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2 Comments

Dave Over a year ago

That ended up working for me. Only thing I'm wondering now is how to do the same thing for floats (output from C++)

Steven Over a year ago

@dave you may have a look at struct

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