Use the `WriteAt` specification to write to a file at offsets.

writeat.go

package main

import (
	"log"
	"os"
)

func main() {
	var (
		fobj *os.File
		err  error
	)

	if fobj, err = os.OpenFile("test.txt", os.O_CREATE|os.O_RDWR, 0644); err != nil {
		log.Fatal(err)
	}
	defer fobj.Close()

	// Write 6 bytes at the beginning of the file
	if _, err = fobj.WriteAt([]byte("abcdef"), 0); err != nil {
		log.Fatal(err)
	}

	// Append 6 bytes at the end of the file
	if _, err = fobj.WriteAt([]byte("ghijkl"), 6); err != nil {
		log.Fatal(err)
	}

	// Write 4 bytes in the middle of the file
	if _, err = fobj.WriteAt([]byte("AAAA"), 4); err != nil {
		log.Fatal(err)
	}

	// Write 3 bytes far after the current data
	if _, err = fobj.WriteAt([]byte("BBB"), 18); err != nil {
		log.Fatal(err)
	}

}

writeat.py

#!/usr/bin/env python3 

import io 


def write_data(fobj, data, offset):
    """
    Writes data to the specified BytesIO fobj at the specified offset, 
    then returns the total length of the resulting fobj for comparison. 
    """
    fobj.seek(offset, io.SEEK_SET)  # Go to the offset from the beginning of the file  
    fobj.write(data)                # Write data to the fobj 
    
    # Return the length of the internal buffer 
    return len(fobj.getbuffer())


def p_write_data(fobj, data, offset):
    """
    Executes write data and pretty prints the result for debugging 
    """
    length = write_data(fobj, data, offset) 
    print("write({: >10}, {:>2}) -> {:>3}  {}".format(repr(data), offset, length, repr(fobj.getvalue())))


# Create empty file-object 
fobj = io.BytesIO() 

# Write 6 bytes at the beginning of the file 
p_write_data(fobj, b'abcdef', 0)

# Append 6 bytes at the end of the file 
p_write_data(fobj, b'ghijkl', 6)

# Write 4 bytes in the middle of the file 
p_write_data(fobj, b'AAAA', 4) 

# Write 3 bytes after the current data 
p_write_data(fobj, b'BBB', 18)