Python3 concatenate 'str' (not "bytes") to 'bytes' TypeError

I want to migrate an existing python 2 script to python 3 the following is working in py2 but not py3:

file_path = "subfolder\a_file.bin"

with file(file_path + ".cap", "wb") as f: f.write(data)

whats done here is just taking a file path and adding an extension with ".cap" which is also located in that sub folder

so I modified it like this:

with open(os.path.abspath(file_path) + ".cap" , 'wb') as f: f.write(data)

I get the error:

TypeError: can only concatenate str (not "bytes") to str

also tried: with open(os.path.abspath(str(file_path)+ ".cap"))

I also tried getting the absolute paths like this:

my_dictonary = {
         "subfolder\a_file.bin" :  ["A3", "B3", "2400"] ,
         "subfolder\b_file.bin" :  ["A4", "B4", "3000"] , 
}

for d in my_dictonary :
    with open(d, "rb") as r: data = r.read()
    
    content= ""

    for line in my_dictonary[d]:
        content= content+ str(line) + "\n"

    file_set = set()

    for filename in glob.iglob('./**/*', recursive=True):
         file_set.add(os.path.abspath(filename))

    f_slice = d.split('\\')
    f_slice = f_slice[1].split(".bin")
    file_n = ""
    for e in file_set:
        if f_slice[0] in e and ".cap" in e:
            file_n = e

with open(file_n, 'wb') as f: f.write(content + data)

I printed the file_n to make sure its the correct file path but even this is throwing the above error. how can add this extra/second file extension to ".bin" and then open that file?

You are reading using the following:

with open(d, "rb") as r: data = r.read()

And trying to write using the following:

with open(file_n, 'wb') as f: f.write(content + data)

There is no problem with that except of this content + data.
You are trying to concatenate str object to byte (content variable declared as follow content = "").

The following code will reproduce the same problem:

>>> byte_like_object = b'This is byte string '
>>> type(byte_like_object)
<class 'bytes'>
>>> string_like_object = 'This is some string type '
>>> type(string_like_object)
<class 'str'>

>>> string_like_object + byte_like_object

Traceback (most recent call last):
  File "<pyshell#13>", line 1, in <module>
    string_like_object + byte_like_object
TypeError: can only concatenate str (not "bytes") to str

In order to solve this issue you need to encode the string object to byte because you are writing to the file with 'wb'.

>>> string_like_object.encode('utf-8') + byte_like_object
b'This is some string type This is byte string'