TypeError: sequence item 0: expected str instance, bytes found

' ' is a string which you're calling its join method with a byte sequence. As the documentation's stated, in python-3.x:

str.joinReturn a string which is the concatenation of the strings in the iterable iterable. A TypeError will be raised if there are any non-string values in iterable, including bytes objects. The separator between elements is the string providing this method.

But in this case since you are dealing with byte objects you cannot use str related methods. The byte object itself comes with a join() method that can be used in the same manner as str.join. You can also use io.BytesIO, or you can do in-place concatenation with a bytearray object. As the documentation's mentioned bytearray objects are mutable and have an efficient overallocation mechanism.

So you can simply add a b prefix to the empty string to make it a byte object:

line = b" ".join(line.split())

Also, if your file is contain strings you can simply open your file in a str mode ('r')instead of byte ('rb').

with open("input.txt", "r") as f:    # Do something with f

Note that despite the separation between str and byte objects in python-3.x, in python-2.x you only have str. You can see this by checking the type of a string with b prefix:

In [2]: type(b'')Out[2]: str

And that's what that makes the following snippet work:

"".join([b'www', b'www'])

You could use str() function:

lines=str(lines)

before running the command to avoid errors.

This way you convert the lines variable to string.

If you came here searching for a solution to join a custom class implemented in C/C++, the simplest method is to add a join method to the class itself and create binding to python.

For example, a class that can have either list or map which should be joinable, example code in pybind11 would be something like this:

py::class_<Data> _data(m, "Data");_data.def(py::init<>())    .def("join", [] (Data &d, const char *j = ' ') {        std::string ret;        if (d.isObject())            for (auto &o: d.object())                ret += o.first + j;        else if (d.isList())            for (auto &o: d.list())                ret += o.stringValue() + j;        return ret;    })

Then in python, it is a simple matter of calling the join method for the class

data.join('_')