From the documentation : The separator between elements is the string providing this method. Maybe not so clear, but it means the iterable in str.join(iterable) will be joined with str as separtor. Some examples: >>> l = ['a', 'b', 'c'] >>> ''.join(l) 'abc' >>> ','.join(l) 'a,b,c' >>> '__'.join(l) 'a__b__c' >>> '! !'.join(l) 'a! !b! !c' More on reddit.com

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A user agent header is a string of text that is sent with HTTP requests to identify the program making the request (the program is called a "user agent"). Web browsers commonly send these in order to identify themselves (so the server can tell if you're using Chrome or Firefox, for example). Many APIs will want you to use a unique user agent string for your program. This is so that if your program misbehaves for some reason, they can more easily identify it, rather than have it look like a browser or just a generic default (this is why they tell you not to spoof a browser). The user agent is something that a human might read at some point in a log somewhere, so it's useful to include something like the name of your program and, in case of Reddit, your username in it. That way, if your program is doing something wrong, the admin reading the server logs can figure out how to talk to you. More on reddit.com

Here's a quick example of how to stream the decompression, since python can't load in an entire ~13 gig file at once into memory.

with open("filename.zst", 'rb') as fh:
    dctx = zstd.ZstdDecompressor(max_window_size=2147483648)
    with dctx.stream_reader(fh) as reader:
        previous_line = ""
        while True:
            chunk = reader.read(2**24)  # 16mb chunks
            if not chunk:
                break

            string_data = chunk.decode('utf-8')
            lines = string_data.split("\n")
            for i, line in enumerate(lines[:-1]):
                if i == 0:
                    line = previous_line + line
                object = json.loads(line)
                # do something with the object here
            previous_line = lines[-1]

Edit: Updated per rhaksw's suggestion below

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