Determine if a file is either in JSON or XML format

Well, if you only need to distinguish between JSON and XML formats I'd keep it simple. If the encoding of that string is known to you (or is ASCII or UTF), then looking at the very first char in the file should be enough. Your code looks pythonic to me, but I'd rather do it this way:

def parse_report_file(report_input_file):
    with open(report_input_file) as unknown_file:
        c = unknown_file.read(1)
        if c != '<':
            return 'Is JSON'
        return 'Is XML'

While it is legal for JSON data structures to begin with null, true, false you can avoid those situations if you already know a bit about your data structures.

If you also need to distinguish between JSON, XML and other random file, you should simply strip any whitespace before you look at the "first" char.

As Dexter said, you could keep it simple and just look at the first character. The flaw being noted with that is it might return "Is JSON" if it's malformed XML and vice-versa, so if you wanted, you could return 1 of three options. So to expand a bit and give you something to build upon. You could use regular expressions to match the first and last characters (and build from here for anything in between).

import 're'

def parse_report_file(report_input_file):
    with open(report_input_file, 'r') as unknown_file:
        # Remove tabs, spaces, and new lines when reading
        data = re.sub(r'\s+', '', unknown_file.read())
        if (re.match(r'^<.+>$', data)):
            return 'Is XML'
        if (re.match(r'^({|[).+(}|])$', data)):
            return 'Is JSON'
        return 'Is INVALID'

The second regex isn't matching the same opening braces with closing braces, so you could do that there. You could also read the entire file in without removing the spaces and then ignore any number of beginning and ending spaces when doing additional regex matches, but I did it this way for a little more readability.

Anyway, just another example. Definitely not the most performant but showing you some options.

Go the way you did already. Taking a cheaper approach like testing the first (non whitespace) character might work on input that always succeeds on parsing. But as soon as you encounter an invalid file you're stuck. You probably won't get the correct error messages.

Implementing a parser for XML and / or JSON with regex only will not work. You will have several edge cases where you achieve false positives or false negatives (e.g. XML recognized as JSON and vice versa). You're parser won't be as efficient as the libraries you're using to handle these formats. You will achieve less performance AND more bugs. Just don't do it!

About the python style guide thing I can't judge as I'm no python guru.