The main issue you have is that you're assigning the method content.split to content, rather than calling it and assigning its return value. If you print out content after that assignment, it will be: <built-in method split of str object at 0x100894200> which is not what you want. Fix it by adding parentheses, to make it a call of the method, rather than just a reference to it:
content = content.split()
I think you might still have an issue after fixing that through. str.split returns a list, which you're then tuning back into a string using str (before trying to substitute commas for spaces). That's going to give you square brackets and quotation marks, which you probably don't want, and you'll get a bunch of extra commas. Instead, I suggest using the str.join method like this:
content = ",".join(content) # joins all members of the list with commas
I'm not exactly sure if this is what you want though. Using split is going to replace all the newlines in the file, so you're going to end up with a single line with many, many words separated by commas.