file name vs file object as a function argument

Question

If a function takes as an input the name of a text file, I can refactor it to instead take a file object (I call it "stream"; is there a better word?). The advantages are obvious - a function that takes a stream as an argument is:

• much easier to write a unit test for, since I don't need to create a temporary file just for the test
• more flexible, since I can use it in situations where I somehow already have the contents of the file in a variable

Are there any disadvantages to streams? Or should I always refactor a function from a file name argument to a stream argument (assuming, of course, the file is text-only)?

Answer 1

... Here is how xml.etree.ElementTree module implements the parse function:

def parse(self, source, parser=None):
    close_source = False
    if not hasattr(source, "read"):
        source = open(source, "rb")
        close_source = True
    ...

As filename is a string, it does not have the read() method (here whatever attribute of that name is checked); however, the open file has it. The four lines makes the rest of code common. The only complication is that you have to remember whether to close the file object (here named source) or not. If it was open inside, then it must be closed. Otherwise, it must not be closed.

Actually, files differ from sreams slightly. Streams are potentially infinite while files usually not (unless some device is mapped as if it were file). The important difference when processing is, that you can never read the stream into memory at once. You have to process it by chunks.