How can I tell if I have a file-like object?

Question

I want to have a function that writes data to a file:

def data_writer(data, file_name):
    spiffy_data = data # ...
    with open(file_name, 'w') as out:
        out.write(spiffy_data)

But sometimes, I have a file object instead of a file name. In this case, I sometimes have a tempfile.TemporaryFile (which creates a file-like object that's writable).

I'd like to be able to write something like:

def data_writer(data, file_thing):
    spiffy_data = data # ...
    if type(file_thing) is file_like:
        file_thing.write(spiffy_data)
    else:
        with open(file_name, 'w') as out:
            out.write(spiffy_data)

What's a good way to do this?

Also, does makes sense to do in Python?

Answer 1

A function should do one thing, and do that one thing well. In the case of data_writer , its one thing is to write data to a file-like object. Let the caller worry about providing such an object. That said, you can also provide that caller in the form of a wrapper that takes a file name and opens it for data_writer .

def data_writer(data, file_obj):
    spiffy_data = data # ...
    file_obj.write(spiffy_data)

def write_data_to_file(data, file_name):
    with open(file_name, "w") as f:
        data_writer(f, file_name)

Answer 2

While your approach is LBYL , it's pythonic to assume it's EAFP . So you could just try to

• write() to the file_thing you received or
• open() it

and except a potential exception, depending on which you feel better represents the default case.

Edit: Cf ShadowRanger's comment for why mixing the exception handling with a context manager is rather unelegant here.