How to capture the output from "subprocess.call" to a file?

In my code I have a line similar to this:

rval = subprocess.call(["mkdir",directoryName], shell=True)

and I can check rval to see if it is 0 or 1, but if it is 1, I would like to have the text from the command "A subdirectory or file ben already exists." in a file format, so I can compare it to another file if I want to make sure the text is the same.

Is it possible to have a line like this, but I know this does not work

rval = subprocess.call(["mkdir",directoryName], shell=True) >> filename

so no matter what happens with the command, the text is captured in filename, and rval still has the return code?