I'm using Knime 3.1.2 on OSX and Linux for OPENMS analysis (Mass Spectrometry).
Currently, it uses static filename.mzML files manually put in a directory. It usually has more than one file pressed in at a time ('Input FileS' module not 'Input File' module) using a ZipLoopStart.
I want these files to be downloaded dynamically and then pressed into the workflow...but I'm not sure the best way to do that. 
Currently, I have a Python script that downloads .gz files (from AWS S3) and then unzips them. I already have variations that can unzip the files into memory using StringIO (and maybe pass them into the workflow from there as data??). 
It can also download them to a directory...which maybe can them be used as the source? But I don't know how to tell the ZipLoop to wait and check the directory after the python script is run. 
I also could have the python script run as a separate entity (outside of knime) and then, once the directory is populated, call knime...HOWEVER there will always be a different number of files (maybe 1, maybe three)...and I don't know how to make the 'Input Files' knime node to handle an unknown number of input files. 
I hope this makes sense. 
Thanks!
There are multiple options to let things work:

Convert the files in-memory to a Binary Object cells using Python, later you can use that in KNIME. (This one, I am not sure is supported, but as I remember it was demoed in one of the last KNIME gatherings.)
Save the files to a temporary folder (Create Temp Dir) using Python and connect the Pyhon node using a flow variable connection to a file reader node in KNIME (which should work in a loop: List Files, check the Iterate List of Files metanode).
Maybe there is already S3 Remote File Handling support in KNIME, so you can do the downloading, unzipping within KNIME. (Not that I know of, but it would be nice.)

I would go with option 2, but I am not so familiar with Python, so for you, probably option 1 is the best. (In case option 3 is supported, that is the best in my opinion.)
Python in Knime: Downloading files and dynamically pressing them into workflow