When you need to open 3 (or more) files in Python3

The problem is I needed to open 3 files:

1. CSV source file to read

2. CSV file to write results on after processing the first file

3. Another CSV file that has metadata resulting from processing the 1st CSV that's different from the 2nd

Solution

There are 2 practical ways to do it. There's a 3rd but it has a very specific use case.

As for Python3, you can basically do this using context managers via the 'with' keyword:

with open('a', 'r') as a_file, open('b', 'w') as b_file, open('c', 'wb') as c_file:    
    do_something()

I believe it this also exist in Python 2.7 but you might need to use the contextlib.nested() function

The other approach is when you need to process files sequentially rather than opening all of them at the same time. Useful if you have variable number of files:

for fname in filenames:
    with open(fname) as f:
        # Process f

You can also use this if you would rather keep the results in memory and only write it to the file when done. 

Reference: 

• https://stackoverflow.com/questions/4617034/how-can-i-open-multiple-files-using-with-open-in-python