Can I use with statements in python with a list?

Question

When I process one file, I can use with statement to ensure it is always closed, on success or failure. Such as

with open('some_file', 'r') as f:
    print f.read()

However, I now need to open a list of files, the number of which is only known at runtime, so I cannot use nested with statements. But can I use it with a list? Kind of like

with [open(fn) for fn in file_names] as files:
     #do something

The above obviously doesn't work. What will work?

Answer 1

As @Marcin mentioned in the comments, I don't believe there is a way to do this using a list and with.

However, with is essentially syntactic sugar for a set of try/except/finally blocks that allows for encapsulation and therefore easier reuse.

While you can specify multiple pairings within a single with, i.e.

with open('foo.txt', 'r') as f, open('bar.txt', 'r') as f2:
    pass    # do stuff here

There's not an allowance for dynamic variables there such as lists or tuples. This form is actually treated as if the comma-separated expressions are nested withs.

However, what you could do instead is essentially ignore the syntactic sugar of with and rely on the original syntax it was meant to replace. That is, encapsulate your logic within a set of try/except/finally blocks. That way, you can open a list of files at runtime, and then ensure via the finally clause that they get cleaned up.

Something of the form:

try:
    files = [open(fn) for fn in file_names]

    # do stuff with files
except:
    # handle exceptions as needed here
finally:
    for f in files:
        f.close();

    # and any other cleanup you want to do

Answer 2

This is exactly what the contextlib.nested manager of the contextlib module is designed for. You can use it like this:

from contextlib import nested

with nested(*[open(fn) for fn in file_names]) as files:
    # files is a list of file objects

Note however that as stated in the documentation, this function has been deprecated since Python2.7 because of two quirks. I will let you judge whether or not you can live with them in the application you have in mind.

I think this is also a good time to mention the little-known multiple manager form of the with statement explained here. Even though it doesn't apply to your case, I think it is good to know it exists.