Python control "with" context manager with conditional

Question

Can Python2.7 use a conditional to control a "with" context manager? My scenario is that if a gzipped file exists, I want to append to it, and if it does not exist, I want to write to a new file. The pseudo code is:

with gzip.open(outfile, 'a+') if os.isfile(outfile) else with open(outfile, 'w') as outhandle:

Or...

if os.isfile(outfile):
    with gzip.open(outfile, 'a+') as outhandle:
        # do stuff
else:
    with open(outfile, 'w') as outhandle:
        # do the same stuff

I don't want to repeat the "do stuff" since it will be the same between them. But how can I use a conditional to control the with context?

Does this answer your question? [Pythonic way to use context manager conditionally](https://stackoverflow.com/questions/33928590/pythonic-way-to-use-context-manager-conditionally) — ideasman42, Oct 14 '21 at 00:21

score 1 · Answer 1 · answered May 07 '19 at 16:32

1

You could try just writing a function for the "do stuff"

def do_stuff():
    #do stuff here 

if os.isfile(outfile):
    with gzip.open(outfile, 'a+') as outhandle:
        do_stuff()
else:
    with open(outfile, 'w') as outhandle:
        do_stuff()

answered May 07 '19 at 16:32

Zander

65
1
9

Good point, and I have done that in other places. But I'm wondering if there is a way to do it in the with statement. – Rusty Lemur May 07 '19 at 16:44

score 1 · Accepted Answer · answered May 30 '19 at 03:46

1

Remember that functions can also be assigned to variables

if os.isfile(outfile):
    open_function = gzip.open
    mode = 'a+'
else:
    open_function = open
    mode = 'w'

with open_function(outfile, mode) as outhandle:
    # do stuff

answered May 30 '19 at 03:46

aydow

3,673
2
23
40

Python control "with" context manager with conditional

2 Answers2