Python file open/close every time vs keeping it open until the process is finished

You should definitely try to open/close the file as little as possible

Because even comparing with file read/write, file open/close is far more expensive

Consider two code blocks:

f=open('test1.txt', 'w')
for i in range(1000):
    f.write('\n')
f.close()

and

for i in range(1000):
    f=open('test2.txt', 'a')
    f.write('\n')
    f.close()

The first one takes 0.025s while the second one takes 0.309s

Use the with statement, it automatically closes the files for you, do all the operations inside the with block, so it'll keep the files open for you and will close the files once you're out of the with block.

with open(inputfile)as f1, open('dog.txt','a') as f2,open('cat.txt') as f3:
   #do something here

EDIT: If you know all the possible filenames to be used before the compilation of your code then using with is a better option and if you don't then you should use your approach but instead of closing the file you can flush the data to the file using writefile1.flush()