Question

In Python:
Let’s say I have a loop, during each cycle of which I produce a list with the following format:
[‘n1′,’n2′,’n3’]
After each cycle I would like to write to append the produced entry to a file (which contains all the outputs from the previous cycles). How can I do that?

Also, is there a way to make a list whose entries are the outputs of this cycle? i.e.
[[],[],[]] where each internal []=[‘n1′,’n2’,’n3] etc

Answer 1

Writing single list as a line to file

Surely you can write it into a file like, after converting it to string:

with open('some_file.dat', 'w') as f:
    for x in xrange(10):  # assume 10 cycles
        line = []
        # ... (here is your code, appending data to line) ...
        f.write('%r\n' % line)  # here you write representation to separate line

Writing all lines at once

When it comes to the second part of your question:

Also, is there a way to make a list whose entries are the outputs of this cycle? i.e. [[],[],[]] where each internal []=['n1','n2','n3'] etc

it is also pretty basic. Assuming you want to save it all at once, just write:

lines = []  # container for a list of lines
for x in xrange(10):  # assume 10 cycles
    line = []
    # ... (here is your code, appending data to line) ...
    lines.append('%r\n' % line)  # here you add line to the list of lines
# here "lines" is your list of cycle results
with open('some_file.dat', 'w') as f:
    f.writelines(lines)

Better way of writing a list to file

Depending on what you need, you should probably use one of the more specialized formats, than just a text file. Instead of writing list representations (which are okay, but not ideal), you could use eg. csv module (similar to Excel’s spreadsheet): http://docs.python.org/3.3/library/csv.html