keeping the last 4 records in a list

[Skaperen]

i'm reading in lines from a file by looping over the open file object so i have 1 line at a time. i need to do tests that check the last 4 lines. i need to keep a sliding window of the last 4 lines. what i have been doing is:

...
buffer = bytearray()
sw4 = [b'baz\n',b'bar\n',b'foo\n']
with open(fn,'rb') as f:
    for line in f:
        buffer += line
        sw4[:0] = line
        sw4[4:] = []
        ...

is this a good way to do that with sw4 having the last 4 lines in reverse order? are there any nice builtin tools to do this better or more clearly?

[Gribouillis]

It is not in the standard library, but more_itertools.windowed() looks like the appropriate ideocyte.

[Skaperen]

it looks like more_itertools.windowed() creates an iterator that gives the various windows in the form of an iterator of windows, from a full iterator such as the file object. so i could apply it to the file object and get a window each time that has the new line and the previous N-1 lines. sweet. this is a script for my own use so i can just install it for myself. thanks!