python - Most efficient way to search the last X lines of a file?

Question

I have a file and I don't know how big it's going to be (it could be quite large, but the size will vary greatly). I want to search the last 10 lines or so to see if any of them match a string. I need to do this as quickly and efficiently as possible and was wondering if there's anything better than:

s = "foo"
last_bit = fileObj.readlines()[-10:]
for line in last_bit:
    if line == s:
        print "FOUND"

Answer 1

# Tail
from __future__ import with_statement

find_str = "FIREFOX"                    # String to find
fname = "g:/autoIt/ActiveWin.log_2"     # File to check

with open(fname, "r") as f:
    f.seek (0, 2)           # Seek @ EOF
    fsize = f.tell()        # Get Size
    f.seek (max (fsize-1024, 0), 0) # Set pos @ last n chars
    lines = f.readlines()       # Read to end

lines = lines[-10:]    # Get last 10 lines

# This returns True if any line is exactly find_str + "
"
print find_str + "
" in lines

# If you're searching for a substring
for line in lines:
    if find_str in line:
        print True
        break