I'm writing a log file viewer for a web application and for that I want to paginate through the lines of the log file. The items in the file are line based with the newest item at the bottom.
So I need a tail() method that can read n lines from the bottom and support an offset. This is hat I came up with:
def tail(f, n, offset=0):
"""Reads a n lines from f with an offset of offset lines."""
avg_line_length = 74
to_read = n + offset
while 1:
try:
f.seek(-(avg_line_length * to_read), 2)
except IOError:
# woops. apparently file is smaller than what we want
# to step back, go to the beginning instead
f.seek(0)
pos = f.tell()
lines = f.read().splitlines()
if len(lines) >= to_read or pos == 0:
return lines[-to_read:offset and -offset or None]
avg_line_length *= 1.3
Is this a reasonable approach? What is the recommended way to tail log files with offsets?
Simple :
with open("test.txt") as f:
data = f.readlines()
tail = data[-2:]
print(''.join(tail)
If reading the whole file is acceptable then use a deque.
from collections import deque
deque(f, maxlen=n)
Prior to 2.6, deques didn't have a maxlen option, but it's easy enough to implement.
import itertools
def maxque(items, size):
items = iter(items)
q = deque(itertools.islice(items, size))
for item in items:
del q[0]
q.append(item)
return q
If it's a requirement to read the file from the end, then use a gallop (a.k.a exponential) search.
def tail(f, n):
assert n >= 0
pos, lines = n+1, []
while len(lines) <= n:
try:
f.seek(-pos, 2)
except IOError:
f.seek(0)
break
finally:
lines = list(f)
pos *= 2
return lines[-n:]
it's so simple:
def tail(fname,nl):
with open(fname) as f:
data=f.readlines() #readlines return a list
print(''.join(data[-nl:]))
Simple and fast solution with mmap:
import mmap
import os
def tail(filename, n):
"""Returns last n lines from the filename. No exception handling"""
size = os.path.getsize(filename)
with open(filename, "rb") as f:
# for Windows the mmap parameters are different
fm = mmap.mmap(f.fileno(), 0, mmap.MAP_SHARED, mmap.PROT_READ)
try:
for i in xrange(size - 1, -1, -1):
if fm[i] == '\n':
n -= 1
if n == -1:
break
return fm[i + 1 if i else 0:].splitlines()
finally:
fm.close()
Several of these solutions have issues if the file doesn't end in \n or in ensuring the complete first line is read.
def tail(file, n=1, bs=1024):
f = open(file)
f.seek(-1,2)
l = 1-f.read(1).count('\n') # If file doesn't end in \n, count it anyway.
B = f.tell()
while n >= l and B > 0:
block = min(bs, B)
B -= block
f.seek(B, 0)
l += f.read(block).count('\n')
f.seek(B, 0)
l = min(l,n) # discard first (incomplete) line if l > n
lines = f.readlines()[-l:]
f.close()
return lines
Another Solution
if your txt file looks like this: mouse snake cat lizard wolf dog
you could reverse this file by simply using array indexing in python '''
contents=[]
def tail(contents,n):
with open('file.txt') as file:
for i in file.readlines():
contents.append(i)
for i in contents[:n:-1]:
print(i)
tail(contents,-5)
result: dog wolf lizard cat
Based on Eyecue answer (Jun 10 '10 at 21:28): this class add head() and tail() method to file object.
class File(file):
def head(self, lines_2find=1):
self.seek(0) #Rewind file
return [self.next() for x in xrange(lines_2find)]
def tail(self, lines_2find=1):
self.seek(0, 2) #go to end of file
bytes_in_file = self.tell()
lines_found, total_bytes_scanned = 0, 0
while (lines_2find+1 > lines_found and
bytes_in_file > total_bytes_scanned):
byte_block = min(1024, bytes_in_file-total_bytes_scanned)
self.seek(-(byte_block+total_bytes_scanned), 2)
total_bytes_scanned += byte_block
lines_found += self.read(1024).count('\n')
self.seek(-total_bytes_scanned, 2)
line_list = list(self.readlines())
return line_list[-lines_2find:]
Usage:
f = File('path/to/file', 'r')
f.head(3)
f.tail(3)
This may be quicker than yours. Makes no assumptions about line length. Backs through the file one block at a time till it's found the right number of '\n' characters.
def tail( f, lines=20 ):
total_lines_wanted = lines
BLOCK_SIZE = 1024
f.seek(0, 2)
block_end_byte = f.tell()
lines_to_go = total_lines_wanted
block_number = -1
blocks = [] # blocks of size BLOCK_SIZE, in reverse order starting
# from the end of the file
while lines_to_go > 0 and block_end_byte > 0:
if (block_end_byte - BLOCK_SIZE > 0):
# read the last block we haven't yet read
f.seek(block_number*BLOCK_SIZE, 2)
blocks.append(f.read(BLOCK_SIZE))
else:
# file too small, start from begining
f.seek(0,0)
# only read what was not read
blocks.append(f.read(block_end_byte))
lines_found = blocks[-1].count('\n')
lines_to_go -= lines_found
block_end_byte -= BLOCK_SIZE
block_number -= 1
all_read_text = ''.join(reversed(blocks))
return '\n'.join(all_read_text.splitlines()[-total_lines_wanted:])
I don't like tricky assumptions about line length when -- as a practical matter -- you can never know things like that.
Generally, this will locate the last 20 lines on the first or second pass through the loop. If your 74 character thing is actually accurate, you make the block size 2048 and you'll tail 20 lines almost immediately.
Also, I don't burn a lot of brain calories trying to finesse alignment with physical OS blocks. Using these high-level I/O packages, I doubt you'll see any performance consequence of trying to align on OS block boundaries. If you use lower-level I/O, then you might see a speedup.
UPDATE
for Python 3.2 and up, follow the process on bytes as In text files (those opened without a "b" in the mode string), only seeks relative to the beginning of the file are allowed (the exception being seeking to the very file end with seek(0, 2)).:
eg: f = open('C:/.../../apache_logs.txt', 'rb')
def tail(f, lines=20):
total_lines_wanted = lines
BLOCK_SIZE = 1024
f.seek(0, 2)
block_end_byte = f.tell()
lines_to_go = total_lines_wanted
block_number = -1
blocks = []
while lines_to_go > 0 and block_end_byte > 0:
if (block_end_byte - BLOCK_SIZE > 0):
f.seek(block_number*BLOCK_SIZE, 2)
blocks.append(f.read(BLOCK_SIZE))
else:
f.seek(0,0)
blocks.append(f.read(block_end_byte))
lines_found = blocks[-1].count(b'\n')
lines_to_go -= lines_found
block_end_byte -= BLOCK_SIZE
block_number -= 1
all_read_text = b''.join(reversed(blocks))
return b'\n'.join(all_read_text.splitlines()[-total_lines_wanted:])
For efficiency with very large files (common in logfile situations where you may want to use tail), you generally want to avoid reading the whole file (even if you do do it without reading the whole file into memory at once) However, you do need to somehow work out the offset in lines rather than characters. One possibility is reading backwards with seek() char by char, but this is very slow. Instead, its better to process in larger blocks.
I've a utility function I wrote a while ago to read files backwards that can be used here.
import os, itertools
def rblocks(f, blocksize=4096):
"""Read file as series of blocks from end of file to start.
The data itself is in normal order, only the order of the blocks is reversed.
ie. "hello world" -> ["ld","wor", "lo ", "hel"]
Note that the file must be opened in binary mode.
"""
if 'b' not in f.mode.lower():
raise Exception("File must be opened using binary mode.")
size = os.stat(f.name).st_size
fullblocks, lastblock = divmod(size, blocksize)
# The first(end of file) block will be short, since this leaves
# the rest aligned on a blocksize boundary. This may be more
# efficient than having the last (first in file) block be short
f.seek(-lastblock,2)
yield f.read(lastblock)
for i in range(fullblocks-1,-1, -1):
f.seek(i * blocksize)
yield f.read(blocksize)
def tail(f, nlines):
buf = ''
result = []
for block in rblocks(f):
buf = block + buf
lines = buf.splitlines()
# Return all lines except the first (since may be partial)
if lines:
result.extend(lines[1:]) # First line may not be complete
if(len(result) >= nlines):
return result[-nlines:]
buf = lines[0]
return ([buf]+result)[-nlines:]
f=open('file_to_tail.txt','rb')
for line in tail(f, 20):
print line
[Edit] Added more specific version (avoids need to reverse twice)
import itertools
fname = 'log.txt'
offset = 5
n = 10
with open(fname) as f:
n_last_lines = list(reversed([x for x in itertools.islice(f, None)][-(offset+1):-(offset+n+1):-1]))
Assumes a unix-like system on Python 2 you can do:
import os
def tail(f, n, offset=0):
stdin,stdout = os.popen2("tail -n "+n+offset+" "+f)
stdin.close()
lines = stdout.readlines(); stdout.close()
return lines[:,-offset]
For python 3 you may do:
import subprocess
def tail(f, n, offset=0):
proc = subprocess.Popen(['tail', '-n', n + offset, f], stdout=subprocess.PIPE)
lines = proc.stdout.readlines()
return lines[:, -offset]
S.Lott's answer above almost works for me but ends up giving me partial lines. It turns out that it corrupts data on block boundaries because data holds the read blocks in reversed order. When ''.join(data) is called, the blocks are in the wrong order. This fixes that.
def tail(f, window=20):
"""
Returns the last `window` lines of file `f` as a list.
f - a byte file-like object
"""
if window == 0:
return []
BUFSIZ = 1024
f.seek(0, 2)
bytes = f.tell()
size = window + 1
block = -1
data = []
while size > 0 and bytes > 0:
if bytes - BUFSIZ > 0:
# Seek back one whole BUFSIZ
f.seek(block * BUFSIZ, 2)
# read BUFFER
data.insert(0, f.read(BUFSIZ))
else:
# file too small, start from begining
f.seek(0,0)
# only read what was not read
data.insert(0, f.read(bytes))
linesFound = data[0].count('\n')
size -= linesFound
bytes -= BUFSIZ
block -= 1
return ''.join(data).splitlines()[-window:]
This is my version of tailf
import sys, time, os
filename = 'path to file'
try:
with open(filename) as f:
size = os.path.getsize(filename)
if size < 1024:
s = size
else:
s = 999
f.seek(-s, 2)
l = f.read()
print l
while True:
line = f.readline()
if not line:
time.sleep(1)
continue
print line
except IOError:
pass
The simplest way is to use deque:
from collections import deque
def tail(filename, n=10):
with open(filename) as f:
return deque(f, n)
Here is a pretty simple implementation:
with open('/etc/passwd', 'r') as f:
try:
f.seek(0,2)
s = ''
while s.count('\n') < 11:
cur = f.tell()
f.seek((cur - 10))
s = f.read(10) + s
f.seek((cur - 10))
print s
except Exception as e:
f.readlines()
If reading the whole file is acceptable then use a deque.
from collections import deque
deque(f, maxlen=n)
Prior to 2.6, deques didn't have a maxlen option, but it's easy enough to implement.
import itertools
def maxque(items, size):
items = iter(items)
q = deque(itertools.islice(items, size))
for item in items:
del q[0]
q.append(item)
return q
If it's a requirement to read the file from the end, then use a gallop (a.k.a exponential) search.
def tail(f, n):
assert n >= 0
pos, lines = n+1, []
while len(lines) <= n:
try:
f.seek(-pos, 2)
except IOError:
f.seek(0)
break
finally:
lines = list(f)
pos *= 2
return lines[-n:]
import time
attemps = 600
wait_sec = 5
fname = "YOUR_PATH"
with open(fname, "r") as f:
where = f.tell()
for i in range(attemps):
line = f.readline()
if not line:
time.sleep(wait_sec)
f.seek(where)
else:
print line, # already has newline
An even cleaner python3 compatible version that doesn't insert but appends & reverses:
def tail(f, window=1):
"""
Returns the last `window` lines of file `f` as a list of bytes.
"""
if window == 0:
return b''
BUFSIZE = 1024
f.seek(0, 2)
end = f.tell()
nlines = window + 1
data = []
while nlines > 0 and end > 0:
i = max(0, end - BUFSIZE)
nread = min(end, BUFSIZE)
f.seek(i)
chunk = f.read(nread)
data.append(chunk)
nlines -= chunk.count(b'\n')
end -= nread
return b'\n'.join(b''.join(reversed(data)).splitlines()[-window:])
use it like this:
with open(path, 'rb') as f:
last_lines = tail(f, 3).decode('utf-8')
Not the first example using a deque, but a simpler one. This one is general: it works on any iterable object, not just a file.
#!/usr/bin/env python
import sys
import collections
def tail(iterable, N):
deq = collections.deque()
for thing in iterable:
if len(deq) >= N:
deq.popleft()
deq.append(thing)
for thing in deq:
yield thing
if __name__ == '__main__':
for line in tail(sys.stdin,10):
sys.stdout.write(line)
On second thought, this is probably just as fast as anything here.
def tail( f, window=20 ):
lines= ['']*window
count= 0
for l in f:
lines[count%window]= l
count += 1
print lines[count%window:], lines[:count%window]
It's a lot simpler. And it does seem to rip along at a good pace.
I found a probably the easiest way to find the first or last N lines of a file
Last N lines of a file(For Ex:N=10)
file=open("xyz.txt",'r")
liner=file.readlines()
for ran in range((len(liner)-N),len(liner)):
print liner[ran]
First N lines of a file(For Ex:N=10)
file=open("xyz.txt",'r")
liner=file.readlines()
for ran in range(0,N+1):
print liner[ran]
you can go to the end of your file with f.seek(0, 2) and then read off lines one by one with the following replacement for readline():
def readline_backwards(self, f):
backline = ''
last = ''
while not last == '\n':
backline = last + backline
if f.tell() <= 0:
return backline
f.seek(-1, 1)
last = f.read(1)
f.seek(-1, 1)
backline = last
last = ''
while not last == '\n':
backline = last + backline
if f.tell() <= 0:
return backline
f.seek(-1, 1)
last = f.read(1)
f.seek(-1, 1)
f.seek(1, 1)
return backline
I had to read a specific value from the last line of a file, and stumbled upon this thread. Rather than reinventing the wheel in Python, I ended up with a tiny shell script, saved as /usr/local/bin/get_last_netp:
#! /bin/bash
tail -n1 /home/leif/projects/transfer/export.log | awk {'print $14'}
And in the Python program:
from subprocess import check_output
last_netp = int(check_output("/usr/local/bin/get_last_netp"))
based on S.Lott's top voted answer (Sep 25 '08 at 21:43), but fixed for small files.
def tail(the_file, lines_2find=20):
the_file.seek(0, 2) #go to end of file
bytes_in_file = the_file.tell()
lines_found, total_bytes_scanned = 0, 0
while lines_2find+1 > lines_found and bytes_in_file > total_bytes_scanned:
byte_block = min(1024, bytes_in_file-total_bytes_scanned)
the_file.seek(-(byte_block+total_bytes_scanned), 2)
total_bytes_scanned += byte_block
lines_found += the_file.read(1024).count('\n')
the_file.seek(-total_bytes_scanned, 2)
line_list = list(the_file.readlines())
return line_list[-lines_2find:]
#we read at least 21 line breaks from the bottom, block by block for speed
#21 to ensure we don't get a half line
Hope this is useful.
I found the Popen above to be the best solution. It's quick and dirty and it works For python 2.6 on Unix machine i used the following
def GetLastNLines(self, n, fileName):
"""
Name: Get LastNLines
Description: Gets last n lines using Unix tail
Output: returns last n lines of a file
Keyword argument:
n -- number of last lines to return
filename -- Name of the file you need to tail into
"""
p = subprocess.Popen(['tail','-n',str(n),self.__fileName], stdout=subprocess.PIPE)
soutput, sinput = p.communicate()
return soutput
soutput will have will contain last n lines of the code. to iterate through soutput line by line do:
for line in GetLastNLines(50,'myfile.log').split('\n'):
print line
There is very useful module that can do this:
from file_read_backwards import FileReadBackwards
with FileReadBackwards("/tmp/file", encoding="utf-8") as frb:
# getting lines by lines starting from the last line up
for l in frb:
print(l)
Update @papercrane solution to python3.
Open the file with open(filename, 'rb') and:
def tail(f, window=20):
"""Returns the last `window` lines of file `f` as a list.
"""
if window == 0:
return []
BUFSIZ = 1024
f.seek(0, 2)
remaining_bytes = f.tell()
size = window + 1
block = -1
data = []
while size > 0 and remaining_bytes > 0:
if remaining_bytes - BUFSIZ > 0:
# Seek back one whole BUFSIZ
f.seek(block * BUFSIZ, 2)
# read BUFFER
bunch = f.read(BUFSIZ)
else:
# file too small, start from beginning
f.seek(0, 0)
# only read what was not read
bunch = f.read(remaining_bytes)
bunch = bunch.decode('utf-8')
data.insert(0, bunch)
size -= bunch.count('\n')
remaining_bytes -= BUFSIZ
block -= 1
return ''.join(data).splitlines()[-window:]
Although this isn't really on the efficient side with big files, this code is pretty straight-forward:
f.\n.It gets the array lists last indexes, using the negative sign to stand for the last indexes, and the : to get a subarray.
def tail(f,n):
return "\n".join(f.read().split("\n")[-n:])
Update for answer given by A.Coady
Works with python 3.
This uses Exponential Search and will buffer only N lines from back and is very efficient.
import time
import os
import sys
def tail(f, n):
assert n >= 0
pos, lines = n+1, []
# set file pointer to end
f.seek(0, os.SEEK_END)
isFileSmall = False
while len(lines) <= n:
try:
f.seek(f.tell() - pos, os.SEEK_SET)
except ValueError as e:
# lines greater than file seeking size
# seek to start
f.seek(0,os.SEEK_SET)
isFileSmall = True
except IOError:
print("Some problem reading/seeking the file")
sys.exit(-1)
finally:
lines = f.readlines()
if isFileSmall:
break
pos *= 2
print(lines)
return lines[-n:]
with open("stream_logs.txt") as f:
while(True):
time.sleep(0.5)
print(tail(f,2))
Posting an answer at the behest of commenters on my answer to a similar question where the same technique was used to mutate the last line of a file, not just get it.
For a file of significant size, mmap is the best way to do this. To improve on the existing mmap answer, this version is portable between Windows and Linux, and should run faster (though it won't work without some modifications on 32 bit Python with files in the GB range, see the other answer for hints on handling this, and for modifying to work on Python 2).
import io # Gets consistent version of open for both Py2.7 and Py3.x
import itertools
import mmap
def skip_back_lines(mm, numlines, startidx):
'''Factored out to simplify handling of n and offset'''
for _ in itertools.repeat(None, numlines):
startidx = mm.rfind(b'\n', 0, startidx)
if startidx < 0:
break
return startidx
def tail(f, n, offset=0):
# Reopen file in binary mode
with io.open(f.name, 'rb') as binf, mmap.mmap(binf.fileno(), 0, access=mmap.ACCESS_READ) as mm:
# len(mm) - 1 handles files ending w/newline by getting the prior line
startofline = skip_back_lines(mm, offset, len(mm) - 1)
if startofline < 0:
return [] # Offset lines consumed whole file, nothing to return
# If using a generator function (yield-ing, see below),
# this should be a plain return, no empty list
endoflines = startofline + 1 # Slice end to omit offset lines
# Find start of lines to capture (add 1 to move from newline to beginning of following line)
startofline = skip_back_lines(mm, n, startofline) + 1
# Passing True to splitlines makes it return the list of lines without
# removing the trailing newline (if any), so list mimics f.readlines()
return mm[startofline:endoflines].splitlines(True)
# If Windows style \r\n newlines need to be normalized to \n, and input
# is ASCII compatible, can normalize newlines with:
# return mm[startofline:endoflines].replace(os.linesep.encode('ascii'), b'\n').splitlines(True)
This assumes the number of lines tailed is small enough you can safely read them all into memory at once; you could also make this a generator function and manually read a line at a time by replacing the final line with:
mm.seek(startofline)
# Call mm.readline n times, or until EOF, whichever comes first
# Python 3.2 and earlier:
for line in itertools.islice(iter(mm.readline, b''), n):
yield line
# 3.3+:
yield from itertools.islice(iter(mm.readline, b''), n)
Lastly, this read in binary mode (necessary to use mmap) so it gives str lines (Py2) and bytes lines (Py3); if you want unicode (Py2) or str (Py3), the iterative approach could be tweaked to decode for you and/or fix newlines:
lines = itertools.islice(iter(mm.readline, b''), n)
if f.encoding: # Decode if the passed file was opened with a specific encoding
lines = (line.decode(f.encoding) for line in lines)
if 'b' not in f.mode: # Fix line breaks if passed file opened in text mode
lines = (line.replace(os.linesep, '\n') for line in lines)
# Python 3.2 and earlier:
for line in lines:
yield line
# 3.3+:
yield from lines
Note: I typed this all up on a machine where I lack access to Python to test. Please let me know if I typoed anything; this was similar enough to my other answer that I think it should work, but the tweaks (e.g. handling an offset) could lead to subtle errors. Please let me know in the comments if there are any mistakes.
Here is my answer. Pure python. Using timeit it seems pretty fast. Tailing 100 lines of a log file that has 100,000 lines:
>>> timeit.timeit('tail.tail(f, 100, 4098)', 'import tail; f = open("log.txt", "r");', number=10)
0.0014600753784179688
>>> timeit.timeit('tail.tail(f, 100, 4098)', 'import tail; f = open("log.txt", "r");', number=100)
0.00899195671081543
>>> timeit.timeit('tail.tail(f, 100, 4098)', 'import tail; f = open("log.txt", "r");', number=1000)
0.05842900276184082
>>> timeit.timeit('tail.tail(f, 100, 4098)', 'import tail; f = open("log.txt", "r");', number=10000)
0.5394978523254395
>>> timeit.timeit('tail.tail(f, 100, 4098)', 'import tail; f = open("log.txt", "r");', number=100000)
5.377126932144165
Here is the code:
import os
def tail(f, lines=1, _buffer=4098):
"""Tail a file and get X lines from the end"""
# place holder for the lines found
lines_found = []
# block counter will be multiplied by buffer
# to get the block size from the end
block_counter = -1
# loop until we find X lines
while len(lines_found) < lines:
try:
f.seek(block_counter * _buffer, os.SEEK_END)
except IOError: # either file is too small, or too many lines requested
f.seek(0)
lines_found = f.readlines()
break
lines_found = f.readlines()
# we found enough lines, get out
# Removed this line because it was redundant the while will catch
# it, I left it for history
# if len(lines_found) > lines:
# break
# decrement the block counter to get the
# next X bytes
block_counter -= 1
return lines_found[-lines:]
abc = "2018-06-16 04:45:18.68"
filename = "abc.txt"
with open(filename) as myFile:
for num, line in enumerate(myFile, 1):
if abc in line:
lastline = num
print "last occurance of work at file is in "+str(lastline)
There are some existing implementations of tail on pypi which you can install using pip:
Depending on your situation, there may be advantages to using one of these existing tools.
Source: Stackoverflow.com