Question

I have 2 simple questions about python:

1.How to get number of lines of a file in python?

2.How to locate the position in a file object to the
last line easily?

Answer 1

lines are just data delimited by the newline char '\n'.

1) Since lines are variable length, you have to read the entire file to know where the newline chars are, so you can count how many lines:

count = 0
for line in open('myfile'):
    count += 1
print count, line # it will be the last line

2) reading a chunk from the end of the file is the fastest method to find the last newline char.

def seek_newline_backwards(file_obj, eol_char='\n', buffer_size=200):
    if not file_obj.tell(): return # already in beginning of file
    # All lines end with \n, including the last one, so assuming we are just 
    # after one end of line char
    file_obj.seek(-1, os.SEEK_CUR)
    while file_obj.tell():
        ammount = min(buffer_size, file_obj.tell())
        file_obj.seek(-ammount, os.SEEK_CUR)
        data = file_obj.read(ammount)
        eol_pos = data.rfind(eol_char)
        if eol_pos != -1:
            file_obj.seek(eol_pos - len(data) + 1, os.SEEK_CUR)
            break
        file_obj.seek(-len(data), os.SEEK_CUR)

You can use that like this:

f = open('some_file.txt')
f.seek(0, os.SEEK_END)
seek_newline_backwards(f)
print f.tell(), repr(f.readline())