Insert line at middle of file with Python?
Question:
Is there a way to do this? Say I have a file that’s a list of names that goes like this:
- Alfred
- Bill
- Donald
How could I insert the third name, “Charlie”, at line x (in this case 3), and automatically send all others down one line? I’ve seen other questions like this, but they didn’t get helpful answers. Can it be done, preferably with either a method or a loop?
Answers:
- Parse the file into a python list using
file.readlines()
or file.read().split('n')
- Identify the position where you have to insert a new line, according to your criteria.
- Insert a new list element there using
list.insert()
.
- Write the result to the file.
This is a way of doing the trick.
with open("path_to_file", "r") as f:
contents = f.readlines()
contents.insert(index, value)
with open("path_to_file", "w") as f:
contents = "".join(contents)
f.write(contents)
index
and value
are the line and value of your choice, lines starting from 0.
You can just read the data into a list and insert the new record where you want.
names = []
with open('names.txt', 'r+') as fd:
for line in fd:
names.append(line.split(' ')[-1].strip())
names.insert(2, "Charlie") # element 2 will be 3. in your list
fd.seek(0)
fd.truncate()
for i in xrange(len(names)):
fd.write("%d. %sn" %(i + 1, names[i]))
You don’t show us what the output should look like, so one possible interpretation is that you want this as the output:
- Alfred
- Bill
- Charlie
- Donald
(Insert Charlie, then add 1 to all subsequent lines.) Here’s one possible solution:
def insert_line(input_stream, pos, new_name, output_stream):
inserted = False
for line in input_stream:
number, name = parse_line(line)
if number == pos:
print >> output_stream, format_line(number, new_name)
inserted = True
print >> output_stream, format_line(number if not inserted else (number + 1), name)
def parse_line(line):
number_str, name = line.strip().split()
return (get_number(number_str), name)
def get_number(number_str):
return int(number_str.split('.')[0])
def format_line(number, name):
return add_dot(number) + ' ' + name
def add_dot(number):
return str(number) + '.'
input_stream = open('input.txt', 'r')
output_stream = open('output.txt', 'w')
insert_line(input_stream, 3, 'Charlie', output_stream)
input_stream.close()
output_stream.close()
If you want to search a file for a substring and add a new text to the next line, one of the elegant ways to do it is the following:
import os, fileinput
old = "A"
new = "B"
for line in fileinput.FileInput(file_path, inplace=True):
if old in line :
line += new + os.linesep
print(line, end="")
There is a combination of techniques which I found useful in solving this issue:
with open(file, 'r+') as fd:
contents = fd.readlines()
contents.insert(index, new_string) # new_string should end in a newline
fd.seek(0) # readlines consumes the iterator, so we need to start over
fd.writelines(contents) # No need to truncate as we are increasing filesize
In our particular application, we wanted to add it after a certain string:
with open(file, 'r+') as fd:
contents = fd.readlines()
if match_string in contents[-1]: # Handle last line to prevent IndexError
contents.append(insert_string)
else:
for index, line in enumerate(contents):
if match_string in line and insert_string not in contents[index + 1]:
contents.insert(index + 1, insert_string)
break
fd.seek(0)
fd.writelines(contents)
If you want it to insert the string after every instance of the match, instead of just the first, remove the else:
(and properly unindent) and the break
.
Note also that the and insert_string not in contents[index + 1]:
prevents it from adding more than one copy after the match_string
, so it’s safe to run repeatedly.
A simple but not efficient way is to read the whole content, change it and then rewrite it:
line_index = 3
lines = None
with open('file.txt', 'r') as file_handler:
lines = file_handler.readlines()
lines.insert(line_index, 'Charlie')
with open('file.txt', 'w') as file_handler:
file_handler.writelines(lines)
Below is a slightly awkward solution for the special case in which you are creating the original file yourself and happen to know the insertion location (e.g. you know ahead of time that you will need to insert a line with an additional name before the third line, but won’t know the name until after you’ve fetched and written the rest of the names). Reading, storing and then re-writing the entire contents of the file as described in other answers is, I think, more elegant than this option, but may be undesirable for large files.
You can leave a buffer of invisible null characters (‘
Is there a way to do this? Say I have a file that’s a list of names that goes like this:
- Alfred
- Bill
- Donald
How could I insert the third name, “Charlie”, at line x (in this case 3), and automatically send all others down one line? I’ve seen other questions like this, but they didn’t get helpful answers. Can it be done, preferably with either a method or a loop?
- Parse the file into a python list using
file.readlines()
orfile.read().split('n')
- Identify the position where you have to insert a new line, according to your criteria.
- Insert a new list element there using
list.insert()
. - Write the result to the file.
This is a way of doing the trick.
with open("path_to_file", "r") as f:
contents = f.readlines()
contents.insert(index, value)
with open("path_to_file", "w") as f:
contents = "".join(contents)
f.write(contents)
index
and value
are the line and value of your choice, lines starting from 0.
You can just read the data into a list and insert the new record where you want.
names = []
with open('names.txt', 'r+') as fd:
for line in fd:
names.append(line.split(' ')[-1].strip())
names.insert(2, "Charlie") # element 2 will be 3. in your list
fd.seek(0)
fd.truncate()
for i in xrange(len(names)):
fd.write("%d. %sn" %(i + 1, names[i]))
You don’t show us what the output should look like, so one possible interpretation is that you want this as the output:
- Alfred
- Bill
- Charlie
- Donald
(Insert Charlie, then add 1 to all subsequent lines.) Here’s one possible solution:
def insert_line(input_stream, pos, new_name, output_stream):
inserted = False
for line in input_stream:
number, name = parse_line(line)
if number == pos:
print >> output_stream, format_line(number, new_name)
inserted = True
print >> output_stream, format_line(number if not inserted else (number + 1), name)
def parse_line(line):
number_str, name = line.strip().split()
return (get_number(number_str), name)
def get_number(number_str):
return int(number_str.split('.')[0])
def format_line(number, name):
return add_dot(number) + ' ' + name
def add_dot(number):
return str(number) + '.'
input_stream = open('input.txt', 'r')
output_stream = open('output.txt', 'w')
insert_line(input_stream, 3, 'Charlie', output_stream)
input_stream.close()
output_stream.close()
If you want to search a file for a substring and add a new text to the next line, one of the elegant ways to do it is the following:
import os, fileinput
old = "A"
new = "B"
for line in fileinput.FileInput(file_path, inplace=True):
if old in line :
line += new + os.linesep
print(line, end="")
There is a combination of techniques which I found useful in solving this issue:
with open(file, 'r+') as fd:
contents = fd.readlines()
contents.insert(index, new_string) # new_string should end in a newline
fd.seek(0) # readlines consumes the iterator, so we need to start over
fd.writelines(contents) # No need to truncate as we are increasing filesize
In our particular application, we wanted to add it after a certain string:
with open(file, 'r+') as fd:
contents = fd.readlines()
if match_string in contents[-1]: # Handle last line to prevent IndexError
contents.append(insert_string)
else:
for index, line in enumerate(contents):
if match_string in line and insert_string not in contents[index + 1]:
contents.insert(index + 1, insert_string)
break
fd.seek(0)
fd.writelines(contents)
If you want it to insert the string after every instance of the match, instead of just the first, remove the else:
(and properly unindent) and the break
.
Note also that the and insert_string not in contents[index + 1]:
prevents it from adding more than one copy after the match_string
, so it’s safe to run repeatedly.
A simple but not efficient way is to read the whole content, change it and then rewrite it:
line_index = 3
lines = None
with open('file.txt', 'r') as file_handler:
lines = file_handler.readlines()
lines.insert(line_index, 'Charlie')
with open('file.txt', 'w') as file_handler:
file_handler.writelines(lines)
Below is a slightly awkward solution for the special case in which you are creating the original file yourself and happen to know the insertion location (e.g. you know ahead of time that you will need to insert a line with an additional name before the third line, but won’t know the name until after you’ve fetched and written the rest of the names). Reading, storing and then re-writing the entire contents of the file as described in other answers is, I think, more elegant than this option, but may be undesirable for large files.
You can leave a buffer of invisible null characters (‘