Appending lines to a file, then reading them - python

I want to append or write multiple lines to a file. I believe the following code appends one line:
with open(file_path,'a') as file:
file.write('1')
My first question is that if I do this:
with open(file_path,'a') as file:
file.write('1')
file.write('2')
file.write('3')
Will it create a file with the following content?
1
2
3
Second question—if I later do:
with open(file_path,'r') as file:
first = file.read()
second = file.read()
third = file.read()
Will that read the content to the variables so that first will be 1, second will be 2 etc? If not, how do I do it?


Question 1: No.
file.write simple writes whatever you pass to it to the position of the pointer in the file. file.write("Hello "); file.write("World!") will produce a file with contents "Hello World!"
You can write a whole line either by appending a newline character ("\n") to each string to be written, or by using the print function's file keyword argument (which I find to be a bit cleaner)
with open(file_path, 'a') as f:
print('1', file=f)
print('2', file=f)
print('3', file=f)
N.B. print to file doesn't always add a newline, but print itself does by default! print('1', file=f, end='') is identical to f.write('1')
Question 2: No.
file.read() reads the whole file, not one line at a time. In this case you'll get
first == "1\n2\n3"
second == ""
third == ""
This is because after the first call to file.read(), the pointer is set to the end of the file. Subsequent calls try to read from the pointer to the end of the file. Since they're in the same spot, you get an empty string. A better way to do this would be:
with open(file_path, 'r') as f: # `file` is a bad variable name since it shadows the class
lines = f.readlines()
first = lines[0]
second = lines[1]
third = lines[2]
Or:
with open(file_path, 'r') as f:
first, second, third = f.readlines() # fails if there aren't exactly 3 lines


The answer to the first question is no. You're writing individual characters. You would have to read them out individually.
Also, note that file.read() returns the full contents of the file.
If you wrote individual characters and you want to read individual characters, process the result of file.read() as a string.
text = open(file_path).read()
first = text[0]
second = text[1]
third = text[2]
As for the second question, you should write newline characters, '\n', to terminate each line that you write to the file.
with open(file_path, 'w') as out_file:
out_file.write('1\n')
out_file.write('2\n')
out_file.write('3\n')
To read the lines, you can use file.readlines().
lines = open(file_path).readlines()
first = lines[0] # -> '1\n'
second = lines[1] # -> '2\n'
third = lines[2] # -> '3\n'
If you want to get rid of the newline character at the end of each line, use strip(), which discards all whitespace before and after a string. For example:
first = lines[0].strip() # -> '1'
Better yet, you can use map to apply strip() to every line.
lines = list(map(str.strip, open(file_path).readlines()))
first = lines[0] # -> '1'
second = lines[1] # -> '2'
third = lines[2] # -> '3'


Writing multiple lines to a file
This will depend on how the data is stored. For writing individual values, your current example is:
with open(file_path,'a') as file:
file.write('1')
file.write('2')
file.write('3')
The file will contain the following:
123
It will also contain whatever contents it had previously since it was opened to append. To write newlines, you must explicitly add these or use writelines(), which expects an iterable.
Also, I don't recommend using file as an object name since it is a keyword, so I will use f from here on out.
For instance, here is an example where you have a list of values that you write using write() and explicit newline characters:
my_values = ['1', '2', '3']
with open(file_path,'a') as f:
for value in my_values:
f.write(value + '\n')
But a better way would be to use writelines(). To add newlines, you could join them with a list comprehension:
my_values = ['1', '2', '3']
with open(file_path,'a') as f:
f.writelines([value + '\n' for value in my_values])
If you are looking for printing a range of numbers, you could use a for loop with range (or xrange if using Python 2.x and printing a lot of numbers).
Reading individual lines from a file
To read individual lines from a file, you can also use a for loop:
my_list = []
with open(file_path,'r') as f:
for line in f:
my_list.append(line.strip()) # strip out newline characters
This way you can iterate through the lines of the file returned with a for loop (or just process them as you read them, particularly if it's a large file).

Related

Python3 - dumping a JSON data into penultimate line of a file [duplicate]

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?

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.

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.

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. %s\n" %(i + 1, names[i]))

The accepted answer has to load the whole file into memory, which doesn't work nicely for large files. The following solution writes the file contents with the new data inserted into the right line to a temporary file in the same directory (so on the same file system), only reading small chunks from the source file at a time. It then overwrites the source file with the contents of the temporary file in an efficient way (Python 3.8+).
from pathlib import Path
from shutil import copyfile
from tempfile import NamedTemporaryFile
sourcefile = Path("/path/to/source").resolve()
insert_lineno = 152 # The line to insert the new data into.
insert_data = "..." # Some string to insert.
with sourcefile.open(mode="r") as source:
destination = NamedTemporaryFile(mode="w", dir=str(sourcefile.parent))
lineno = 1
while lineno < insert_lineno:
destination.file.write(source.readline())
lineno += 1
# Insert the new data.
destination.file.write(insert_data)
# Write the rest in chunks.
while True:
data = source.read(1024)
if not data:
break
destination.file.write(data)
# Finish writing data.
destination.flush()
# Overwrite the original file's contents with that of the temporary file.
# This uses a memory-optimised copy operation starting from Python 3.8.
copyfile(destination.name, str(sourcefile))
# Delete the temporary file.
destination.close()
EDIT 2020-09-08: I just found an answer on Code Review that does something similar to above with more explanation - it might be useful to some.

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()

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.

location_of_line = 0
with open(filename, 'r') as file_you_want_to_read:
#readlines in file and put in a list
contents = file_you_want_to_read.readlines()
#find location of what line you want to insert after
for index, line in enumerate(contents):
if line.startswith('whatever you are looking for')
location_of_line = index
#now you have a list of every line in that file
context.insert(location_of_line, "whatever you want to append to middle of file")
with open(filename, 'w') as file_to_write_to:
file_to_write_to.writelines(contents)
That is how I ended up getting whatever data I want to insert to the middle of the file.
this is just pseudo code, as I was having a hard time finding clear understanding of what is going on.
essentially you read in the file to its entirety and add it into a list, then you insert your lines that you want to that list, and then re-write to the same file.
i am sure there are better ways to do this, may not be efficient, but it makes more sense to me at least, I hope it makes sense to someone else.

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)

I write this in order to reutilize/correct martincho's answer (accepted one)
! IMPORTANT: This code loads all the file into ram and rewrites content to the file
Variables index, value may be what you desire, but pay attention to making value string and end with '\n' if you don't want it to mess with existing data.
with open("path_to_file", "r+") as f:
# Read the content into a variable
contents = f.readlines()
contents.insert(index, value)
# Reset the reader's location (in bytes)
f.seek(0)
# Rewrite the content to the file
f.writelines(contents)
See the python docs about file.seek method: Python docs

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 ('\0') at the insertion location to be overwritten later:
num_names = 1_000_000 # Enough data to make storing in a list unideal
max_len = 20 # The maximum allowed length of the inserted line
line_to_insert = 2 # The third line is at index 2 (0-based indexing)
with open(filename, 'w+') as file:
for i in range(line_to_insert):
name = get_name(i) # Returns 'Alfred' for i = 0, etc.
file.write(F'{i + 1}. {name}\n')
insert_position = file.tell() # Position to jump back to for insertion
file.write('\0' * max_len + '\n') # Buffer will show up as a blank line
for i in range(line_to_insert, num_names):
name = get_name(i)
file.write(F'{i + 2}. {name}\n') # Line numbering now bumped up by 1.
# Later, once you have the name to insert...
with open(filename, 'r+') as file: # Must use 'r+' to write to middle of file
file.seek(insert_position) # Move stream to the insertion line
name = get_bonus_name() # This lucky winner jumps up to 3rd place
new_line = F'{line_to_insert + 1}. {name}'
file.write(new_line[:max_len]) # Slice so you don't overwrite next line
Unfortunately there is no way to delete-without-replacement any excess null characters that did not get overwritten (or in general any characters anywhere in the middle of a file), unless you then re-write everything that follows. But the null characters will not affect how your file looks to a human (they have zero width).

Printing letters from a File 3 Letters at a Time in Python

I am trying to write a function that opens a filename, reads the contents of the file, and then prints the contents 3 letters at a time.
Here is what I have tried:
def trigram_printer(filename):
open_file = open(filename)
copy = open_file
three_letters = copy.read(4)
for contents in copy:
print(three_letters)
open_file.close

There are several things I’d change about this code:
You never update the three_letters variable, which is why it prints the same thing repeatedly. You need to update the value of three_letters (by reading three more characters from the file) after you print it.
You copy the open_file object, when I’d just use it directly.
By doing .read(4), you print the contents 4 letters at a time, not 3.
You’re using the f = open(filename); ...; f.close() construction, rather than the more conventional with open(filename) as f; ....
With those thoughts in mind, here’s how I’d write your trigram printer:
def trigram_printer(filename):
"""Prints the contents of <filename>, three characters at a time."""
with open(filename, 'r') as f:
three_letters = f.read(3)
while three_letters:
print(three_letters)
three_letters = f.read(3)
The key part is that every time three_letters gets printed, this function reads the next three characters from the file. When it runs out of characters, three_letters will be an empty string and the while loop will stop.

copy simply points to open_file and your printing the letters for the number of lines in copy:
with open('test.txt') as open_file:
data = open_file.read(3)
while data != '':
print(data) # print 3-gram
data = open_file.read(3)
Just use a continuous loop to test if the file buffer is empty and print the data during the iteration.

.split() creating a blank line in python3

I am trying to convert a 'fastq' file in to a tab-delimited file using python3.
Here is the input: (line 1-4 is one record that i require to print as tab separated format). Here, I am trying to read in each record in to a list object:
#SEQ_ID
GATTTGGGGTT
+
!''*((((***
#SEQ_ID
GATTTGGGGTT
+
!''*((((***
using this:
data = open('sample3.fq')
fq_record = data.read().replace('#', ',#').split(',')
for item in fq_record:
print(item.replace('\n', '\t').split('\t'))
Output is:
['']
['#SEQ_ID', 'GATTTGGGGTT', '+', "!''*((((***", '']
['#SEQ_ID', 'GATTTGGGGTT', '+', "!''*((((***", '', '']
I am geting a blank line at the begining of the output, which I do not understand why ??
I am aware that this can be done in so many other ways but I need to figure out the reason as I am learning python.
Thanks

When you replace # with ,#, you put a comma at the beginning of the string (since it starts with #). Then when you split on commas, there is nothing before the first comma, so this gives you an empty string in the split. What happens is basically like this:
>>> print ',x'.split(',')
['', 'x']
If you know your data always begins with #, you can just skip the empty record in your loop. Just do for item in fq_record[1:].

You can also go line-by-line without all the replacing:
fobj = io.StringIO("""#SEQ_ID
GATTTGGGGTT
+
!''*((((***
#SEQ_ID
GATTTGGGGTT
+
!''*((((***""")
data = []
entry = []
for raw_line in fobj:
line = raw_line.strip()
if line.startswith('#'):
if entry:
data.append(entry)
entry = []
entry.append(line)
data.append(entry)
data looks like this:
[['#SEQ_ID', 'GATTTGGGGTTy', '+', "!''*((((***"],
['#SEQ_ID', 'GATTTGGGGTTx', '+', "!''*((((***"]]

Thank you all for your answers. As a beginner, my main problem was the occurrence of a blank line upon .split(',') which I have now understood conceptually. So my first useful program in python is here:
# this script converts a .fastq file in to .fasta format
import sys
# Usage statement:
print('\nUsage: fq2fasta.py input-file output-file\n=========================================\n\n')
# define a function for fasta formating
def format_fasta(name, sequence):
fasta_string = '>' + name + "\n" + sequence + '\n'
return fasta_string
# open the file for reading
data = open(sys.argv[1])
# open the file for writing
fasta = open(sys.argv[2], 'wt')
# feed all fastq records in to a list
fq_records = data.read().replace('#', ',#').split(',')
# iterate through list objects
for item in fq_records[1:]: # this is to avoid the first line which is created as blank by .split() function
line = item.replace('\n', '\t').split('\t')
name = line[0]
sequence = line[1]
fasta.write(format_fasta(name, sequence))
fasta.close()
Other things suggested in the answers would be more clear to me as I learn more.
Thanks again.

Splitting lines in python based on some character

Input:
!,A,56281,12/12/19,19:34:12,000.0,0,37N22.714,121W55.576,+0013!,A,56281,12/1
2/19,19:34:13,000.0,0,37N22.714,121W55.576,+0013!,A,56281,12/12/19,19:34:14,000.
0,0,37N22.714,121W55.576,+0013!,A,56281,12/12/19,19:34:15,000.0,0,37N22.714,121W
55.576,+0013!,A,56281,12/12/19,19:34:16,000.0,0,37N22.714,121W55.576,+0013!,A,56
281,12/12/19,19:34:17,000.0,0,37N22.714,121W55.576,+0013!,A,56281,12/12/19,19:34
:18,000.0,0,37N22.714,121W55.576,+0013!,A,56281,12/12/19,19:34:19,000.0,0,37N22.
Output:
!,A,56281,12/12/19,19:34:12,000.0,0,37N22.714,121W55.576,+0013
!,A,56281,12/12/19,19:34:13,000.0,0,37N22.714,121W55.576,+0013
!,A,56281,12/12/19,19:34:14,000.0,0,37N22.714,121W55.576,+0013
!,A,56281,12/12/19,19:34:15,000.0,0,37N22.714,121W55.576,+0013
!,A,56281,12/12/19,19:34:16,000.0,0,37N22.714,121W55.576,+0013
!,A,56281,12/12/19,19:34:17,000.0,0,37N22.714,121W55.576,+0013
!,A,56281,12/12/19,19:34:18,000.0,0,37N22.714,121W55.576,+0013
!,A,56281,12/12/19,19:34:19,000.0,0,37N22.
'!' is the starting character and +0013 should be the ending of each line (if present).
Problem which I am getting:
Output is like :
!,A,56281,12/12/19,19:34:12,000.0,0,37N22.714,121W55.576,+0013
!,A,56281,12/1
2/19,19:34:13,000.0,0,37N22.714,121W55.576,+0013
!,A,56281,12/12/19,19:34:14,000.
0,0,37N22.714,121W55.576,+0013
!,A,56281,12/12/19,19:34:15,000.0,0,37N22.714,121W
Any help would be highly appreciated...!!!
My code:
file_open= open('sample.txt','r')
file_read= file_open.read()
file_open2= open('output.txt','w+')
counter =0
for i in file_read:
if '!' in i:
if counter == 1:
file_open2.write('\n')
counter= counter -1
counter= counter +1
file_open2.write(i)

You can try something like this:
with open("abc.txt") as f:
data=f.read().replace("\r\n","") #replace the newlines with ""
#the newline can be "\n" in your system instead of "\r\n"
ans=filter(None,data.split("!")) #split the data at '!', then filter out empty lines
for x in ans:
print "!"+x #or write to some other file
.....:
!,A,56281,12/12/19,19:34:12,000.0,0,37N22.714,121W55.576,+0013
!,A,56281,12/12/19,19:34:13,000.0,0,37N22.714,121W55.576,+0013
!,A,56281,12/12/19,19:34:14,000.0,0,37N22.714,121W55.576,+0013
!,A,56281,12/12/19,19:34:15,000.0,0,37N22.714,121W55.576,+0013
!,A,56281,12/12/19,19:34:16,000.0,0,37N22.714,121W55.576,+0013
!,A,56281,12/12/19,19:34:17,000.0,0,37N22.714,121W55.576,+0013
!,A,56281,12/12/19,19:34:18,000.0,0,37N22.714,121W55.576,+0013
!,A,56281,12/12/19,19:34:19,000.0,0,37N22.

Could you just use str.split?
lines = file_read.split('!')
Now lines is a list which holds the split data. This is almost the lines you want to write -- The only difference is that they don't have trailing newlines and they don't have '!' at the start. We can put those in easily with string formatting -- e.g. '!{0}\n'.format(line). Then we can put that whole thing in a generator expression which we'll pass to file.writelines to put the data in a new file:
file_open2.writelines('!{0}\n'.format(line) for line in lines)
You might need:
file_open2.writelines('!{0}\n'.format(line.replace('\n','')) for line in lines)
if you find that you're getting more newlines than you wanted in the output.
A few other points, when opening files, it's nice to use a context manager -- This makes sure that the file is closed properly:
with open('inputfile') as fin:
lines = fin.read()
with open('outputfile','w') as fout:
fout.writelines('!{0}\n'.format(line.replace('\n','')) for line in lines)

Another option, using replace instead of split, since you know the starting and ending characters of each line:
In [14]: data = """!,A,56281,12/12/19,19:34:12,000.0,0,37N22.714,121W55.576,+0013!,A,56281,12/1
2/19,19:34:13,000.0,0,37N22.714,121W55.576,+0013!,A,56281,12/12/19,19:34:14,000.
0,0,37N22.714,121W55.576,+0013!,A,56281,12/12/19,19:34:15,000.0,0,37N22.714,121W
55.576,+0013!,A,56281,12/12/19,19:34:16,000.0,0,37N22.714,121W55.576,+0013!,A,56
281,12/12/19,19:34:17,000.0,0,37N22.714,121W55.576,+0013!,A,56281,12/12/19,19:34
:18,000.0,0,37N22.714,121W55.576,+0013!,A,56281,12/12/19,19:34:19,000.0,0,37N22.""".replace('\n', '')
In [15]: print data.replace('+0013!', "+0013\n!")
!,A,56281,12/12/19,19:34:12,000.0,0,37N22.714,121W55.576,+0013
!,A,56281,12/12/19,19:34:13,000.0,0,37N22.714,121W55.576,+0013
!,A,56281,12/12/19,19:34:14,000.0,0,37N22.714,121W55.576,+0013
!,A,56281,12/12/19,19:34:15,000.0,0,37N22.714,121W55.576,+0013
!,A,56281,12/12/19,19:34:16,000.0,0,37N22.714,121W55.576,+0013
!,A,56281,12/12/19,19:34:17,000.0,0,37N22.714,121W55.576,+0013
!,A,56281,12/12/19,19:34:18,000.0,0,37N22.714,121W55.576,+0013
!,A,56281,12/12/19,19:34:19,000.0,0,37N22.

Just for some variance, here is a regular expression answer:
import re
outputFile = open('output.txt', 'w+')
with open('sample.txt', 'r') as f:
for line in re.findall("!.+?(?=!|$)", f.read(), re.DOTALL):
outputFile.write(line.replace("\n", "") + '\n')
outputFile.close()
It will open the output file, get the contents of the input file, and loop through all the matches using the regular expression !.+?(?=!|$) with the re.DOTALL flag. The regular expression explanation & what it matches can be found here: http://regex101.com/r/aK6aV4
After we have a match, we strip out the new lines from the match, and write it to the file.

Let's try to add a \n before every "!"; then let python splitlines :-) :
file_read.replace("!", "!\n").splitlines()

I will actually implement as a generator so that you can work on the data stream rather than the entire content of the file. This will be quite memory friendly if working with huge files
>>> def split_on_stream(it,sep="!"):
prev = ""
for line in it:
line = (prev + line.strip()).split(sep)
for parts in line[:-1]:
yield parts
prev = line[-1]
yield prev
>>> with open("test.txt") as fin:
for parts in split_on_stream(fin):
print parts
,A,56281,12/12/19,19:34:12,000.0,0,37N22.714,121W55.576,+0013
,A,56281,12/12/19,19:34:13,000.0,0,37N22.714,121W55.576,+0013
,A,56281,12/12/19,19:34:14,000.0,0,37N22.714,121W55.576,+0013
,A,56281,12/12/19,19:34:15,000.0,0,37N22.714,121W55.576,+0013
,A,56281,12/12/19,19:34:16,000.0,0,37N22.714,121W55.576,+0013
,A,56281,12/12/19,19:34:17,000.0,0,37N22.714,121W55.576,+0013
,A,56281,12/12/19,19:34:18,000.0,0,37N22.714,121W55.576,+0013
,A,56281,12/12/19,19:34:19,000.0,0,37N22.

Insert line at middle of file with Python?

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?

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.

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.

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. %s\n" %(i + 1, names[i]))

The accepted answer has to load the whole file into memory, which doesn't work nicely for large files. The following solution writes the file contents with the new data inserted into the right line to a temporary file in the same directory (so on the same file system), only reading small chunks from the source file at a time. It then overwrites the source file with the contents of the temporary file in an efficient way (Python 3.8+).
from pathlib import Path
from shutil import copyfile
from tempfile import NamedTemporaryFile
sourcefile = Path("/path/to/source").resolve()
insert_lineno = 152 # The line to insert the new data into.
insert_data = "..." # Some string to insert.
with sourcefile.open(mode="r") as source:
destination = NamedTemporaryFile(mode="w", dir=str(sourcefile.parent))
lineno = 1
while lineno < insert_lineno:
destination.file.write(source.readline())
lineno += 1
# Insert the new data.
destination.file.write(insert_data)
# Write the rest in chunks.
while True:
data = source.read(1024)
if not data:
break
destination.file.write(data)
# Finish writing data.
destination.flush()
# Overwrite the original file's contents with that of the temporary file.
# This uses a memory-optimised copy operation starting from Python 3.8.
copyfile(destination.name, str(sourcefile))
# Delete the temporary file.
destination.close()
EDIT 2020-09-08: I just found an answer on Code Review that does something similar to above with more explanation - it might be useful to some.

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()

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.

location_of_line = 0
with open(filename, 'r') as file_you_want_to_read:
#readlines in file and put in a list
contents = file_you_want_to_read.readlines()
#find location of what line you want to insert after
for index, line in enumerate(contents):
if line.startswith('whatever you are looking for')
location_of_line = index
#now you have a list of every line in that file
context.insert(location_of_line, "whatever you want to append to middle of file")
with open(filename, 'w') as file_to_write_to:
file_to_write_to.writelines(contents)
That is how I ended up getting whatever data I want to insert to the middle of the file.
this is just pseudo code, as I was having a hard time finding clear understanding of what is going on.
essentially you read in the file to its entirety and add it into a list, then you insert your lines that you want to that list, and then re-write to the same file.
i am sure there are better ways to do this, may not be efficient, but it makes more sense to me at least, I hope it makes sense to someone else.

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)

I write this in order to reutilize/correct martincho's answer (accepted one)
! IMPORTANT: This code loads all the file into ram and rewrites content to the file
Variables index, value may be what you desire, but pay attention to making value string and end with '\n' if you don't want it to mess with existing data.
with open("path_to_file", "r+") as f:
# Read the content into a variable
contents = f.readlines()
contents.insert(index, value)
# Reset the reader's location (in bytes)
f.seek(0)
# Rewrite the content to the file
f.writelines(contents)
See the python docs about file.seek method: Python docs

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 ('\0') at the insertion location to be overwritten later:
num_names = 1_000_000 # Enough data to make storing in a list unideal
max_len = 20 # The maximum allowed length of the inserted line
line_to_insert = 2 # The third line is at index 2 (0-based indexing)
with open(filename, 'w+') as file:
for i in range(line_to_insert):
name = get_name(i) # Returns 'Alfred' for i = 0, etc.
file.write(F'{i + 1}. {name}\n')
insert_position = file.tell() # Position to jump back to for insertion
file.write('\0' * max_len + '\n') # Buffer will show up as a blank line
for i in range(line_to_insert, num_names):
name = get_name(i)
file.write(F'{i + 2}. {name}\n') # Line numbering now bumped up by 1.
# Later, once you have the name to insert...
with open(filename, 'r+') as file: # Must use 'r+' to write to middle of file
file.seek(insert_position) # Move stream to the insertion line
name = get_bonus_name() # This lucky winner jumps up to 3rd place
new_line = F'{line_to_insert + 1}. {name}'
file.write(new_line[:max_len]) # Slice so you don't overwrite next line
Unfortunately there is no way to delete-without-replacement any excess null characters that did not get overwritten (or in general any characters anywhere in the middle of a file), unless you then re-write everything that follows. But the null characters will not affect how your file looks to a human (they have zero width).

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