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Add 1 word after readlines()

I am still learning python and have a question about the function readlines() The following is a part of my script:
f = open("demofile.txt", "r")
text = "".join(f.readlines())
print(text)
demofile.txt contains:
This is the first line
This is the second line
This is the third line
Now I want to add a single word to this so I get:
This is the first line
This is the second line
This is the third line
Example
I thought of something easy way of doing it:
f = open("demofile.txt", "r")
text = "".join(f.readlines())."Example"
print(text)
But that doesn't work (of course) I googled and looked around here but didn't really have the good keywords to search for this issue. Hopefully someone can point me in the right direction.

.readlines() returns list you can append() to it:
with open("demofile.txt") as txt:
lines = txt.readlines()
lines.append("Example")
text = "".join(lines)
print(text)
or you can unpack the file object txt, since its an iterator to a new list with the word you wanted to add:
with open("demofile.txt") as txt:
text = "".join([*txt, "Example"])
print(text)

Firstly, the open function in python opens a file in read mode by default. Thus, you do not need to specify the mode r when opening the file. Secondly, you should always close a file after you are done with it. A with statement in python handles this for you. Moreover, instead of using . to add Example onto the end of the string, you should use the concatenation operator in python to add a newline character, \n, and the string, Example.
with open("demofile.txt") as f:
text = "".join(f.readlines()) + "\nExample"
print(text)

This should help you. While dealing with files. It is always recommended to use with open('filename','r') as f instead of f=open('filename','r'). Using ContextManager during file open is the idea that this file will be open in any case whether everything is ok or any exception is raised. And you don't need to explicitly close the file i.e f.close().
end_text='Example'
with open('test.txt','r') as f:
text=''.join(f.readlines())+'\n'+end_text
print(text)

Reading through a .m File and Python keeps reading a character in the .m File as a line?

I am trying to read the text within a .m file in Python and Python keeps reading a single character within the .m file as a line when I use file.readline(). I've also had issues with trying to remove certain parts of the line before adding it to a list.
I've tried adjusting where the readline is on for loops that I have set up since I have to read through multiple files in this program. No matter where I put it, the string always comes out separated by character. I'm new to Python so I'm trying my best to learn what to do.
# Example of what I did
with open('MyFile.m') as f:
for line in f:
text = f.readline()
if text.startswith('%'):
continue
else:
my_string = text.strip("=")
my_list.append(my_string)
This has only partially worked as it will still return parts of lines that I do not want and when trying to format the output by putting spaces between new lines it output like so:
Expected: "The String"
What happened: "T h e S t r i n g"

Without your input file I've had to make some guesses here
Input file:
%
The
%
String
%
Solution:
my_list = []
with open('MyFile.m') as f:
for line in f:
if not line.startswith('%'):
my_list.append(line.strip("=").strip())
print(' '.join(my_list))
The readLine() call was unnecessary as the for loop already gets you the line. The empty if was negated to only catch the part that you cared about. Without your actual input file I can't help with the '=' part. If you have any clarifications I'd be glad to help further.

As suggested by Xander, you shouldn't call readline since the for line in f does that for you.
my_list = []
with open('MyFile.m') as f:
for line in f:
line = line.strip() # lose the \n if you want to
if line.startswith('%'):
continue
else:
my_string = line.strip("=")
my_list.append(my_string)

While reading text file some lines are not detected?

text_file.txt
I am getting the output for first print statement but not for second print statement.Please sugget me the correct code is there anything i have to encode or decode? please help me i m new to python3

Here's a more straightforward implementation of what you're trying to achieve. You can read the file into a Python list and reference each line by a Python list index
with open('text_file.txt','r') as f: # automatically closes the file
input_file = f.readlines() # Read all lines into a Python list
for line_num in range(len(input_file)):
if "INBOIS BERCUKAI" in input_file[line_num]:
print(input_file[line_num + 2]) # offset by any number you want
# same for other if statements

python search for string in file return entire line + next line into new text file

I have a very large text file (50,000+ lines) that should always be in the same sequence. In python I want to search the text file for each of the $INGGA lines and join this line with the subsequent $INHDT to create a new text file. I need to do this without reading into memory as this causes it to crash every time. I can find return the $INGGA line but I'm not sure of the best way of then getting the next line and joining into a new string that is memory efficient
Thanks
Phil
=~=~=~=~=~=~=~=~=~=~=~= PuTTY log 2016.05.06 09:11:34 =~=~=~=~=~=~=~=~=~=~=~= > $PRDID,2.15,-0.10,31.87*6E
$INGGA,091124.00,5249.8336,N,00120.9619,W,1,20,0.6,95.0,M,49.4,M,,*50
$INHDT,31.9,T*1E $INZDA,091124.0055,06,05,2016,,*7F
$INVTG,22.0,T,,M,4.4,N,8.1,K,A*24 $PRDID,2.13,-0.06,34.09*6C
$INGGA,091124.20,5249.8338,N,00120.9618,W,1,20,0.6,95.0,M,49.4,M,,*5D
$INHDT,34.1,T*13 $INZDA,091124.2055,06,05,2016,,*7D
$INVTG,24.9,T,,M,4.4,N,8.1,K,A*2B $PRDID,2.16,-0.03,36.24*61
$INGGA,091124.40,5249.8340,N,00120.9616,W,1,20,0.6,95.0,M,49.4,M,,*5A
$INHDT,36.3,T*13 $INZDA,091124.4055,06,05,2016,,*7B
$INVTG,27.3,T,,M,4.4,N,8.1,K,A*22 $PRDID,2.11,-0.05,38.33*68
$INGGA,091124.60,5249.8343,N,00120.9614,W,1,20,0.6,95.1,M,49.4,M,,*58
$INHDT,38.4,T*1A $INZDA,091124.6055,06,05,2016,,*79
$INVTG,29.5,T,,M,4.4,N,8.1,K,A*2A $PRDID,2.09,-0.02,40.37*6D
$INGGA,091124.80,5249.8345,N,00120.9612,W,1,20,0.6,95.1,M,49.4,M,,*56
$INHDT,40.4,T*15 $INZDA,091124.8055,06,05,2016,,*77
$INVTG,31.7,T,,M,4.4,N,8.1,K,A*21 $PRDID,2.09,0.02,42.42*40
$INGGA,091125.00,5249.8347,N,00120.9610,W,1,20,0.6,95.1,M,49.4,M,,*5F
$INHDT,42.4,T*17

You can just read a line of file and write to another new file.
Like this:
import re
#open new file with append
nf = open('newfile', 'at')
#open file with read
with open('file', 'rt') as f:
for line in f:
r = re.match(r'\$INGGA', line)
if r is not None:
nf.write(line)
nf.write("$INHDT,31.9,T*1E" + '\n')
You can use at to append write and wt to read line!
I have 150,000 lines file, It's run well!

I suggest using a simple regex that will parse and capture the parts you care about. Here is an example that will capture the piece you care about:
(\$INGGA.*\n\$INHDT.*\n)
https://regex101.com/r/tK1hF0/3
As in my above link, you'll notice that I used the "global" g setting on the regex, telling it to capture all groups that match. Otherwise, it'll stop after the first match.
I also had trouble determining where the actual line breaks exist in your above example file, so you can tweak the above to match exactly where the breaks occur.
Here is some starter python example code:
import re
test_str = # load your file here
p = re.compile(ur'(\$INGGA.*\n\$INHDT.*\n)')
matches = re.findall(p, test_str)

In the example PuTTY log you give, its all one line separated with space.
So in this case you can use this to replace the space with new line and gets new file -
cat large_file | sed 's/ /\n/g' > new_large_file
To iterate over the file separated with new line, run this -
cat new_large_file | python your_script.py
Your script get line by line so your computer should not crash.
your_script.py -
import sys
INGGA_line = ""
for line in sys.stdin:
line_striped = line.strip()
if line_striped.startswith("$INGGA"):
INGGA_line = line_striped
elif line_striped.startswith("$INZDA"):
print line_striped, INGGA_line
else:
print line_striped

This answer is aimed at python 3.
According to this other answer (and the docs), you can iterate your file line-by-line memory-efficiently:
with open(filename, 'r') as f:
for line in f:
...process...
An example of how you could fulfill your above criteria could be
# Target file write-only, source file read-only
with open(targetfile, 'w') as tf, open(sourcefile, 'r') as sf:
# Flag for whether we are looking for 1st or 2nd part
look_for_ingga = True
for line in sf:
if look_for_ingga:
if line.startswith('$INGGA,'):
tf.write(line)
look_for_ingga = False
elif line.startswith('$INHDT,'):
tf.write(line)
look_for_ingga = True
In the case where you have multiple '$INGGA,' prior to the '$INHDT,', this grabs the first one and disregards the rest. In case you want to take only the last '$INGGA,' before the '$INHDT,', store the last '$INGGA,' in a variable instead of writing it to disk. Then, when you find your '$INHDT,', store both.
In case you meant that you want to write to a separate new file for each INGGA-INHDT pair, the target file with-statement should be nested inside for line in sf instead, or the results should be buffered in a list for later storage.
Refer to the docs for introductions to with-statements and file reading/writing.

.txt file does not print after being written with a join lines command in python

I'm new to Python and I have the following question:
I am writing the first 72 lines of a .txt-file to another .txt-file, textA.txt.
textA = open('textA.txt', 'w')
textA.write('\n'.join(lines[1:72]))
textA.close
Now, as I intended, the textA file contains 72 sentences, each starting at a new line.
However, when I do a line count or I am trying to print the file through
f=open ('textA.txt','r')
print f.read()
nothing happens (and the nonblank lines count is zero).
Can someone help me out?

It looks like you haven't closed the file handle, and the write may not have finished. close function needs to be called :: textA.close().
To not have to worry about remembering to close files, use the with statement.
with open('textA.txt', 'w') as f:
f.write('\n'.join(lines[1:72]))
And then, read back your file, as required
with open('textA.txt') as f:
print f.readlines()