Develop Reference

Why does file appear empty when it's clearly not?

This is the code (sorry if I did it wrong I have no idea how to do that):
with open( 'output', 'r' ) as f:
print("this is line one: " + f.readline(1))
And this is what it prints:
this is line one:
I have checked the file so many times the one says "0". I've checked it in vscode. I've checked it by just opening it in the folder it definitely says 0, but it is just not working and yes it's on line 1.
I've looked all over google and stack overflow and nothing is at all helpful when it comes to my question.

Just to add explanation for the answer above, from python docs of readline
f.readline() reads a single line from the file; a newline character
(\n) is left at the end of the string, and is only omitted on the last
line of the file if the file doesn’t end in a newline. This makes the
return value unambiguous; if f.readline() returns an empty string, the
end of the file has been reached.
where its argument is similar to the size argument of the read function:
size is an optional numeric argument. When size is omitted or
negative, the entire contents of the file will be read and returned
Meaning: when you passed 1 the the readline function you actually read 1 byte from the line, instead of reading the entire line. when passing no arguments, the entire line is returned as a string up to the newline character \n

with open('output', 'r') as f:
print("this is line one: " + f.readline(1))
readline does not take an argument, but readlines takes a size (readlines(size:int) argument.
Though in your case you want to read one line, so readline() should do fine.
with open('output', 'r') as f:
print("this is line one: " + f.readline())

The syntax file.readline(size) read from the file a string of that size. If no argument is specified the readline() method will return the entire line from the given file.
So your function should be:
with open( 'output', 'r' ) as f:
print("this is line one: " + f.readline())
If you want to read all the lines in the file you can modify the function to this:
with open( 'output', 'r' ) as f:
for count, line in enumerate(f):
print("Line {}: {}".format(count, line))

Writing to file with unwanted empty lines

I have a piece of code that's removing some unwanted lines from a text file and writing the results to a new one:
f = open('messyParamsList.txt')
g = open('cleanerParamsList.txt','w')
for line in f:
if not line.startswith('W'):
g.write('%s\n' % line)
The original file is single-spaced, but the new file has an empty line between each line of text. How can I lose the empty lines?

You're not removing the newline from the input lines, so you shouldn't be adding one (\n) on output.

Either strip the newlines off the lines you read or don't add new ones as you write it out.

Just do:
f = open('messyParamsList.txt')
g = open('cleanerParamsList.txt','w')
for line in f:
if not line.startswith('W'):
g.write(line)
Every line that you read from original file has \n (new line) character at the end, so do not add another one (right now you are adding one, which means you actually introduce empty lines).

My guess is that the variable "line" already has a newline in it, but you're writing an additional newline with the g.write('%s*\n*' % line)

line has a newline at the end.
Remove the \n from your write, or rstrip line.

Jython code for deleting spaces from Text file

I am trying to write a jython code for deleting spaces from Text file.I have a following scenario.
I have a text file like
STARTBUR001 20120416
20120416MES201667 20120320000000000201203210000000002012032200000000020120323000000000201203240000000002012032600000000020120327000000000201203280000000002012032900000000020120330000000000
20120416MES202566 2012030500000000020120306000000000201203070000000002012030800000000020120309000000000201203100000000002012031100000000020120312000000000201203130000000002012031400000000020
20120416MES275921 20120305000000000201203060000000002012030700000000020120308000000000201203090000000002012031000000000020120311000000000201203120000000002012031300000000020120314000000000
END 0000000202
Here all lines are single lines.
But what i want is like
STARTBUR001 20120416
20120416MES201667 20120320000000000201203210000000002012032200000000020120323000000000201203240000000002012032600000000020120327000000000201203280000000002012032900000000020120330000000000
20120416MES202566 2012030500000000020120306000000000201203070000000002012030800000000020120309000000000201203100000000002012031100000000020120312000000000201203130000000002012031400000000020
20120416MES275921 20120305000000000201203060000000002012030700000000020120308000000000201203090000000002012031000000000020120311000000000201203120000000002012031300000000020120314000000000
END 0000000202
So in all i want to start checking from second line till i encounter END and delete all spaces at tyhe end of each line.
Can someone guide me for writing this code??
tried like:
srcfile=open('d:/BUR001.txt','r')
trgtfile=open('d:/BUR002.txt','w')
readfile=srcfile.readline()
while readfile:
trgtfile.write(readfile.replace('\s',''))
readfile=srcfile.readline()
srcfile.close()
trgtfile.close()
Thanks,
Mahesh

You can use fact that those special lines starts with special values:
line = srcfile.readline()
while line:
line2 = line
if not line2.startswith('START') and not line2.startswith('END'):
line2 = line2.replace(' ','')
trgtfile.write(line2)
line = srcfile.readline()
Also note that with readline() result strings ends with \n (or are empty at end of input file), and this code removes all spaces from the line, not only those at end of the line.
If I understood your example all you want is to remove empty lines, so instead of reading file line by line read it at once:
content = srcfile.read()
and then remove empty lines from content:
while '\n\n' in content:
content = content.replace('\n\n', '\n')

python reading files

I need to get a specific line number from a file that I am passing into a python program I wrote. I know that the line I want will be line 5, so is there a way I can just grab line 5, and not have to iterate through the file?

If you know how many bytes you have before the line you're interested in, you could seek to that point and read out a line. Otherwise, a "line" is not a first class construct (it's just a list of characters terminated by a character you're assigning a special meaning to - a newline). To find these newlines, you have to read the file in.
Practically speaking, you could use the readline method to read off 5 lines and then read your line.
Why are you trying to do this?

you can to use linecache
import linecache
get = linecache.getline
print(get(path_of_file, number_of_line))

I think following should do :
line_number=4
# Avoid reading the whole file
f = open('path/to/my/file','r')
count=1
for i in f.readline():
if count==line_number:
print i
break
count+=1
# By reading the whole file
f = open('path/to/my/file','r')
lines = f.read().splitlines()
print lines[line_number-1] # Index starts from 0
This should give you the 4th line in the file.

editing a single .txt line in python 3.1

i have some data stored in a .txt file in this format:
----------|||||||||||||||||||||||||-----------|||||||||||
1029450386abcdefghijklmnopqrstuvwxy0293847719184756301943
1020414646canBeFollowedBySpaces 3292532113435532419963
don't ask...
i have many lines of this, and i need a way to add more digits to the end of a particular line.
i've written code to find the line i want, but im stumped as to how to add 11 characters to the end of it. i've looked around, this site has been helpful with some other issues i've run into, but i can't seem to find what i need for this.
it is important that the line retain its position in the file, and its contents in their current order.
using python3.1, how would you turn this:
1020414646canBeFollowedBySpaces 3292532113435532419963
into
1020414646canBeFollowedBySpaces 329253211343553241996301846372998

As a general principle, there's no shortcut to "inserting" new data in the middle of a text file. You will need to make a copy of the entire original file in a new file, modifying your desired line(s) of text on the way.
For example:
with open("input.txt") as infile:
with open("output.txt", "w") as outfile:
for s in infile:
s = s.rstrip() # remove trailing newline
if "target" in s:
s += "0123456789"
print(s, file=outfile)
os.rename("input.txt", "input.txt.original")
os.rename("output.txt", "input.txt")

Check out the fileinput module, it can do sort of "inplace" edits with files. though I believe temporary files are still involved in the internal process.
import fileinput
for line in fileinput.input('input.txt', inplace=1, backup='.orig'):
if line.startswith('1020414646canBeFollowedBySpaces'):
line = line.rstrip() + '01846372998' '\n'
print(line, end='')
The print now prints to the file instead of the console.
You might want to back up your original file before editing.

target_chain = '1020414646canBeFollowedBySpaces 3292532113435532419963'
to_add = '01846372998'
with open('zaza.txt','rb+') as f:
ch = f.read()
x = ch.find(target_chain)
f.seek(x + len(target_chain),0)
f.write(to_add)
f.write(ch[x + len(target_chain):])
In this method it's absolutely obligatory to open the file in binary mode 'b' for some reason linked to the treatment of the end of lines by Python (see Universal Newline, enabled by default)
The mode 'r+' is to allow the writing as well as the reading
In this method, what is before the target_chain in the file remains untouched. And what is after the target_chain is shifted ahead. As said by Greg Hewgill, there is no possibility to move apart bits on a hard drisk to insert new bits in the middle.
Evidently, if the file is very big, reading all of its content in ch could be too much memory consuming and the algorithm should then be changed: reading line after line until the line containing the target_chain, and then reading the next line before inserting, and then continuing to do "reading the next line - re-writing on the current line" until the end of the file in order to shift progressively the content from the line concerned with addition.
You see what I mean...

Copy the file, line by line, to another file. When you get to the line that needs extra chars then add them before writing.