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Read() function erases text in file [duplicate]

Started Python a week ago and I have some questions to ask about reading and writing to the same files. I've gone through some tutorials online but I am still confused about it. I can understand simple read and write files.
openFile = open("filepath", "r")
readFile = openFile.read()
print readFile
openFile = open("filepath", "a")
appendFile = openFile.write("\nTest 123")
openFile.close()
But, if I try the following I get a bunch of unknown text in the text file I am writing to. Can anyone explain why I am getting such errors and why I cannot use the same openFile object the way shown below.
# I get an error when I use the codes below:
openFile = open("filepath", "r+")
writeFile = openFile.write("Test abc")
readFile = openFile.read()
print readFile
openFile.close()
I will try to clarify my problems. In the example above, openFile is the object used to open file. I have no problems if I want write to it the first time. If I want to use the same openFile to read files or append something to it. It doesn't happen or an error is given. I have to declare the same/different open file object before I can perform another read/write action to the same file.
#I have no problems if I do this:
openFile = open("filepath", "r+")
writeFile = openFile.write("Test abc")
openFile2 = open("filepath", "r+")
readFile = openFile2.read()
print readFile
openFile.close()
I will be grateful if anyone can tell me what I did wrong here or is it just a Pythong thing. I am using Python 2.7. Thanks!

Updated Response:
This seems like a bug specific to Windows - http://bugs.python.org/issue1521491.
Quoting from the workaround explained at http://mail.python.org/pipermail/python-bugs-list/2005-August/029886.html
the effect of mixing reads with writes on a file open for update is
entirely undefined unless a file-positioning operation occurs between
them (for example, a seek()). I can't guess what
you expect to happen, but seems most likely that what you
intend could be obtained reliably by inserting
fp.seek(fp.tell())
between read() and your write().
My original response demonstrates how reading/writing on the same file opened for appending works. It is apparently not true if you are using Windows.
Original Response:
In 'r+' mode, using write method will write the string object to the file based on where the pointer is. In your case, it will append the string "Test abc" to the start of the file. See an example below:
>>> f=open("a","r+")
>>> f.read()
'Test abc\nfasdfafasdfa\nsdfgsd\n'
>>> f.write("foooooooooooooo")
>>> f.close()
>>> f=open("a","r+")
>>> f.read()
'Test abc\nfasdfafasdfa\nsdfgsd\nfoooooooooooooo'
The string "foooooooooooooo" got appended at the end of the file since the pointer was already at the end of the file.
Are you on a system that differentiates between binary and text files? You might want to use 'rb+' as a mode in that case.
Append 'b' to the mode to open the file in binary mode, on systems
that differentiate between binary and text files; on systems that
don’t have this distinction, adding the 'b' has no effect.
http://docs.python.org/2/library/functions.html#open

Every open file has an implicit pointer which indicates where data will be read and written. Normally this defaults to the start of the file, but if you use a mode of a (append) then it defaults to the end of the file. It's also worth noting that the w mode will truncate your file (i.e. delete all the contents) even if you add + to the mode.
Whenever you read or write N characters, the read/write pointer will move forward that amount within the file. I find it helps to think of this like an old cassette tape, if you remember those. So, if you executed the following code:
fd = open("testfile.txt", "w+")
fd.write("This is a test file.\n")
fd.close()
fd = open("testfile.txt", "r+")
print fd.read(4)
fd.write(" IS")
fd.close()
... It should end up printing This and then leaving the file content as This IS a test file.. This is because the initial read(4) returns the first 4 characters of the file, because the pointer is at the start of the file. It leaves the pointer at the space character just after This, so the following write(" IS") overwrites the next three characters with a space (the same as is already there) followed by IS, replacing the existing is.
You can use the seek() method of the file to jump to a specific point. After the example above, if you executed the following:
fd = open("testfile.txt", "r+")
fd.seek(10)
fd.write("TEST")
fd.close()
... Then you'll find that the file now contains This IS a TEST file..
All this applies on Unix systems, and you can test those examples to make sure. However, I've had problems mixing read() and write() on Windows systems. For example, when I execute that first example on my Windows machine then it correctly prints This, but when I check the file afterwards the write() has been completely ignored. However, the second example (using seek()) seems to work fine on Windows.
In summary, if you want to read/write from the middle of a file in Windows I'd suggest always using an explicit seek() instead of relying on the position of the read/write pointer. If you're doing only reads or only writes then it's pretty safe.
One final point - if you're specifying paths on Windows as literal strings, remember to escape your backslashes:
fd = open("C:\\Users\\johndoe\\Desktop\\testfile.txt", "r+")
Or you can use raw strings by putting an r at the start:
fd = open(r"C:\Users\johndoe\Desktop\testfile.txt", "r+")
Or the most portable option is to use os.path.join():
fd = open(os.path.join("C:\\", "Users", "johndoe", "Desktop", "testfile.txt"), "r+")
You can find more information about file IO in the official Python docs.

Reading and Writing happens where the current file pointer is and it advances with each read/write.
In your particular case, writing to the openFile, causes the file-pointer to point to the end of file. Trying to read from the end would result EOF.
You need to reset the file pointer, to point to the beginning of the file before through seek(0) before reading from it

You can read, modify and save to the same file in python but you have actually to replace the whole content in file, and to call before updating file content:
# set the pointer to the beginning of the file in order to rewrite the content
edit_file.seek(0)
I needed a function to go through all subdirectories of folder and edit content of the files based on some criteria, if it helps:
new_file_content = ""
for directories, subdirectories, files in os.walk(folder_path):
for file_name in files:
file_path = os.path.join(directories, file_name)
# open file for reading and writing
with io.open(file_path, "r+", encoding="utf-8") as edit_file:
for current_line in edit_file:
if condition in current_line:
# update current line
current_line = current_line.replace('john', 'jack')
new_file_content += current_line
# set the pointer to the beginning of the file in order to rewrite the content
edit_file.seek(0)
# delete actual file content
edit_file.truncate()
# rewrite updated file content
edit_file.write(new_file_content)
# empties new content in order to set for next iteration
new_file_content = ""
edit_file.close()

An external increment?

I am trying to create a file "varstore.dat" (that does not exist prior to running this) that should initially contain the value 0. Then every time I execute the script, I want to increment this value by 1.
So essentially I am trying to create the file one time, read from the file, and then rewrite(or overwrite) the file upon each execution. However, my problem is that each time I run the program, it will always initialize to 0 and output 1. I am trying to rewrite into varstore.dat and for the new value to become the old value the next time I execute the script.
def get_var_value(filename="varstore.dat"):
with open(filename, "a+") as f:
val = int(f.read() or 0) + 1
f.seek(0)
f.truncate()
f.write(str(val))
return val
your_counter = get_var_value()
print("This script has been run {} times.".format(your_counter))

You just need to do f.seek(0) before you get the val.
def get_var_value(filename="varstore.dat"):
with open(filename, "a+") as f:
f.seek(0)
val = int(f.read() or 0) + 1
f.seek(0)
f.truncate()
f.write(str(val))
return val
The original code actually works in Python 2. Python2 opens the file(in a+ mode) at 0 whereas Python3 opens it at the end(hence "append" mode). It appears that the reason it's different is because Python2 is based on C's stdio.h: https://linux.die.net/man/3/fopen
http://bugs.python.org/msg229514

When you open the file in "a+" mode (or any a mode), it puts the file position at the end of the file. To read it again you'd have to seek back to byte zero, as Cory Madden's answer suggests.
However, I don't understand why you're opening in "a+" mode if you're reading first, then overwriting. Just make two open calls.
with open(filename, 'r') as f:
val = int(f.read() or 0) + 1
with open(filename, 'w') as f:
f.write(str(val))

The issue is with the 'a+' mode that's being used to open the file. In Python 3, that mode starts you at the end of the file (where there's nothing left to read).
I'm not sure why it didn't work that way in Python 2. Instead it starts you at position 0 and warps you to the end of the file whenever you try to write to it (at least, that's how it works on my Windows system, the documentation suggests it may be OS dependent).
To make your code work correctly, you should use mode 'r+' to open the file. That will allow you to both read and write the file from the beginning. And in fact, it might let you get rid of the truncate() call, since the new number you're going to be writing will always be at least as long as the number that was already in the file.

python write method append

I'm stuck in a very basic problem of I/O in python. I'd like to insert some line in existing file (called ofe, output file), extracted from an source file (called ife, input file) according to arguments passed by user as stored in an list called lineRange (which has an index idx and values lineNumber).
This is the result:
for ifeidx,ifeline in enumerate(ife,1): #for each line of the input file...
with open(outFile,'r+') as ofe:
for idx,lineNumber in enumerate(lineRange,1): #... check if it's present in desired list of lines...
if (ifeidx == lineNumber): #...if found...
ofeidx = 0
for ofeidx, ofeline in enumerate(ofe,1):
if (ofeidx == idx): #...just scroll the the output file and find which is the exact position in desired list...
ofe.write(ifeline) #...put the desired line in correct order. !!! This is always appending at the end of out file!!!!
break
Problem is, the write() method is always pointing to the end of file, appending the lines instead of inserting them when scrolling the output file.
I really don't understand what's happening since the file is open in read+write (r+) mode, neither append (a) nor read+append (r+a) mode, .
I'm also aware that code will (should) overwrite the output file lines. Additional information are the OS WIndow7, Python version 2.7 and development tool is Eclipse with PyDev 3.7.1.xx
Any suggestion on what I'm doing wrong?

You can start by reading the whole file with readlines(), which will return a list. After that you just need to do list.insert(index, value) and write it again back to the file.
with open(outFile, "r") as f:
data = f.readlines()
data.insert(index, value)
with open(outFile, "w+") as f:
f.write(data)
Of course you should change this approach if you are dealing with a huge file.
By the way, if you are not using the with statement you should close the file in the end.

Python CSV seek not working?

file_handle = open("/var/www/transactions.csv", "a")
c = csv.writer(file_handle);
oldamount = amount / 1.98
file_handle.seek(0);
c.writerow( [addre, oldamount, "win"])
Here is my code
I wish to write [addre, oldamount, "win"]) to the start of my CSV file, however it's not working. It's still going to the bottom.

You are opening the file in append ("a") mode. The documentation for open() points out this behavior explicitly: "all writes append to the end of the file regardless of the current seek position".
It isn't possible to "just insert" text at the beginning of a file like you want to. You can either read the whole file, add your data in the front, and write it back out, or you live with the fact that the data goes at the end.
Example for rewriting:
with open("/var/www/transactions.csv", "r+") as f:
olddata = f.read()
f.seek(0)
c = csv.writer(f);
c.writerow([addre, oldamount, "win"])
f.write(olddata)
Note that this can corrupt your file if something goes wrong while writing. If you want to minimize that possibility, write to a new file, then os.rename() it to overwrite the old one.

How to erase the file contents of text file in Python?

I have text file which I want to erase in Python. How do I do that?

In python:
open('file.txt', 'w').close()
Or alternatively, if you have already an opened file:
f = open('file.txt', 'r+')
f.truncate(0) # need '0' when using r+

Opening a file in "write" mode clears it, you don't specifically have to write to it:
open("filename", "w").close()
(you should close it as the timing of when the file gets closed automatically may be implementation specific)

Not a complete answer more of an extension to ondra's answer
When using truncate() ( my preferred method ) make sure your cursor is at the required position.
When a new file is opened for reading - open('FILE_NAME','r') it's cursor is at 0 by default.
But if you have parsed the file within your code, make sure to point at the beginning of the file again i.e truncate(0)
By default truncate() truncates the contents of a file starting from the current cusror position.
A simple example

As #jamylak suggested, a good alternative that includes the benefits of context managers is:
with open('filename.txt', 'w'):
pass

When using with open("myfile.txt", "r+") as my_file:, I get strange zeros in myfile.txt, especially since I am reading the file first. For it to work, I had to first change the pointer of my_file to the beginning of the file with my_file.seek(0). Then I could do my_file.truncate() to clear the file.

Writing and Reading file content
def writeTempFile(text = None):
filePath = "/temp/file1.txt"
if not text: # If not provided return file content
f = open(filePath, "r")
slug = f.read()
return slug
else:
f = open(filePath, "a") # Create a blank file
f.seek(0) # sets point at the beginning of the file
f.truncate() # Clear previous content
f.write(text) # Write file
f.close() # Close file
return text
It Worked for me

If security is important to you then opening the file for writing and closing it again will not be enough. At least some of the information will still be on the storage device and could be found, for example, by using a disc recovery utility.
Suppose, for example, the file you're erasing contains production passwords and needs to be deleted immediately after the present operation is complete.
Zero-filling the file once you've finished using it helps ensure the sensitive information is destroyed.
On a recent project we used the following code, which works well for small text files. It overwrites the existing contents with lines of zeros.
import os
def destroy_password_file(password_filename):
with open(password_filename) as password_file:
text = password_file.read()
lentext = len(text)
zero_fill_line_length = 40
zero_fill = ['0' * zero_fill_line_length
for _
in range(lentext // zero_fill_line_length + 1)]
zero_fill = os.linesep.join(zero_fill)
with open(password_filename, 'w') as password_file:
password_file.write(zero_fill)
Note that zero-filling will not guarantee your security. If you're really concerned, you'd be best to zero-fill and use a specialist utility like File Shredder or CCleaner to wipe clean the 'empty' space on your drive.

You have to overwrite the file. In C++:
#include <fstream>
std::ofstream("test.txt", std::ios::out).close();

You can also use this (based on a few of the above answers):
file = open('filename.txt', 'w')
file.close()
of course this is a really bad way to clear a file because it requires so many lines of code, but I just wrote this to show you that it can be done in this method too.
happy coding!

You cannot "erase" from a file in-place unless you need to erase the end. Either be content with an overwrite of an "empty" value, or read the parts of the file you care about and write it to another file.

Assigning the file pointer to null inside your program will just get rid of that reference to the file. The file's still there. I think the remove() function in the c stdio.h is what you're looking for there. Not sure about Python.

Since text files are sequential, you can't directly erase data on them. Your options are:
The most common way is to create a new file. Read from the original file and write everything on the new file, except the part you want to erase. When all the file has been written, delete the old file and rename the new file so it has the original name.
You can also truncate and rewrite the entire file from the point you want to change onwards. Seek to point you want to change, and read the rest of file to memory. Seek back to the same point, truncate the file, and write back the contents without the part you want to erase.
Another simple option is to overwrite the data with another data of same length. For that, seek to the exact position and write the new data. The limitation is that it must have exact same length.
Look at the seek/truncate function/method to implement any of the ideas above. Both Python and C have those functions.

This is my method:
open the file using r+ mode
read current data from the file using file.read()
move the pointer to the first line using file.seek(0)
remove old data from the file using file.truncate(0)
write new content and then content that we saved using file.read()
So full code will look like this:
with open(file_name, 'r+') as file:
old_data = file.read()
file.seek(0)
file.truncate(0)
file.write('my new content\n')
file.write(old_data)
Because we are using with open, file will automatically close.