While doing some Python tutorials, the author of the book asked for the following (opening a file and reading its content):
#we could do this in one line, how?
in_file = open(from_file)
indata = in_file.read()
How can I do this in one line?
You can get all the contents of the file from a file handler by by simply extending what you have with your path and reading from it.
indata = open(from_file).read()
However, it's more obvious what's happening and easier to extend if you use a with block.
with open(from_file) as fh: # start a file handler that will close itself!
indata = fh.read() # get all of the contents of in_file in one go as a string
Beyond that, you should protect your file opening and closing from IOErrors if (for example) your file path does not exist.
Finally, files are by default opened read-only and raise an error if you (or someone else later) attempts to write to it, which will safeguard data blocks. You can change this from 'r' to a variety of other options depending on your needs.
Here is a fairly complete example with the above concepts.
def get_file_contents(input_path):
try:
with open(input_path, 'r') as fh:
return fh.read().strip()
except IOError:
return None
Related
I am trying to copy the content of one file to another.
The script successfully copies content to the file but when I try to run READ command with the output file to print the output, it is blank.
from sys import argv
script, inputFile, outputFile = argv
inFile = open(inputFile)
inData = inFile.read()
outFile = open(outputFile, 'w+')
outFile.write(inData)
print("The new data is:\n",outFile.read())
inFile.close()
outFile.close()
After the write operation the file pointer is at the end of file so you'd need to reset it to the start. Also, the filesystem IO buffers may not have been flushed at that point (you haven't closed the file yet)...
Simple solution: close the outFile and reopen it for reading.
As a side note: always make sure you DO close your files whatever happens, specially when writing, else you may end up with corrupted data. The simplest way is the with statement:
with open(...) as infile, (...) as outfile:
outfile.write(infile.read())
# at this point both files have been automagically closed
You forgot to return to the beginning of outFile after writing to it.
So inserting outFile.seek(0) should fix your issues.
After you are done writing, file pointer is at the end of the file, so no data is there. Reposition pointer to start of the file.
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()
I am creating a tempFile and then adding text to it by opening it with an editor(so that user can enter his message), and then save it to read the text from save NamedTemporaryFile.
with tempfile.NamedTemporaryFile(delete=False) as f:
f.close()
if subprocess.call([editor.split(" ")[0], f.name]) != 0:
raise IOError("{} exited with code {}.".format(editor.split(" ")[0], rc))
with open(f.name) as temp_file:
temp_file.seek(0)
for line in temp_file.readlines():
print line
But every time it is coming out to be blank. why is it so ?
If you are using SublimeText as the editor, you will need to pass in the -w or --wait argument to make sure that the Python program waits until you actually close SublimeText again.
Otherwise, you would just start to edit the file, and immediately try to read its contents which at that point are empty.
You could also verify that, by putting in a print before reading the file.
E.g.:
editor = ['/path/to/sublime_text', '-w']
with tempfile.NamedTemporaryFile(delete=False) as f:
f.close()
if subprocess.call(editor + [f.name]) != 0:
raise IOError()
# read file…
Here's a solution without editor interaction since I don't have enough information to mimic your editor setup.
import tempfile
with tempfile.NamedTemporaryFile(delete=False) as f:
f.close()
with open(f.name, f.mode) as fnew:
# Replace with editor interaction
fnew.write('test')
fnew.seek(0)
for line in fnew.readlines():
print line
At least on my system and version (2.7.2), I had to make these changes to allow the file object to be properly reopened and interacted with, and it builds correctly in Sublime. f.name is the temporary file path, and f.mode is the mode used by the already closed file (in this case it should default to 'w+b').
In the case of a chained method when opening a file, eg.:
indata = open(from_file).read()
Is it necessary (or possible) to close the file handle opened with the open() function?
If not, is it best practice to instead do:
infile = open(from_file)
indata = infile.read()
infile.close()
In the case of a chained method when opening a file
This is the pitfall of chained method to open files, so the suggested solution is to use the with clause. The lifetime of an object is within the with block and the fileObj is closed automatically
with open(from_file) as fin:
indata = fin.read()
Why it is wrong?
You are at the mercy of GC to close files
If exception happens, you may not know where the exception happened, while opening or while reading
The other code piece
infile = open(from_file)
indata = infile.read()
infile.close()
Has also its pitfall.
If an exception happens, the file may not be closed
The code might take a different route and the close statement may not be exceuted
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.