for some reason the readline() function in my following code seems to print nothing.
fileName = input()
fileName += ".txt"
fileA = open(fileName, 'a+')
print("Opened", fileA.name)
line = fileA.readline()
print(line)
fileA.close()
I'm using PyCharm, and I've been attempting to access 'file.txt' which is located inside my only PyCharm project folder. It contains the following:
Opened file!!
I have no idea what is wrong, and I can't find any relevant information for my problem whatsoever. Any help is appreciated.
Because you opened the file in a+ mode, the file pointer starts at the end of the file. After all, that is where you would normally append text.
If you want to read from the top, you need to place fileA.seek(0) just before you call readline:
fileA.seek(0)
line = fileA.readline()
Doing so sets the pointer to the top of the file.
Note: After reading the comments, it appears that you only need to do this if you are running a Windows machine. Those using a *nix system should not have this problem.
Related
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 have a simple code to write a file in a specific folder. System creates the file in the folder but couldn't write on it. It is on windows and I checked the IDE write access (Pycharm) they seems fine. File is empty.
Following with code is to read whether I could write or ensure the previous one is finished. It is not writing the short string to the file. I have tried it on command line but it didn't work there also.
with open ('C:/Users/***/Desktop/***/output.log',mode ='w', encoding ='utf-8') as a_file:
a_file.write ="test"
with open ('C:/Users/***/Desktop/***/output.log', encoding ='utf-8') as a_file:
print(a_file.read())
The write is a method (function), you need to call, instead of assigning to it.
with open ('C:/Users/***/Desktop/***/output.log', mode='w', encoding='utf-8') as a_file:
a_file.write("test") # <---
I am writing a script to log into a switch, write the config to a file, and then rename the file. I have the parts working separately. The issue is that I cannot figure out how to get all parts with in the same function so that I can use the function on a list of devices. I get a file not open for reading in the for 'line in f' statement. when as far as i can see the file is still open.
I have tried writing a function to rename the file that works on its own, but not when in this script with the other parts.
I have another script that i wrote that has the rename portion outside of the function which works, but will not work to rename the file if multiple hosts are called with the Exscript 'quickstart' module.
Thanks for any help,
from Exscript.util.start import quickstart
import os
import datetime
import time
time = datetime.datetime.now().strftime("%d-%m-%Y")
tm = 'c:/test/tmp.txt'
def do_something(job, host, conn):
f = open(tm, 'w+') #opens File with read and write permissions
conn.execute('term len 0')
conn.execute('sh run')
f.write(conn.response)
conn.execute('quit')
#this is the part where the error comes
for line in f:
if "hostname" in line:
host = line.strip()
test = 'c:/test/' + host[9:] + 'on' + time + '.txt'
os.rename(tm, test)
quickstart('ssh://x.x.x.x', do_something)
According to the manual, mode w+ truncates (removes all the content from) the file. If you want to open the file for both reading and writing without destroying its contents, use mode r+ or a+.
::edit:: Note, I'm not sure how this works on Windows.
You have to test the file pointer at the beginning of the file using f.seek(0). Or first write to the file then close it then reopen it for reading. But you dont need a file at all - you can as well work on a local variable.
If you are simply planning to write to a file using a python script as show below:
#!/usr/bin/python
count = 1
fo = open('DbCount.txt', 'w')
fo.write(str(count))
#fo.flush()
fo.close()
The Dbcount.txt file which was placed in the same folder as the script(attempting to modify the Dbcount.txt). i dont see any change in the txt file and no error is shown by the interpreter, its very strange, any help ?
first of all, always use the with statement variant, that will always close the file, even on errors:
#!/usr/bin/python
count = 1
with open('DbCount.txt', 'w') as fo:
fo.write(str(count))
then the 'w' overwrites your file each time you write to it. If you want to append, use 'a'.
About your specific problem, did you look only in the directory of your script, or in the current directory you're calling the script from? As you wrote your code, the file's path you write to is relative to where you execute your code from.
try:
import os
count = 1
with open(os.path.join(os.path.dirname(__file__), 'DbCount.txt'), 'w') as fo:
fo.write(str(count))
then it should output DbCount.txt in the same path as your script.
I'm attempting to do a "find and replace" in a file on a Mac OS X computer. Although it appears to work correctly. It seems that the file is somehow altered. The text editor that I use (Text Wrangler) is unable to even open the file once this is completed.
Here is the code as I have it:
import fileinput
for line in fileinput.FileInput("testfile.txt",inplace=1):
line = line.replace("newhost",host)
print line,
When I view the file from the terminal, it does say "testfile" may be a binary file. See it anyway? Is there a chance that this replace is corrupting the file? Do I have another option for this to work? I really appreciate the help.
Thank you,
Aaron
UPDATE: the actual file is NOT a .txt file it is a .plist file which is preference file in Mac OS X if that makes any difference
LINK to plist file:
http://www.queencitytech.com/plist.zip
Your code worked for me fine. However, I would suggest a different approach: don't try overwriting the file directly. I never like changing the file directly because if you have a bug or something like that the file is lost. Generate a new file then copy it over manually (or within python, if you really want to).
PATH = 'testfile.txt'
FILE = open(PATH)
OUT_FILE = open('out_' + PATH, 'w')
for line in FILE.readlines():
print >> OUT_FILE, line.replace('newhost', host),
Try using sys.stdout.write instead of print. readlines() retains the new line characters at the end of the read line. The print statement adds an additional new line character, so it's likely double spacing the file.