Develop Reference

Nesting 'r' after opening a file in write mode

so I had an exercise asking to write to a file (using newlines) and then open it in reading mode. I did exactly that and the console outputs the right result. What happened was that I tried to do it within the 'w' mode code block and the console outputs nothing.
For example:
with open('test.txt', 'w') as wf:
nicknames = ["Big Tuna", "Prison Mike", "Booster Seat"]
wf.write('\n'.join(nicknames))
with open('test.txt', 'r') as rf:
print(rf.read())
I understand that the program only closes the file after the with statement, but I need clarification on my understanding of what's happening here.
So, what I understood so far is that the program first creates the test.txt file (test.txt did not exist in my file path) and then proceeds to convert and write the given contents into the test.txt file. After that, the nested with tries to open a file named test.txt in reading mode, but the console will not output anything because the program is trying to open a file that is already opened, that's why it cannot read into an already opened file.
Please correct me if I'm misunderstood the above because I am unsure whether or not I've understood it correctly, thank you.

That’s not what’s happening. Unix systems, at least, will happily let you open a file multiple times.
However, Python’s IO is buffered by default. You need to flush the data you’ve written out to the file before you can read the data from it. See https://docs.python.org/3/library/io.html#io.IOBase.flush for more information about this. (Summary: put wf.flush() after the wf.write(…) call and before attempting to read from it.

Python Win 32 error while trying to rename a file

I have a folder with several csv-files in it. I have to change the filename of every file with a string that I find in the file. So I tried the script below. It looks like it is working until I try to rename the file.
What did I try:
First I didn't have the file.close() line in the program, but did didn't fix the problem
I added a line print(file.closed) to see if the file was actually closed
I tried to get the os.rename out of the indented 'with' block. But I keep getting the same error
I tried to get the os.rename out of any block. But then I get a Winerror 123, where it sais that the filename , directoryname etc. is incorrect.
I also read the questions WindowsError 32 while trying to os.rename and Windows Error: 32 when trying to rename file in python.
I understood that maybe I had to close the file with f.close since this is the handler, but that didn't work as well.
The code that I tried:
for f in glob.glob("/path/*.csv"):
with open(f, "r") as file:
#read the lines in the csv-file
data = file.read()
#search the lines that have been read for a pattern and save that in "search"
search = re.findall("some_pattern", data)
#The result was a list. With this line I tried to change it into a string
file.close()
Listtostring = ''.join([str(elem) for elem in search])
#I only want to use a part of the match in the new file name
name = Listtostring.replace("part_of_string", "")
os.rename(f,f+name)
I hope somebody can give me some tips and explain what I am doing wrong. Pretty new to Python, so if you can give me some insight in my mistakes, than it's appreciated!

Thank you for your comments and time. It seemed that one of the files that was opened was still busy in some process and therefore the code didn’t work. I first closed all the applications that were running, but that didn’t work. After that I restarted the computer and the script worked fine!

Parsing a .lis file in Python

I am trying to parse in a .lis file into python to perform further analysis on the data but every time I get the following error,
<_io.TextIOWrapper name='Data.lis' mode='r' encoding='cp1252'>
I am parsing in the file with the standard command,
open(fileName)
Is there a certain package I need to install or is my parsing method incorrect?

What you got as an output doesn't appear to be an error, it is just telling you that python opened the file, and you have a file type object now.
Further, the operation you performed only got you part of the way. When reading a file, you need to:
Open the file
Store it as a variable (usually)
Read the variable a line at a time
Parse the result of your reading
Close the file
I usually start by trying to open the file in a program like Notepad++. That way I can get an idea of what I am trying to parse.
Let's walk through an example:
filename = 'myfile.lis'
with open(filename) as f:
for line in f:
print(line)
The code above opens the .lis file, and then prints the file to the console one line at a time. The with statement ensure that the file gets closed after we're done.
However, you could just as well replace the print() command with a parse() command of your own choosing:
def parse(input_line):
if 'text' in input_line:
print('I found \'text\' in line \'{}\''.format(input_line))
Hopefully that will get you started. If you are able to provide more detail about what the contents of your .lis file is, or what you are looking to extract from that file, I'm sure many around here can provide better guidance.

Creating a new file in Python

I am a beginner, writing a python script in which I need it to create a file that I can write information to. However, I am having problems getting it to create a new, not previously existing file.
for example, I have:
file = open(coordinates.kml, 'w')
which it proceeds to tell me:
nameerror: name 'coordinates' is not defined.
Of course it isn't defined, I'm trying to make that file.
Everything I read on creating a new file says to take this route, but it simply will not allow me. What am I doing wrong?
I even tried to flat out define it...
file = coordinates.kml
file_open = open(file, 'w')
... and essentially got the same result.

You need to pass coordinates.kml as a string, so place them in quotes (single or double is fine).
file = open("coordinates.kml", "w")

In addition to the above answer,
If you want to create a file in the same path, then no problem or else you need to specify the path as well in the quotes.
But surely opening a file with read permission will throw an error as you are trying to access an nonexistent file.

To be future proof and independent of the platforms you can read and write files in binaries. For example if this is Python on Windows, there could be some alternations done to the end of line. Hence reading and writing in Binary mode should help, using switches "rb" and "wb"
file = open("coordinates.kml", "wb")
And also remember to close the file session, else can throw errors while re running the script.

Issues reading and writing txt files line by line

The script is written using PyQt4.10.1 and Python2.7
I have been working on a simple tool to do allow a user to search for paths and then save them out to a config file for another program to read later. If there is already a config file then the script reads it and displays the existing paths for the user to edit or add to. I wrote a gui to make it as user friendly as possible. There are a couple issues I am having with it.
First, when I read in the config file I am using the following code:
try:
self.paths = open(configFile, "r")
self.data = self.paths.readlines()
self.paths.close()
except:
self.data = None
if self.data is not None:
for line in self.data:
print line
#self.listDelegate is the model for my QListView
self.listDelegate.insertRows(0, 1, line)
When I do that I get the following in my gui:
This (above) is how it looks when you first input the data (before the data is saved and then reopened)
This (above) is how the data looks after the config file is saved and then read back in (note the extra space below the path).
The config file is only read in when the script is first opened.
the following is how the config file looks when it is written out.
C:\Program Files
C:\MappedDrives
C:\NVIDIA
Now all of that wouldnt be a big deal but when I open the config file to edit it with this tool then the extra space in the gui is read as another line break. so the config file is then printed as:
C:\Program Files
C:\MappedDrives
C:\NVIDIA
Then the problem just gets bigger and bigger every time I edit the file.
This issue leads me to the second issue (which I think may be the culprit). When I write the lines from the gui to the config file I use the following code:
rowCount = self.listDelegate.rowCount()
if rowCount > 0:
myfile = open(configFile, 'w')
for i in range(rowCount):
myfile.write(str(self.listDelegate.index(i).data(role = QtCore.Qt.DisplayRole).toPyObject()))
myfile.write("\n")
myfile.close()
I am assuming that the issue with the extra line breaks is because I am adding the line breaks in manually. The problem is that I need each path to be on its own line for the config file to be usable later. I don't have a lot of experience writing out text files and everyone says that the easiest way to write them out line by line is to add in the line breaks by hand. If anyone has any better ideas I would love to hear them.
Sorry for the long winded explanation. If I am not being clear enough please tell me and I will try to explain myself better.
Thanks for the help!

The problem is that every time you read the file the line break remains at the end of the line. From the description 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.
If you try
self.paths = open(configFile, "r")
self.data = self.paths.readlines()
for line in self.data:
print repr(line)
which prints the representation of every line as python code you will get something like
'C:\\Program Files\n'
'C:\\MappedDrives\n'
'C:\\NVIDIA\n'
As you later insert further newlines the easiest fix is probably to remove the trailing newline:
for line in self.data:
strippedLine = line.rstrip('\n')