File.readlines() not returning any strings; possible file not opening

Question

I'm working on building a script for configuring disks automatically on linux systems. One of my functions is not working and for the life of me I cannot figure out why.

I've tested the code on pythontutor.com with no errors. When I run the code on my machine, I get no errors, but instead of the function returning data from the file "disks.txt" I get empty dictionaries.

I have tried to add print statements at different points in the process to see what is going on, but nothing prints.

Here is my function:

def check_for_disks():
    cmd = 'lsblk -l -o name,mountpoint'.split()
    driveDict = {}
    finalDict = {}
    diskFile = open("disks.txt", "w+")
    subprocess.Popen(cmd, stdout=diskFile)
    diskFile.close()
    diskFile = open("disks.txt", "r+")
    for line in diskFile.readlines():
        dictItem = line.split()
        try:
            driveDict[dictItem[0]] = dictItem[1]
        except(IndexError):
            driveDict[dictItem[0]] = "No MountPoint Defined"
    diskFile.close()
    for k, v in driveDict.items():
        if 'sd' in k:
            finalDict[k] = v
        else:
            pass
    return finalDict

This part works flawlessly and creates the file I want, with the relevant information:

def check_for_disks():
    cmd = 'lsblk -l -o name,mountpoint'.split()
    driveDict = {}
    finalDict = {}
    diskFile = open("disks.txt", "w+")
    subprocess.Popen(cmd, stdout=diskFile)
    diskFile.close()

This part fails:

diskFile = open("disks.txt", "r+")
for line in diskFile.readlines():

It is seemingly just not opening the file. I've checked the file with ls -la and seen its permissions are fine aka -rw-rw-r--

I've tried with open("disks.txt", "a+") as diskFile:

I've tried the options "r", "r+", "a+"

I've run the script sudo

Any help is appreciated

PLEASE SAVE ME I'M GOING NUTS

score 0 · Accepted Answer · answered May 29 '20 at 02:21

TLDR: the file is empty when you open it.

The following command opens a new thread, and the new thread, starts to write this file.

subprocess.Popen(cmd, stdout=diskFile)

But at the same time, as the file is begin created you start to read the file. Two threads makes this command faster, but I don't think you need that. Simply, wait until the file is finished before reading it.

something like this, should do what you want

p = subprocess.Popen(cmd, stdout=diskFile)
p_status = p.wait() #finish writing the file before starting to read it

Let me know if you really need to multiple threads, or if this snippet has any issues.

Thank you so much! This is my first time using subprocess and so I am unfamiliar with how things work. Your comment led me to search and find this: https://stackoverflow.com/questions/7681715/whats-the-difference-between-subprocess-popen-and-call-how-can-i-use-them which explains in more detail what is going on and how to use the different parts of subprocess. THANKS AGAIN! — Citizen356964, May 29 '20 at 02:38

File.readlines() not returning any strings; possible file not opening

1 Answers1