Day 2–101 Days of DevOps -Python Script to check if the file or directory exists

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Prashant Lakhera

Welcome to Day 2 of 101 Days of DevOps. On Day 1, we discussed OS Module.

101 Days of DevOps

At the end of Day 1, I gave you an assignment on checking if the file or directory exists. If you already finished the assignment, great job. If not, continue reading the blog or watch the below video.

Now the main question is why you want to check if the file or directory exists?

Checking If the file exists or not is very important, as you want to make sure that file exists before you open it or before you are writing to that file, or before you overwrite that file. In Python, there are different ways to check whether the file exists, but I am going to cover three main topics

1. open()
2. os.path()
3. pathlib

Approach 1:

Let start with open()

• In your python console, try to open a non-existing file using open(). You will get a FileNotFoundError.

>>> open("abc.txt")
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
FileNotFoundError: [Errno 2] No such file or directory: 'abc.txt'

• Try to open a file to which you don’t have permission. You can create a file and then change the permission via chmod.

touch filetest
chmod 100 filetest
ls -l filetest
---x------  1 plakhera  staff  0 Jun 27 16:50 filetest

Now in your Python console, try to open that file.

>>> open("filetest")
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
PermissionError: [Errno 13] Permission denied: 'filetest'

This time you got a different error PermissionError.

• Let’s explore one more use case in which you try to open a directory. This time you got IsADirectoryError.

>>> open("/etc")
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IsADirectoryError: [Errno 21] Is a directory: '/etc'

We need to write our script in such a manner to handle these exceptions gracefully. To do this, we can use try/except block.

try:
with open("abc.txt") as f:
print(f.read())
except Exception as e:
print(e)

• If you try to execute the above code, you will get the below output

[Errno 2] No such file or directory: 'abc.txt'

Approach 2:

Use of os.path() sub-module.

• We will start with os.path.exists(). I already covered os.path() in my youtube video and on Day 1, so I will quickly breeze over it. os.path.exists() will return True if the file/directory exist else False.

>>> import os
>>> os.path.exists("/etc/hosts")
True
>>> os.path.exists("/etc/abc.txt")
False

NOTE: os.path.exists() works for both files as well as for directory().

In some cases, after checking the file or directory exist, we need to make a distinction between file or directory. To do that, we need to use os.path.isfile() for file and os.path.isdir() for directory. It will return True if the file or directory exists; else, it will return False.

>>> os.path.isfile("/etc/hosts")
True
>>> os.path.isfile("/etc")
False
>>> os.path.isdir("/etc")
True

Now, if you need to write your code, you need to use a combination of both. Using os.path.exists() first, check if the file exists and then check if it's a file if you are checking for file or dir if you are checking for a directory using and.

import os
filename="/etc/hosts"

if os.path.exists(filename) and os.path.isfile(filename):
    print("File exist")
else:
    print("File doesn't exist")

• If you save the above script in test_file.py and run it, it will return File exists as an output as /etc/hosts file exist.

python3 test_file.py
File exist

Approach 3:

Use of pathlib module

• This is the newest module and available starting from Python 3.4. It provides a clean object-oriented interface when dealing with Paths.

>>> path = pathlib.Path("/etc/hosts")
>>> path.exists()
True
>>> path.is_file()
True
>>> path.is_dir()
False

Assignment:

1. Write a simple script to find a file in the filesystem. , let say you are searching for file /etc/hosts in your system.

BONUS

1. Try to make your script user-friendly so that it will accept arguments like the path to search(e.g.,/etc.) and then filename(, hosts) on the command line.

Hint: You need to use argparse module for that

I am looking forward to you guys joining the amazing journey.

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