In my early days as a QA engineer / Python developer, I was working on test-related projects a lot. For such projects, the focus is on logging and the question: “How do I open a log file in Python?” comes up on a daily basis. Let’s look into possible solutions in this post.

How Do I Open A Log File In Python?

In Python, you can use the open() function to open a log file.

This function needs to know the file path and the mode (like ‘read’ or ‘write’) that you want to use.

In Python, the r mode is used to open a file in read mode.

with open('log.txt', 'r') as log_file:
    content = log_file.read()

print('content: ', content)

This means that when you open a file with this mode, you can read the data from the file, but you cannot write to it.

If you try to read from a file that you’ve opened in write mode (using the w mode), you will get an io.UnsupportedOperation error.

Traceback (most recent call last):
  File "/Users/robertsgreibers/projects/pythonic.me/how_do_i_open_a_log_file_in_python/1.py", line 10, in <module>
    content = log_file.read()
io.UnsupportedOperation: not readable

Also, keep in mind r mode will raise an error if the file doesn’t exist, so you should make sure the file you’re trying to open exists before using this mode.

Traceback (most recent call last):
  File "/Users/robertsgreibers/projects/pythonic.me/how_do_i_open_a_log_file_in_python/1.py", line 9, in <module>
    with open('not_existing.txt', 'r') as log_file:
FileNotFoundError: [Errno 2] No such file or directory: 'not_existing.txt'

⚠️ This is a very simple answer to a How Do I Open A Log File In Python? question. I’ll give you way more real Python project examples I’ve gathered over the years in the following sections of this post.

How Do I Read A Log File From A Command Line?

When you’re trying to read a log file not always Python needs to be involved.

Sometimes, it’s easier to just use common command line tools to read a log file.

One of the easiest ways to to read a log file from the command line, is to use the cat command.  

This command allows you to display the contents of a file on the command line, but it’s not always the pretties format.  (depends on the type of log file you’re reading)

(pythonic.me) robertsgreibers@MacBook-Pro-2 pythonic.me % cat log_file.txt
this is the content
this is the content
this is the content
this is the content
(pythonic.me) robertsgreibers@MacBook-Pro-2 pythonic.me % 

Take for example a log file provided by one of my students. 

File content comes from a real situation where one of my students had to analyze log files at her day job. 

And it would not make sense to read it with cat command as it’s a very large file. 

What can you do instead?

Well, we decided to create a log parser in Python. 🎉

I’m not going to share the code in this post, but the idea is very similar to what I have already explained in this post. 

And when you run Python executable through a command line, the log parser we wrote outputs a summary of the results: 

So, it really depends on your situation.

You can use cat command for a quick read if the file is not too large or you can develop your own log parser in python, make it output a summary of the results you’re looking for and keep executing it through command line.

I’ve done both and like I said, it entirely depends on how often you need to read the log file and how large is the file you’re trying to read.

How Do I Search And Open A File In Python?

If you don’t know the exact path of the file you want to open, you can use the os.walk() method to search for the file in the current directory and all subdirectories.

The os.walk() method returns a generator object that yields a tuple containing the current directory, a list of subdirectories in the current directory, and a list of files in the current directory.

You can iterate over the generator object to search for the file you want.

For example, to search for a file named log_file.txt in the current directory and all subdirectories, you can use the following code:

import os

for root, dirs, files in os.walk('.'):
    if 'log_file.txt' in files:
        # File found!
        file_path = os.path.join(root, 'log_file.txt')
        break

This code uses a for loop to iterate over the generator object returned by os.walk().

For each iteration, the for loop checks if the current directory contains a file named log_file.txt using the in keyword.

If the file is found, the code constructs the full path to the file using the os.path.join() method and assigns it to a variable named file_path.

The break statement is used to exit the for loop once the file is found.

Once you have the full path to the file, you can use the open() function to open it, as described in the previous answer.

For example:

f = open(file_path, 'r')

This code opens the file in read-only mode and assigns the resulting file object to a variable named f.

You can then use the methods of the file object to read the contents of the file or write to it, as described in the previous answer.

How Do I Search For A File Type In Python?

Another way to search for files in Python is to use the glob module, which provides a way to use wildcards to search for files in a directory.

The glob.glob() method takes a pattern as an argument and returns a list of files that match the pattern.

The pattern can include wildcards such as *, which matches any sequence of characters, or ?, which matches any single character.

For example, to search for a file named log_file.txt in the current directory, you can use the following code:

import glob

file_list = glob.glob('log_file.txt')

This code uses the glob.glob() method to search for a file named log_file.txt in the current directory. If the file is found, it will be added to the file_list variable.

You can also use wildcards in the pattern to search for multiple files at once.

For example, to search for all files in the current directory that end with the “.txt” extension, you can use the following code:

import glob

file_list = glob.glob('*.txt')

This code uses the glob.glob() method to search for all files that end with the “.txt” extension in the current directory.

If any files are found, they will be added to the file_list variable.

Once you have a list of files that match the pattern you specified, you can iterate over the list and use the open() function to open each file.

For example:

import glob

file_list = glob.glob('*.txt')

for file_name in file_list:
    f = open(file_name, 'r')

This code uses a for loop to iterate over the list of files returned by glob.glob().

For each file, the code opens the file in read-only mode and assigns the resulting file object to a variable named f.

You can then use the methods of the file object to read the contents of the file or write to it, as described in the previous answer.

Keep in mind that the glob module only searches for files in the current directory, so if you want to search for files in subdirectories, you will need to use the os.walk() method instead, as described in the previous answer.

What Do You Do When A File Is Not Found In Python?

It’s possible for files to not be found when using the os.walk() or glob modules in Python.

The os.walk() method returns a generator object that yields a tuple containing the current directory, a list of subdirectories in the current directory, and a list of files in the current directory.

If the file you’re looking for doesn’t exist in the current directory or any of its subdirectories, it won’t be included in the list of files returned by os.walk().

Similarly, the glob.glob() method returns a list of files that match a specified pattern.

If the file you’re looking for doesn’t exist in the current directory or doesn’t match the pattern you specified, it won’t be included in the list of files returned by glob.glob().

In either case, it’s a good idea to handle the case where a file is not found gracefully by providing a helpful error message and, if appropriate, exiting the program or returning to a point where the user can try again.

For example, if you’re trying to open a file using the open() function and the file doesn’t exist, you can use a try/except block to handle the FileNotFoundError that is raised by the open() function.

try:
    f = open('log_file.txt', 'r')
except FileNotFoundError:
    print('Error: File not found.')
    return

This code tries to open a file named log_file.txt in read-only mode using the open() function.

If the file doesn’t exist, a FileNotFoundError is raised and caught by the except block.

Inside the except block, the code prints an error message and returns from the current function, which will typically exit the program or return to a point where the user can try again.

Alternatively, if you’re using the os.path.exists() method to check if a file exists before trying to open it, you can handle the case where the file doesn’t exist by simply checking the return value of os.path.exists() and providing a helpful error message if the file doesn’t exist.

For example:

import os

if not os.path.exists('log_file.txt'):
    print('Error: File not found.')
    return

This code checks if a file named log_file.txt exists in the current directory using the os.path.exists() method.

If the file doesn’t exist, the code prints an error message and returns from the current function, which will typically exit the program or return to a point where the user can try again.