Python write line number

How to print line numbers of a file in Python?

Use the Python enumerate function to loop through a text or csv file and then, for each line print the line number and the line values. Here’s a simple snippet:

with open (your_file_path, 'r') as f: for i, line in enumerate (f): print(f'Line number:; Content: '.strip()) 

Create an Example file

Assuming that you have the following multi-line text written into a text file in your operating system (either Windows, Linux or macOS)

This is our log file.
It is located at the C:\MyWork directory.
This file was built with Python.

Find the line numbers and print the line contents

Our task is to print for each line of the file the line number and the line contents.

# import the path library - ships from Python 3.4 and onwards from pathlib import Path # replace with path of file in your environment file_path = Path('C:\WorkDir\multi_line_file.txt') # Verify that file exists if file_path.is_file(): #Open file and loop through lines, print line number and content with open (file_path, 'r') as f: for i, line in enumerate (f): print(f'Line number:; Content: '.strip()) else: print("The file doesn't exist.")

This will return the following result:

Line number:1; Content: This is our log file. Line number:2; Content: It is located at the C:\MyWork directory. Line number:3; Content: This file was built with Python.

Print specific lines from a file

Next we would like to search for a specific string in the file contents and print only the specific line which contains the string. We will use the re (regular expressions) module to easily parse the file.

 with open (file_path, 'r') as f: for i, line in enumerate (f): # search for words ending with the string 'thon' if (re.search(r'thon\b', line)): print(f'Line number:; Content: '.strip()) 

This will render the following result:

Line number:3; Content: This file was built with Python.

Get number of lines in a Python file

To count the number of lines in a text file with Python, proceed as following:

file_path = Path('C:\WorkDir\multi_line_file.txt') with open (file_path, 'r') as f: for l in file: l += 1 print (f'The file line count is : ') 

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Simple, efficient way to writing to a specific line number in python

Unless the new line is exactly as long as the old line, you best option would be to rewrite the file, anyway.

The easiest way to do so is to use the fileinput module:

import sys, fileinput for count, line in enumerate(fileinput.input(filename, inplace=True)): if count == 69: # Python counts from 0 line = newvalue sys.stdout.write(line) 

This creates a backup of the file, and output goes to a new file replacing the old. Writing to sys.stdout writes to than new file (the alternative is to use print but line still includes the newline).

In the special case where your lines are always the same length you can use

with open('myfile.txt', 'rb+') as f: f.seek(FIXED_LINE_LENGTH * line_number) f.write(new_line) 

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Get the Filename and a Line Number in Python

When working on real-world applications or side projects, we often have to retrieve line numbers and filenames for debugging purposes. Generally, this is done to understand what code is getting executed when or to analyse the flow of control of any application. In this article, we will learn how to get a line number and the filename of the Python script using Python.

Get the Filename and a Line Number in Python

To get the filename and a line number from the executing Python script, we can use the inspect module Python. The inspect module contains several utilities to fetch information about objects, classes, methods, functions, frame objects, and code objects. This library has a getframeinfo() method that retrieves information about a frame or a traceback object. This method accepts a frame argument about which it retrieves details. The currentFrame() method returns the frame object for the caller’s stack frame. We can use these utilities for our use case. Refer to the following Python code to understand the usage.

from inspect import currentframe, getframeinfo  frame = getframeinfo(currentframe()) filename = frame.filename line = frame.lineno print("Filename:", filename) print("Line Number:", line) 

Filename: full/path/to/file/main.py Line Number: 3 

As we can see, the filename attribute will return the full path to the Python file. In my case, the Python file’s name was main.py ; hence, it shows main.py in the output. And, the lineno attribute return the line number at which this frame = getframeinfo(currentframe()) statement was executed. The mentioned statement was execute at line 3 ; hence the output has a 3 after the Line Number label.

Vaibhav is an artificial intelligence and cloud computing stan. He likes to build end-to-end full-stack web and mobile applications. Besides computer science and technology, he loves playing cricket and badminton, going on bike rides, and doodling.

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