Read only one line in file python

How to Read only the First Line of the File?

In this article, we will learn how one can read the first line only from a file in Python. We will use some built-in functions, some simple approaches, and some custom codes as well to better understand the topic.

Python handles various file operations. In the case of reading files, if the user wants to read only the first line or maybe a header, Python provides readline() function for it. Let us discuss three different methods to read the first line of the file. We will read the first line of the given sample.txt file.

Sample Text File

It is an exciting time to be a book reviewer. Once confined to print newspapers and journals, reviews now dot many corridors of the Internet forever helping others discover their next great read.

Example: Read the First Line using next()

We use the sample.txt file to read the first line. We open the file in read mode and uses next() to store the first line in a variable. The below code uses strip() function to remove extra newline characters. You can remove it according to your need.

with open('sample.txt', 'r') as f: first_line = next(f).strip() print(first_line)

It is an exciting time to be a book reviewer.

Example: Read the First Line using readlines()

We use the sample.txt file to read the contents. This method uses readlines() to store the first line. readlines() uses the slicing technique. As you can see in the below example, firstline[0].strip() , it denotes the stripping of newlines spaces from index 0. This is a much more powerful solution as it generalizes to any line. The drawback of this method is that it works fine for small files but can create problems for large files.

with open('sample.txt', 'r') as f: first_line = f.readlines() print(first_line[0].strip())

It is an exciting time to be a book reviewer.

Example: Read the First Line using readline()

We use the sample.txt file to read the contents. This is an efficient and pythonic way of solving the problem. This even works for in-memory uploaded files while iterating over file objects. This simply uses readline() to print the first line.

with open('sample.txt', 'r') as f: first_line = f.readline().strip() print(first_line)

It is an exciting time to be a book reviewer.

Note: Observe your output when you run the above code snippets without strip() function. You will notice an empty line along with the first line.

Conclusion

In this article, we learned to read the first line of the file by using several built-in functions such as next() , readlines() , readline() and different examples to read the first line from the given file.

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How to read only the first line of a file with Python?

In Python, file handling is a big topic spanning several scores of operations; one such operation is one of reading the first line of a file and this allows you to quickly access and retrieve specific information without reading the entire file. In this present article, we will examine several ways of executing the process of reading only the first line of a file using Python. You can follow the lucid and descriptive explanations and code examples below to understand and master the process of reading the first line of a file.

Using the readline() Method

To be able to read the first line of a file, you follow these steps:

• Step 1: You make use of the open() function to open the file in read mode.
• Step 2: You then assign the returned file object to a variable.
• Step 3: You also use the readline() method to read the first line of the file and keep it in a variable.
• Step 4: You next process the first line of the file (e.g., print it or perform other operations).
• Step 5: At last, you can close the file to ensure proper handling of system resources.

Here is the code snippet given below that showcases the process:

Example

Suppose we have a file name myfile.txt with following contents

#myfile.txt This is a test file # Open the file for reading file = open('myfile.txt', 'r') # Read the first line of the file first_line = file.readline() # Process the first line (e.g., print it) print("First line:", first_line) # Close the file file.close()

When the above code is run, and when we open the myfile.txt this is the output that we see

Output

First line: This is a test file

Using a Loop to Read the First Line

In an alternate method, you can achieve the same purpose by using a loop to read the first line of a file. Even though this method is not so efficient, it still allows you to handle different situations where the first line may contain special characters or it may require additional processing.

Let us consider an example code snippet that is given below:

Example

Suppose we have a file name myfile.txt with following contents

#myfile.txt This is a test file # Open the file for reading file = open('myfile.txt', 'r') # Read the first line of the file using a loop for line in file: first_line = line break # Process the first line (e.g., print it) print("First line:", first_line) # Close the file file.close()

When the above code is run, and when we open the myfile.txt this is the output that we see

Output

First line: This is a test file

Using Context Managers to Read the First Line

We have already learned that Python provides a method of conveniently reading files using context managers. This is how you can use a context manager to read the first line of a file:

• Step 1: You use the ‘with statement’ and the open() function to open the file in read mode.
• Step 2: Next assign the returned file object to a variable within the with block.
• Step 3: Use the readline() method to read the first line of the file and keep it in a variable.
• Step 4: Lastly, process the first line (e.g., print it or perform other operations).

Let us take a look at an example code snippet as is given below:

Example

Suppose we have a file name myfile.txt with following contents

#myfile.txt This is a test file # Open the file for reading using a context manager with open('myfile.txt', 'r') as file: # Read the first line of the file first_line = file.readline() # Process the first line (e.g., print it) print("First line:", first_line)

When the above code is run, and when we open the myfile.txt this is the output that we see

Output

First line: This is a test file

Using the next() Function to Read the First Line

To be able to read the first line of a file, you have another method of doing so; you can also use the next() function in combination with the open() function. This approach makes it possible for you to directly access the first line without using a ‘for loop’. Follow these steps:

• Step 1: Use the open() function to open the file in read mode.
• Step 2: Assign the returned file object to a variable.
• Step 3: Use the next() function with the file object as the argument to retrieve the first line.
• Step 4: Process the first line (e.g., print it or perform other operations).
• Step 5: At last, close the file to ensure proper handling of system resources.

Let us consider an example code snippet:

Example

Suppose we have a file name myfile.txt with following contents

#myfile.txt This is a test file # Open the file for reading file = open('myfile.txt', 'r') # Get the first line of the file using the next() function first_line = next(file) # Process the first line (e.g., print it) print("First line:", first_line) # Close the file file.close()

When the above code is run, and when we open the myfile.txt this is the output that we see

Output

First line: This is a test file

Reading and Stripping the First Line

It is found sometimes that; the first line of a file may contain leading or trailing whitespace characters. To remove such whitespace characters, you make use of the strip() method. Let us look at an example code snippet:

Example

Here, we make use of the readline() method to read the first line of the file and immediately apply the strip() method to remove any leading or trailing whitespace characters. This makes it sure that the processed first line does not contain any unnecessary spaces.

Suppose we have a file name myfile.txt with following contents

#myfile.txt This is a test file # Open the file for reading file = open('myfile.txt', 'r') # Read the first line of the file first_line = file.readline().strip() # Process the first line (e.g., print it) print("First line:", first_line) # Close the file file.close()

When the above code is run, and when we open the myfile.txt this is the output that we see

Output

First line: This is a test file

By now you must have figured out that the operation of reading only the first line of a file is a very routine and common operation in Python file handling. Here, in this article, we went through some examples that gave an illustration of how to read the first line of a file using different approaches. We also gave additional examples that made clear how to read the first line using the next() function and how to strip any leading or trailing whitespace characters from the first line. By following the clear and lucid explanations and code snippets, you must have gained a fair understanding of reading the first line of a file in Python. It is better we remember to close, without fail, the file after reading it, to ensure proper handling of system resources.

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Read specific line of file in Python without filling memory

I’m trying to read through a very large text file (> 1.5gb) line by line but would like to avoid loading the whole file into memory, is there a way to just read a specific line at once without loading everything first?

It’s easy enough to do this without loading the whole file at once, but probably impossible to do it without at some point loading every part of the file up to the line you want, as there’s no way to figure out where the n th line is without reading all the lines before it.

6 Answers 6

To read every line one by one you can do

with open('file.txt') as file: for line in file: print(line) 

Actually when you open a file, you will just get a file handle of the file. The file is never fully loaded in memory unless you specifically want to do that.

If you know what byte offset the line exists at, you can use seek to move the file handle’s current position to just the line you want. Of course, to find that offset, you’ll need to count newlines or the like.

One way is to just iterate over the file:

from itertools import count with open('myfile') as f: line_index = 10 c = count() while next(c) < line_index-1: next(f) my_line = next(f) 

This would only load in one line at the time, and throw away the result until you reach the desired line.

Unless you have an index built or fixed length records, you'll probably need to read the file, one line at a time, throwing out early values until you get the one you want.

In fact, readlines could also do this, the default readlines without arguments will keep reading until got an EOF, but provided the extra arguments, it will read the size data at most.

Print only one specified line without loading the whole file:

line = 19 with open(full_name, encoding='utf8') as f: [next(f) for x in range(line-1)] print(next(f)) 

Print only first n lines without loading the whole file:

n = 5 with open(r'C:\Temp\test.txt', encoding='utf8') as f: head = [next(f) for x in range(n)] print(*head, sep='') 

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