For those wanting to delve into the versatile world of Python programming, understanding Python file operations is crucial. Of the many file operations, one that particularly stands out as useful and engaging is the function of Python’s readline(). In this tutorial, we will explore this function and learn how you can use it to manipulate and extract content from a text file.
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What is Python readline()?
Simply put, readline() is a Python function used for reading a single line from an open text file. To be more technically specific, it reads a line from the file, updates the current position in the file and then returns the line that was read.
What is readline() used for?
The readline() functionality finds its use in various applications, particularly when there’s a need to analyze and extract specific data from text files. Whether it’s reading game scores from a text file, organizing high score lists, or parsing out dialogue in a text-based RPG, readline() forms an essential part of the process.
Why should I learn it?
If you aim to master Python programming, understanding functions like readline() will prove to be of significant value. It not only allows you to handle files efficiently but also offers you the ability to enrich your projects, such as game development or data organization tools, with more functionality and depth. Furthermore, the complexity of the applications you can build becomes expanded once you’re familiarized with these concepts. Understanding readline() would be your stepping stone to more advanced Python topics like file manipulation and data analysis.
How to Use readline()
Let’s start with the basics. When you want to use readline(), first you need to open a file. Once the file is open, you can use the readline() function. Below is an example:
file = open('sample.txt', 'r') print(file.readline())
In this case, sample.txt is our file name and ‘r’ signifies that we are opening the file in read mode. When we call readline(), it reads the first line from the file.
Let’s see another example of how you would print out multiple lines from a text file using the readline() function.
file = open('sample.txt', 'r') print(file.readline()) # reads 1st line print(file.readline()) # reads 2nd line print(file.readline()) # reads 3rd line
This will output the first 3 lines of text from your file.
The readline() function will return an empty string once it has reached the end of the file. Trying to read beyond this point without checking may result in an error. To avoid this, we can use a simple if condition to check if the line is empty (which means end of file), as shown below:
file = open('sample.txt', 'r') while True: line = file.readline() if not line: break print(line)
In the above example, the loop will continue until readline() returns an empty string. As soon as it encounters an empty string, it breaks the loop, hence preventing any potential errors.
Removing the Newline Character
Python’s readline() function includes a newline character at the end of each line it reads, which can result in blank lines when printing out text. To avoid this, you can use the rstrip() function to remove the newline character:
file = open('sample.txt', 'r') while True: line = file.readline().rstrip('\n') if not line: break print(line)
In this example, rstrip(‘\n’) is used to remove the newline character from the text read from the file. Thus, there will be no blank lines between the lines of text.
Reading a Specific Number of Characters
One of Python’s convenient features is that readline() can take a number as an argument, representing the number of characters to read from the current line. Let’s see how this works:
file = open('sample.txt', 'r') print(file.readline(10)) # reads first 10 characters of the first line file.close()
This will print out the first 10 characters of the first line in the file.
Python makes it easy to count the number of lines in a file—simply use readline() inside a loop. Here’s an example:
file = open('sample.txt', 'r') count = 0 while file.readline(): count += 1 print("Number of lines in the file: ", count)
In this example, the loop continues until readline() returns an empty string (indicating the end of the file), while incrementing count with each iteration.
Using readline() with Other Functions
Python readline() works nicely with other file functions. For instance, you can use the tell() function, which returns the current position within the file, so you can see how the file position changes as you read lines:
file = open('sample.txt', 'r') print(file.readline()) print("Current file position: ", file.tell()) file.close()
This code will print out the first line in the file and then print the current position with file.tell(), showing where the next readline() call would begin reading from.
Reading Lines in Reverse
Python’s readline() function is meant to read lines sequentially, starting from the top. But what if we want to read lines from the bottom? For this purpose, we can use the reversed() function. Here’s how:
file = open('sample.txt', 'r') lines = file.readlines() for line in reversed(lines): print(line.rstrip())
In this example, we use readlines() to read all lines into a list, then reversed() to process those lines in reverse order. Note that we still use rstrip() to remove extra newline characters.
Where to go next?
Learning Python’s readline() function is only one stepping stone on your journey to mastering this highly versatile language. So, what’s next?
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Why choose Zenva’s Python Mini-Degree?
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