Read Specific Lines From a File in Python

This article lets you know how to read a specific lines from a file by line number in Python.

Steps To Read Specific Lines From A File

Example: Read specific lines from file by line number

The following code shows how to read a text file by line number in Python. See the attached file used in the example and an image to show the file’s content for reference.

In this example, we are reading line number 4 and 7 and storing it in a list variable.

with open(r"E:\demos\files\read_demo.txt", 'r') as fp:
    # lines to read
    line_numbers = [4, 7]
    # To store lines
    lines = []
    for i, line in enumerate(fp):
        # read line 4 and 7
        if i in line_numbers:
            lines.append(line.strip())
        elif i > 7:
            # don't read after line 7 to save time
            break
print(lines)

Output:

['Fifth line', 'Eighth Line']

linecache Module Read line from a file by line number

In this section, we’ll see how to read file by line number using a linecache module.

Python’s linecache is another performance-optimized way to jump to a particular line in a text file. Assume if you have a large text file, then linecache is the correct choice.

The linecache module allows one to get any line from a Python source file, while attempting to optimize internally, using a cache, the common case where many lines are read from a single file. This is used by the traceback module to retrieve source lines for inclusion in the formatted traceback.

Python Documentation

Use the linecache.getline() method to read specific line from a file.

linecache.getline(filename, lineno, module_globals=None)

• Get line lineno from a file named filename. This function will not return any error if the line is not present in a file instead, it will return an empty string.
• Change the lineno to your desired line number, and you’re ready to go.

Example

import linecache

# read fifth line
line = linecache.getline(r"E:\demos\files\read_demo.txt", 5)
print(line)

Output:

Fifth line

Note: The linache reads the whole file in memory. So, if random access to line number is more important than performance, then use linache.

If you want to read more than one line number from a file using linecache use the below example.

import linecache

line_numbers = [2, 4]
lines = []
for i in line_numbers:
    x = linecache.getline(r"E:\demos\files\read_demo.txt", i).strip()
    lines.append(x)
print(lines)

Use readlines() to Read the range of line from the File

If your file size is small and you are not concerned with performance, then the readlines() method is best suited.

Reading a file in Python is fast if the file size is in a few MB.

The readlines() method reads all lines from a file and stores it in a list. You can use an index number as a line number to extract a set of lines from it.

This is the most straightforward way to read a specific line from a file in Python. We read the entire file using this way and then pick specific lines from it as per our requirement.

Use readlines()[start:end] to read range of lines.

• the start is the starting line number
• the end is the last line number
• To read from line number 3 to 5 use readlines()[2:5]
• To read a single line use fp.readlines()[2]. this will read the third line.

Example: Read line from 3 to 5

with open(r"E:\demos\files\read_demo.txt", 'r') as fp:
    # read line number 3 to 5
    # index starts from 0
    x = fp.readlines()[2:5]
    print(x)

Example: Read line 8

with open(r"E:\demos\files\read_demo.txt", 'r') as fp:
    # read line 8
    x = fp.readlines()[7]
    print(x)

You can also use the readline() method to read a file line by line, stop when you’ve gotten to the lines you want. Using this technique, we don’t need to read the entire file.

Example 3:

lines = [2, 5]
result = []
i = 0

with open("read_demo.txt", "r+") as fp:
    # access each line
    while True:
        line = fp.readline()
        # check line number
        if i in lines:
            result.append(line.strip())
        # line number greater than needed exit the loop
        # lines[-1] give last item from list
        if i > lines[-1]:
            break;
        i = i + 1
print(result)

Generator to Read Lines from a file by line number

A fast and compact solution could be a generator expression to read a file using the line numbers.

If the number of lines to be returned from a file vast, you should use the generator.

This solution accepts file pointer and line numbers to be read returns a generator object to iterate using a loop to get each line.

# read by line numbers
def get_lines(fp, line_numbers):
    return (x for i, x in enumerate(fp) if i in line_numbers)

with open(r"E:\demos\files\read_demo.txt", 'r') as fp:
    # read line 4 and 7
    lines = get_lines(fp, [3, 6])
    # print each line
    for line in lines:
        print(line.strip())

Output:

Fourth line
Seventh Line

for Loop in fileobject to Read Specific Lines in Python

If your file size is small and you are not concerned with performance, then use looping technique.

• Use for loop to iterate each line from a file object
• Use the if condition in each iteration of a loop to check the line number. If it matches, then save that line into a list.

Note: This will load entire file in memory.

Example:

lines = [2, 5]
result = []
i = 0

with open("read_demo.txt", "r+") as fp:
    # access each line
    for line in fp:
        # check line number
        if i in lines:
            result.append(line.strip())
        i = i + 1
print(result)

Output:

['Third line', 'Sixth Line']

Conclusion

• Use readlines() or readline() and loop to iterate lines from a file object if the file size is small.
• Use linecache for a more clean solution. It is fast if you are reading repeatedly or reading different lines from multiple files.
• Use a loop and enumerate() for large files because we don’t need to load the entire file in memory.