How to read specific lines from a file in Python

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How to read specific lines from a file in Python

Author：JIYIK Last Updated：2025/05/05 Views：

A common way to read files in Python is to read the file completely and then process specific lines. Reading files in Python is fast, for example, it takes about 0.67 seconds to write a 100MiB file. However, if the file size exceeds 100MB, it will cause memory issues when reading it into memory.

Python has 3 built-in methods to read specific lines from a file, as described in the following sections.

`fileobject.readlines()`Read specific lines of a small file

fileobject.readlines()Reads all file contents into memory. It can use list slicing to read specific lines.

If we only need to read line 10,

with open("file.txt") as f:
    data = f.readlines()[10]
print(data)

If we need to read 10 to 100 rows,

with open("file.txt") as f:
    data = f.readlines()[10:100]
print(data)

Loop over file object `for`to read specific lines in Python

For file objects, for lineit is also a fast solution for small files.

lines = [10, 100]
data = []
i = 0

with open("file.txt", "r+") as f:
    for line in f:
        if i in lines:
            data.append(line.strip)

        i = i + 1

print(data)

`linecache`Module to read specific lines in Python

linecacheFor reading many files, you can repeat or extract many lines:

import linecache

data = linecache.getline("file.txt", 10).strip()

String method strip()Returns a string with spaces removed from both ends.

linecacheThe module allows you to get any line from a Python source file while using caching for internal optimizations, a common practice for reading multiple lines from a single file. The traceback module uses it to retrieve source code lines for inclusion in a formatted traceback.

`enumerate`Function to read specific lines from a large file in Python

Larger files may cause problems such as out of memory when reading the file. In this case, we can use enumerate():

with open("file.txt") as f:
    for i, line in enumerate(f):
        pass  # process line i

Note that for row n, i = n-1.

enumerate()Functions are used to combine traversable data objects (such as lists, tuples, or strings) into indexed sequences, listing both the data and the data subscripts, typically forusing a loop in .

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