How Can I Efficiently Read Specific Lines from a Large File in Python?

Line-Specific File Reading in Python

When processing large text files, it's often necessary to read only specific lines rather than the entire file. This can optimize performance and save memory. Python offers ways to achieve this without loading the complete file into memory.

Reading Specific Lines Using Line Number

Suppose you want to read line 26 and line 30 from a large text file. A straightforward approach is to open the file and use a for loop to iterate over the lines:

fp = open("file")
for i, line in enumerate(fp):
    if i == 25:
        # 26th line
    elif i == 29:
        # 30th line
    elif i > 29:
        break
fp.close()

Note that i == n - 1 for the nth line. This allows you to specify the desired line numbers precisely.

Alternatively, if you're using Python 2.6 or later, you can use the following syntax:

with open("file") as fp:
    for i, line in enumerate(fp):
        if i == 25:
            # 26th line
        elif i == 29:
            # 30th line
        elif i > 29:
            break

This approach automatically handles file closing and is more concise.

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