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

Reading Specific File Lines Using Line Numbers

When processing large files, it is often necessary to read only specific lines. To achieve this, Python provides several methods that allow you to navigate a file and read lines based on their line numbers.

Using a For Loop

If the file is small or memory constraints are not an issue, you can use a for loop to iterate through the file and read lines by their line numbers. For example, to read lines 26 and 30 from a file:

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 the line numbers in Python start from 0, so line number 26 corresponds to the 27th line in the file.

Using the with Statement (Python 2.6 or later)

Another approach, introduced in Python 2.6, involves using the with statement to open the file and iterate through its lines:

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 method ensures that the file is automatically closed when the loop exits, even in the event of an exception.

By utilizing these techniques, you can efficiently read specific lines from a file without having to parse the entire file or use external libraries.

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