What's the fastest way to split a text file using Python?


Splitting a text file in Python can be done in various ways, depending on the size of the file and the desired output format. In this article, we will discuss the fastest way to split a text file using Python, taking into consideration both the performance and readability of the code.

split() method

One of the most straightforward ways to split a text file is by using the built-in split() function in Python. Based on a specified delimiter this function splits a string into a list of substrings.

For example, the following code splits a text file by newline characters and returns a list of lines −

with open('file.txt', 'r') as f:
   lines = f.read().split('\n')

Here,

  • The built-in split() function splits a text file by newline characters and returns a list of lines.

  • The code starts by opening the file using the open() function, with 'r' as the mode, which stands for reading. This returns a file object, which is stored in the variable f.

  • Next, the read() method is used on the file object to read the entire contents of the file into memory as a single string.

  • The split() function is then called on this string, with the newline character \n passed as the delimiter. This splits the string into a list of substrings, where each substring corresponds to a line in the original file. Finally, the result is stored in the variable lines.

readline() method

The previous method is simple and easy to read, but it can be slow for large files as it reads the entire file into memory before splitting it. If you are working with a large file, you may want to consider using the readline() method instead, which reads one line at a time.

with open('file.txt', 'r') as f: lines = [] for line in f: lines.append(line)

From the example,

  • The code starts by opening the file in the same way as the previous example.

  • Then we create an empty list called lines. Next, we use a for loop to iterate over the file object.

  • The readline() method is called on the file object inside the for loop, which reads one line at a time from the file and assigns it to the variable line. This variable is then appended to the lines list.

  • This way the entire file is read line by line and the lines are stored in the list.

This method is faster than the previous one as it reads one line at a time, and it does not require loading the entire file into memory. However, it still reads the entire file and can be slow for very large files.

mmap module

Another option is to use the mmap module in Python, which allows you to memory-map a file, giving you an efficient way to access the file as if it were in memory. Here's an example of how to use mmap to split a text file −

import mmap with open('file.txt', 'r') as f: # memory-map the file mmapped_file = mmap.mmap(f.fileno(), 0, access=mmap.ACCESS_READ) # split the file by newline characters lines = mmapped_file.read().split('\n')

This method is the most efficient for large files, as it allows you to access the file as if it were in memory without actually loading the entire file into memory.

  • The code starts by importing the mmap module.

  • Next, the file is opened in the same way as before, and the fileno() method is called on the file object to get the file descriptor for the file.

  • This is passed as the first argument to the mmap() function, along with 0 and mmap.ACCESS_READ as the second and third arguments, respectively. This memory maps the file, and the result is stored in the variable mmapped_file.

  • The read() method is then called on the memory-mapped file, which reads the entire contents of the file into a single string, as before.

  • The split() function is then called on this string, again with the newline character \n passed as the delimiter. This splits the string into a list of substrings, where each substring corresponds to a line in the original file. Finally, the result is stored in the variable lines.

Conclusion

In conclusion, the fastest way to split a text file using Python depends on the size of the file. If the file is small, the split() function or the readline() method can be used. However, for large files, the mmap module should be used to memory-map the file, providing a fast and efficient way to access the file.

Updated on: 01-Feb-2023

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