Modifying a single line in a file [duplicate]

Thi开发者_如何转开发s question already has answers here: Editing specific line in text file in Python (11 answers) Closed 4 months ago.

Is there a way, in Python, to modify a single line in a file without a for loop looping through all the lines?

The exact positions within the file that need to be modified are unknown.

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This should work -

f = open(r'full_path_to_your_file', 'r')    # pass an appropriate path of the required file
lines = f.readlines()
lines[n-1] = "your new text for this line"    # n is the line number you want to edit; subtract 1 as indexing of list starts from 0
f.close()   # close the file and reopen in write mode to enable writing to file; you can also open in append mode and use "seek", but you will have some unwanted old data if the new data is shorter in length.

f = open(r'full_path_to_your_file', 'w')
f.writelines(lines)
# do the remaining operations on the file
f.close()

However, this can be resource consuming (both time and memory) if your file size is too large, because the f.readlines() function loads the entire file, split into lines, in a list.
This will be just fine for small and medium sized files.

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Unless we're talking about a fairly contrived situation in which you already know a lot about the file, the answer is no. You have to iterate over the file to determine where the newline characters are; there's nothing special about a "line" when it comes to file storage -- it all looks the same.

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Yes, you can modify the line in place, but if the length changes, you will have to rewrite the remainder of the file.

You'll also need to know where the line is, in the file. This usually means the program needs to at least read through the file up to the line that needs to be changed.

There are exceptions - if the lines are all fixed length, or you have some sort of index on the file for example