Efficient way to edit the last line of a text file in Python

Question

I have a Python script which creates a large SQL insert file of about 300,000 lines. The problem with this is that the last line of the file ends up looking like this:

'),

Which results in a SQL error as it is expecting a closing semicolon.

I am looking for an efficient way to open the file to replace the comma with a semicolon on the last line.

Define 'efficient'. Time, CPU, memory efficient? Can't the Python script be altered to not emit that comma? — Martijn Pieters, Feb 01 '16 at 07:46
Your could seek to a character before end-of-file, see https://docs.python.org/2/tutorial/inputoutput.html. But it would be better to fix the original script which creates the SQL. — cdarke, Feb 01 '16 at 07:52

score 1 · Answer 1 · answered Feb 01 '16 at 09:10

1

The simplest way is to use file.seek which is built into the standard library. Take a look at https://docs.python.org/2/library/stdtypes.html?highlight=seek#file.seek

You simply need to set offset to 0 and the whence to os.SEEK_END.

answered Feb 01 '16 at 09:10

Josh Leeb-du Toit

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score 0 · Answer 2 · answered Apr 28 '22 at 13:26

I found this neat solution which did exactly what I needed:

# Get the last line in the file and replace the comma with a semicolon to close the SQL statement.
with open(file_to_execute, 'rb+') as filehandle:
    filehandle.seek(-1, os.SEEK_END)
    filehandle.truncate()
    filehandle.write(";")
    filehandle.close()

This was originally added to Revision 2 of the question.

Efficient way to edit the last line of a text file in Python

2 Answers2

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