Python Help To Debug An Error When Modify A File In A Stream Mode

I have the following problem. I am reading a file x,y,z as: 481492.93 6244326.24 26.56 481493.03 6244325.60 25.06 481493.17 6244324.68 22.89 481493.50 6244322.52 17.80 481492.84 62

Solution 1:

opening the line in r+ mode means that you read a line, ie 38 characters read. Then you modify those 38 characters Then, at the current file position (character 39) you over write the existing data

I would guess this is not what you want

Hope this helps

Solution 2:

This just isn't going to work. As Martijn said,

I

file objects have a buffer position. Every time you read a character the buffer position advances by 1. Suppose you read a line that's 10 characters long:

>>>myfile = open('some_file.txt')>>>myfile.tell() #gets the buffer position
0
>>>myfile.readline()
'012345678\n'

Now the buffer position is advanced by len(line) characters:

>>>myfile.tell()
10

This means that when you call myfile.write(), it starts writing at position 10.

II

You simply can't "insert" characters into a file without overwriting something, or appending characters to the end (assuming that the buffer position is at the end of the file).

So what do you do?

You can create a temporary file, and simultaneously read from your input file, and write to your temp file. Afterwards (if you should wish), you can replace your original file with the temporary one:

withopen(input_file) as infile, open(output_temp_file, "w") as outfile:
    for line in infile:
        x, y, z = line.split()
        new_line = ' '.join([x, y, z] + [function_of_xyz(x, y, z)]) + '\n'
        outfile.write(new_line)

You should also check out the csv module.