python read a file, save a new column for each line ad save the same file

问题

I have a file with x,y,z values. I wish to find an elegant way to open and add a new value id to each line and save again the same file.

def get_point_grid_id(x,y,x_min,y_max,x_dist,y_dist):
    col = int((x - x_min)/x_dist)
    row = int((y_max - y)/y_dist)
    return (row, col)

1 1 10
2 2 10
3 3 10

the id will be

get_point_grid_id(1,1,0,10,1,1)
(9, 1)
get_point_grid_id(2,2,0,10,1,1)
(8, 2)
get_point_grid_id(3,3,0,10,1,1)
(7, 3)

the new file will be

1 1 10 (9, 1)
2 2 10 (8, 2)
3 3 10 (7, 3)

i am reading in Stackoverflow several approach and i tested several approaches. i am honest to say that i have tried and failed to save the new file.

i had tried the followig solution

with open(file_temp, "r+") as f:
    for line in open(file_temp):
        x,y,z = line.split()
        id = get_point_grid_id(float(x),float(y),0,10,1,1)
        element = [x,y,z,id]
        newelement = " ".join([str(e) for e in element])+ "\n"
        f.write(newelement)

but i get this error message

Traceback (most recent call last):
  File "<editor selection>", line 3, in <module>
ValueError: too many values to unpack

where newelement (real data) is

'481499.55 6244324.75 19.15 (377, 2909)\n'

回答1:

You can emulate the required behavior via the fileinput module but bear in mind that it will create a backup copy of your original 10GB+ file in the background:

#! /usr/bin/env python
import fileinput

def get_point_grid_id(x,y,x_min,y_max,x_dist,y_dist):
    col = int((x - x_min)/x_dist)
    row = int((y_max - y)/y_dist)
    return (row, col)

input_file = "test.dat"
#
# Add mode='rb' to the arguments of fileinput.input() if you are
# using a binary file on operating systems that differentiate 
# between binary and text files (e.g. Microsoft Windows). 
#
for line in fileinput.input(input_file, inplace=True):
    columns = line.split()
    if 3 == len(columns):
        x, y, z = columns
        id = get_point_grid_id(float(x),float(y),0,10,1,1)
        print "{0} {1} {2} {3}".format(x, y, z, id)

The inplace parameter passed to fileinput.input triggers the magic.

来源：https://stackoverflow.com/questions/15030038/python-read-a-file-save-a-new-column-for-each-line-ad-save-the-same-file

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python

performance

save

overwrite