How do I write a list of lists to a file in python?

Question

The code I have is this. I am trying to radix sort a list of numbers into a list of lists and then write that list of lists to a file, with each individual list on a different line. If I try to use f.write, it gives me a character buffer expected error.

from math import log

b = open("radix.in.txt").readlines()
a = [int(i) for i in b]

f = open("radix.out.txt", "w")
def getIndex(num, base, digit_num):
    return (num // base ** digit_num) % base  

def newLists(base):
    return [ [] for i in range(base) ]  

def order(q, base, digit_num):
    placehold = newLists(base)
    for num in a:
        placehold[getIndex(num, base, digit_num)].append(num)  
    return placehold

def radixSort(a, base):
    passes = 3
    new_list = a
    for digit_num in range(passes):
    new_list = order(new_list, base, digit_num)
    list_c = [str(i) for i in new_list]
    print list_c


radixSort(a, 10)

Putting f.write(list_c) in after the print list_c gives this traceback

Traceback (most recent call last):
  File "C:\Users\Nolan Caldwell\Desktop\HW5.py", line 29, in <module>
    radixSort(a, 10)
  File "C:\Users\Nolan Caldwell\Desktop\HW5.py", line 26, in radixSort
    f.write(list_c)
TypeError: expected a character buffer object

Can you post the full error?

thegrinner
– thegrinner

2013-04-16 20:13:22 +00:00
Commented Apr 16, 2013 at 20:13 — thegrinner
– thegrinner, Commented Apr 16, 2013 at 20:13

Robᵩ · Accepted Answer · 2013-04-16 20:19:08Z

3

list_c is a list-of-str. To convert that to a single str, use str.join():

f.write('\n'.join(list_c)+'\n')

Or, if you prefer, you can write each str individually:

for s in list_c:
  f.write(s+'\n')

answered Apr 16, 2013 at 20:19

Robᵩ

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4 Comments

Nolan Caldwell Over a year ago

Both of these are just creating an empty text file.

Robᵩ Over a year ago

@NolanCaldwell - With all due respect, I don't believe you. The first one can't produce an empty text file -- it will always print at least one blank line. Further, when I test these, they produce output files with 10 lines, the contents of which depend upon the input given. Perhaps the lines you add aren't being executed?

Nolan Caldwell Over a year ago

I totally believe that it should work. I am stumped as to why it doesn't. I added the code you supplied to the radixSort method before the print statement which would surely execute them, no? For both of your code samples, a text file named "radix.out.txt" is created but nothing(it could be an empty line) is in the file.

Hai Vu Over a year ago

There are two possibilities: you write to the wrong file, foo.txt, thinking that you wrote to bar.txt (less likely), or your list is empty (most likely). That means list_c is empty, thus new_list is empty, which means order() returned an empty list. Investigate your code from there.

Aswin Murugesh · Accepted Answer · 2013-04-16 20:25:15Z

0

convert it to a string and use literal_eval to get it as an array while reading

f=open('file.txt','w')
a=[[1,2],[3,4],[4,5]]
b=str(a)
f.write(b)

and when you read,

import ast
f=open('file.txt','r')
a=f.read()
b=ast.literal_eval(a)

because the write() function can process only strings. Or convert the list of lists to a string and use write.

edited Apr 16, 2013 at 20:25

answered Apr 16, 2013 at 20:15

Aswin Murugesh

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Comments

thkang · Accepted Answer · 2013-04-16 20:26:31Z

0

Sounds like it is a great chance to use csv module.

import csv
with open('numbers.csv', 'wb') as csvfile:
    writer = csv.writer(csvfile)
    for lst in list_of_lists:
        writer.writerow(lst)

answered Apr 16, 2013 at 20:26

thkang

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Comments

tdelaney · Accepted Answer · 2013-04-16 20:35:29Z

0

As others have pointed out, you need to serialize the data in some way before you can write it to a file. You can pickle, json encode, xml encode, stick it in a database like sqlite, write it line by line, put it in a CSV file, and etc... The "best" method (there is no best) depends on what you plan to do with that file later. Do people need to read it? Do non-python programs need to read it? Is it large enough to be concerned about the amount of space it takes?

If you just want to save a list of integers and want it to be readable in many programming languages, I like json

import json
(... your other stuff here...)
new_list = order(new_list, base, digit_num)
json.dump(new_list, open('myfile.json', 'w'))

answered Apr 16, 2013 at 20:35

tdelaney

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