如何在Python中将列表列表写入文件？

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

Answer 1

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')

Answer 2

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.

Answer 3

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)

Answer 4

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'))