Free memory during loop

Question

I'm facing a memory error in my code. My parser could be summarized like that:

# coding=utf-8
#! /usr/bin/env python
import sys
import json
from collections import defaultdict


class MyParserIter(object):

    def _parse_line(self, line):
        for couple in line.split(","):
            key, value = couple.split(':')[0], couple.split(':')[1]
            self.__hash[key].append(value)

    def __init__(self, line):
        # not the real parsing just a example to parse each
        # line to a dict-like obj
        self.__hash = defaultdict(list)
        self._parse_line(line)

    def __iter__(self):
        return iter(self.__hash.values())

    def to_dict(self):
        return self.__hash

    def __getitem__(self, item):
        return self.__hash[item]

    def free(self, item):
        self.__hash[item] = None

    def free_all(self):
        for k in self.__hash:
            self.free(k)

    def to_json(self):
        return json.dumps(self.to_dict())


def parse_file(file_path):
    list_result = []
    with open(file_path) as fin:
        for line in fin:
            parsed_line_obj = MyParserIter(line)
            list_result.append(parsed_line_obj)
    return list_result


def write_to_file(list_obj):
    with open("out.out", "w") as fout:
        for obj in list_obj:
            json_out = obj.to_json()
            fout.write(json_out + "\n")
            obj.free_all()
            obj = None

if __name__ == '__main__':
        result_list = parse_file('test.in')
        print(sys.getsizeof(result_list))
        write_to_file(result_list)
        print(sys.getsizeof(result_list))
        # the same result for memory usage result_list
        print(sys.getsizeof([None] * len(result_list)))
        # the result is not the same :(

The aim is to parse (large) file, each line transformed to a json object that will be written back to a file.

My goal is to reduce the footprint because in some case this code raises a Memory error. After each fout.write i would like delete (free memory) obj reference.

I tried to set obj to None of call the method obj.free_all() but none of them free the memory. I also used simplejson rather than json, that have reduced the footprint but still to too large in some cases.

test.in is looking like:

test1:OK,test3:OK,...
test1:OK,test3:OK,...
test1:OK,test3:OK,test4:test_again...
....

Answer 1

Don't store many instance of class in array, instead do it inline. Example.

% cat test.in
test1:OK,test3:OK
test1:OK,test3:OK
test1:OK,test3:OK,test4:test_again

% cat test.py 
import json

with open("test.in", "rb") as src:
    with open("out.out", "wb") as dst:
        for line in src:
            pairs, obj = [x.split(":",1) for x in line.rstrip().split(",")], {}
            for k,v in pairs:
                if k not in obj: obj[k] = []
                obj[k].append(v)
            dst.write(json.dumps(obj)+"\n")

% cat out.out
{"test1": ["OK"], "test3": ["OK"]}
{"test1": ["OK"], "test3": ["OK"]}
{"test1": ["OK"], "test3": ["OK"], "test4": ["test_again"]}

If it is slow, don't write to file line by line, but store dumped json string in array and do dst.write("\n".join(array))