[python] How to save a dictionary to a file?

I have problem with changing a dict value and saving the dict to a text file (the format must be same), I only want to change the member_phone field.

My text file is the following format:

memberID:member_name:member_email:member_phone

and I split the text file with:

mdict={}
for line in file:
    x=line.split(':')
    a=x[0]
    b=x[1]
    c=x[2]
    d=x[3]
    e=b+':'+c+':'+d

    mdict[a]=e

When I try change the member_phone stored in d, the value has changed not flow by the key,

def change(mdict,b,c,d,e):
    a=input('ID')
    if a in mdict:
        d= str(input('phone'))
        mdict[a]=b+':'+c+':'+d
    else:
        print('not')

and how to save the dict to a text file with same format?

I'm not sure what your first question is, but if you want to save a dictionary to file you should use the json library. Look up the documentation of the loads and puts functions.

Save and load dict to file:

def save_dict_to_file(dic):
    f = open('dict.txt','w')
    f.write(str(dic))
    f.close()

def load_dict_from_file():
    f = open('dict.txt','r')
    data=f.read()
    f.close()
    return eval(data)

For a dictionary of strings such as the one you're dealing with, it could be done using only Python's built-in text processing capabilities.

(Note this wouldn't work if the values are something else.)

with open('members.txt') as file:
    mdict={}
    for line in file:
        a, b, c, d = line.strip().split(':')
        mdict[a] = b + ':' + c + ':' + d

a = input('ID: ')
if a not in mdict:
    print('ID {} not found'.format(a))
else:
    b, c, d = mdict[a].split(':')
    d = input('phone: ')
    mdict[a] = b + ':' + c + ':' + d  # update entry
    with open('members.txt', 'w') as file:  # rewrite file
        for id, values in mdict.items():
            file.write(':'.join([id] + values.split(':')) + '\n')

I would suggest saving your data using the JSON format instead of pickle format as JSON's files are human-readable which makes your debugging easier since your data is small. JSON files are also used by other programs to read and write data. You can read more about it here

You'll need to install the JSON module, you can do so with pip:

pip install json


# To save the dictionary into a file:
json.dump( data, open( "myfile.json", 'w' ) )

This creates a json file with the name myfile.

# To read data from file:
data = json.load( open( "myfile.json" ) )

This reads and stores the myfile.json data in a data object.

As Pickle has some security concerns and is slow (source), I would go for JSON, as it is fast, built-in, human-readable, and interchangeable:

import json
data = {'another_dict': {'a': 0, 'b': 1}, 'a_list': [0, 1, 2, 3]}
# e.g. file = './data.json' 
with open(file, 'w') as f: 
    json.dump(data, f)

Reading is similar easy:

with open(file, 'r') as f:
    data = json.load(f)

This is similar to this answer, but implements the file handling correctly.

Unless you really want to keep the dictionary, I think the best solution is to use the csv Python module to read the file. Then, you get rows of data and you can change member_phone or whatever you want ; finally, you can use the csv module again to save the file in the same format as you opened it.

Code for reading:

import csv

with open("my_input_file.txt", "r") as f:
   reader = csv.reader(f, delimiter=":")
   lines = list(reader)

Code for writing:

with open("my_output_file.txt", "w") as f:
   writer = csv.writer(f, delimiter=":")
   writer.writerows(lines)

Of course, you need to adapt your change() function:

def change(lines):
    a = input('ID')
    for line in lines:
      if line[0] == a:
        d=str(input("phone"))
        line[3]=d
        break
    else:
      print "not"

Pickle is probably the best option, but in case anyone wonders how to save and load a dictionary to a file using NumPy:

import numpy as np

# Save
dictionary = {'hello':'world'}
np.save('my_file.npy', dictionary) 

# Load
read_dictionary = np.load('my_file.npy',allow_pickle='TRUE').item()
print(read_dictionary['hello']) # displays "world"

FYI: NPY file viewer

We can also use the json module in the case when dictionaries or some other data can be easily mapped to JSON format.

import json

# Serialize data into file:
json.dump( data, open( "file_name.json", 'w' ) )

# Read data from file:
data = json.load( open( "file_name.json" ) )

This solution brings many benefits, eg works for Python 2.x and Python 3.x in an unchanged form and in addition, data saved in JSON format can be easily transferred between many different platforms or programs. This data are also human-readable.

file_name = open("data.json", "w")
json.dump(test_response, file_name)
file_name.close()

or use context manager, which is better:

with open("data.json", "w") as file_name:
    json.dump(test_response, file_name)

Python has the pickle module just for this kind of thing.

These functions are all that you need for saving and loading almost any object:

def save_obj(obj, name ):
    with open('obj/'+ name + '.pkl', 'wb') as f:
        pickle.dump(obj, f, pickle.HIGHEST_PROTOCOL)

def load_obj(name ):
    with open('obj/' + name + '.pkl', 'rb') as f:
        return pickle.load(f)

These functions assume that you have an obj folder in your current working directory, which will be used to store the objects.

Note that pickle.HIGHEST_PROTOCOL is a binary format, which could not be always convenient, but is good for performance. Protocol 0 is a text format.

In order to save collections of Python there is the shelve module.

I haven't timed it but I bet h5 is faster than pickle; the filesize with compression is almost certainly smaller.

import deepdish as dd
dd.io.save(filename, {'dict1': dict1, 'dict2': dict2}, compression=('blosc', 9))