使用pickle来保存python中的字典

Question

I have a file hash_db.pickle that when I created it I saved a dictionary on it:

v = {hash_value:{"file name":file_name,"file size":file_size,"last scanned time":scanned_time}}

{123dfre345:{"file name":calc.pdf,"file size":234,"last scanned time":12:23 24/12/2013}}
{3gcdshj754:{"file name":star.pdf,"file size":10,"last scanned time":10:30 10/10/2013}}

so if I want to change from the file only last scanned time for 3gcdshj754

how could I do that?

Answer 1

You can use pickle .

import pickle
d = pickle.load(open('hash_db.pickle', 'rb'))
d['3gcdshj754']['last scanned time'] = '11:30 11/10/2015'
pickle.dump(d, open('hash_db.pickle', 'wb'))

But you might find the shelve module a little more convenient than direct use of pickle. It provides a persistent dictionary which seems to be exactly what you want. Sample usage:

import shelve
from datetime import datetime, timedelta

# create a "shelf"
shelf = shelve.open('hash_db.shelve')
shelf['123dfre345'] = {"file name": 'calc.pdf', "file size": 234, "last scanned time": datetime(2013, 12, 24, 12, 23)}
shelf['3gcdshj754'] = {"file name": 'star.pdf', "file size": 10, "last scanned time": datetime(2013, 10, 10, 10, 30)}
shelf.close()

# open, update and close
shelf = shelve.open('hash_db.shelve')
file_info = shelf['3gcdshj754']
file_info['last scanned time'] += timedelta(hours=+1, minutes=12)
shelf['3gcdshj754'] = file_info
shelf.close()

And that's it.

Answer 2

Using pickle is pretty simple, when writing, use

dct = {'3gcdshj754': {'file name': 'star.pdf', 'last scanned time': '10:30 10/10/2014', 'file size': '10'}}

import pickle
pickle.dump(dct, open("save.p", "wb"))

and then, when reading it, use

import pickle
dct_read = pickle.load(open("save.p", "rb"))

Note that either time, you have to open the file in binary mode ( b flag ).

Editing the content is now simple:

dct_read.values()[0]["last scanned time"] = '10:10 10/10/2010'

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Alternatively, as @mhawke suggests in his answer , you can use shelve .