Radioactive Head Radioactive Head - 3 months ago 9
Python Question

Python writing dict to txt file and reading dict from txt file

I am new to Python and am trying to write a dictionary to a txt file. Then read the dict values by typing the keys with raw_input. I feel like I am just missing one step but I have been looking for awhile now so any help would be appreciated.
EDIT: I get this error

File "name.py", line 24, in reading
print whip[name]
TypeError: string indices must be integers, not str

#!/usr/bin/env python
from sys import exit

class Person(object):
def __init__(self):
self.name = ""
self.address = ""
self.phone = ""
self.age = ""
self.whip = {}

def writing(self):
self.whip[p.name] = p.age, p.address, p.phone
target = open('deed.txt', 'a')
target.write(str(self.whip))
print self.whip



def reading(self):
self.whip = open('deed.txt', 'r').read()
name = raw_input("> ")
if name in self.whip:
print self.whip[name]
p = Person()

while True:
print "Type:\n\t*read to read data base\n\t*write to write to data base\n\t*exit to exit"
action = raw_input("\n> ")
if "write" in action:
p.name = raw_input("Name?\n> ")
p.phone = raw_input("Phone Number?\n> ")
p.age = raw_input("Age?\n> ")
p.address = raw_input("Address?\n>")
p.writing()
elif "read" in action:
p.reading()
elif "exit" in action:
exit(0)

Answer

Your code is almost right! You are right, you are just missing one step. When you read in the file, you are reading it as a string; but you want to turn the string back into a dictionary.

The error message you saw was because self.whip was a string, not a dictionary.

I first wrote that you could just feed the string into dict() but that doesn't work! You need to do something else.

Example

Here is the simplest way: feed the string into eval(). Like so:

def reading(self):
    s = open('deed.txt', 'r').read()
    self.whip = eval(s)

You can do it in one line, but I think it looks messy this way:

def reading(self):
    self.whip = eval(open('deed.txt', 'r').read())

But eval() is sometimes not recommended. The problem is that eval() will evaluate any string, and if someone tricked you into running a really tricky string, something bad might happen. In this case, you are just running eval() on your own file, so it should be okay.

But because eval() is useful, someone made an alternative to it that is safer. This is called literal_eval and you get it from a Python module called ast.

import ast

def reading(self):
    s = open('deed.txt', 'r').read()
    self.whip = ast.literal_eval(s)

ast.literal_eval() will only evaluate strings that turn into the basic Python types, so there is no way that a tricky string can do something bad on your computer.

EDIT

Actually, best practice in Python is to use a with statement to make sure the file gets properly closed. Rewriting the above to use a with statement:

import ast

def reading(self):
    with open('deed.txt', 'r') as f:
        s = f.read()
        self.whip = ast.literal_eval(s)

In the most popular Python, known as "CPython", you usually don't need the with statement as the built-in "garbage collection" features will figure out that you are done with the file and will close it for you. But other Python implementations, like "Jython" (Python for the Java VM) or "PyPy" (a really cool experimental system with just-in-time code optimization) might not figure out to close the file for you. It's good to get in the habit of using with, and I think it makes the code pretty easy to understand.

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