I am working through the chapter exercises in Tony Gaddis's "Starting Out With Python" 3rd edition from a class I have taken previously. I'm in chapter 9 and Exercise 8 requires me to write a program that pickles a dictionary (name:email) to a file when it closes and unpickles that file retaining the data when it is opened. I have read every word in that chapter and I still don't understand how you can do both in the same file. When you use the open function it creates a file which, in my understanding, is a new file with no data. I'm thinking it may be a sequencing issue, as in where to put the dump and load lines of code but that doesn't make sense either. Logic dictates you have to open the file before you can dump to it.
If the 'open' function creates a file object and associates it with a file and this function appears early in the code (as in def main), what keeps it from zeroing out the file each time that line is called?
This is not a homework assignment. I have completed that class. I am doing this for my own edification and would appreciate any explanation which would help me to understand it. I have included my attempt at the solution which is reflected in the code below and will keep gnawing at it until I find the solution. I just thought since the gene pool is deeper here I would save myself some time and frustration. Thank you very much to those that choose to reply and if I am lacking in any pertinent data that would help to clarify this issue, please let me know.
import pickle
#global constants for menu choices
ADDNEW = 1
LOOKUP = 2
CHANGE = 3
DELETE = 4
EXIT = 5
#create the main function
def main():
#open the previously saved file
friends_file = open('friends1.txt', 'rb')
#friends = pickle.load(friends_file)
end_of_file = False
while not end_of_file:
try:
friends = pickle.load(friends_file)
print(friends[name])
except EOFError:
end_of_file = True
friends_file.close()
#initialize variable for user's choice
choice = 0
while choice != EXIT:
choice = get_menu_choice() #get user's menu choice
#process the choice
if choice == LOOKUP:
lookup(friends)
elif choice == ADDNEW:
add(friends)
elif choice == CHANGE:
change(friends)
elif choice == DELETE:
delete(friends)
#menu choice function displays the menu and gets a validated choice from the user
def get_menu_choice():
print()
print('Friends and Their Email Addresses')
print('---------------------------------')
print('1. Add a new email')
print('2. Look up an email')
print('3. Change a email')
print('4. Delete a email')
print('5. Exit the program')
print()
#get the user's choice
choice = int(input('Enter your choice: '))
#validate the choice
while choice < ADDNEW or choice > EXIT:
choice = int(input('Enter a valid choice: '))
#return the user's choice
return choice
#the add function adds a new entry into the dictionary
def add(friends):
#open a file to write to
friends_file = open('friends1.txt', 'wb')
#loop to add data to dictionary
again = 'y'
while again.lower() == 'y':
#get a name and email
name = input('Enter a name: ')
email = input('Enter the email address: ')
#if the name does not exist add it
if name not in friends:
friends[name] = email
else:
print('That entry already exists')
print()
#add more names and emails
again = input('Enter another person? (y/n): ')
#save dictionary to a binary file
pickle.dump(friends, friends1.txt)
friends1.close()
#lookup function looks up a name in the dictionary
def lookup(friends):
#get a name to look up
name = input('Enter a name: ')
#look it up in the dictionary
print(friends.get(name, 'That name was not found.'))
#the change function changes an existing entry in the dictionary
def change(friends):
#get a name to look up
name = input('Enter a name: ')
if name in friends:
#get a new email
email = input('Enter the new email address: ')
#update the entry
friends[name] = email
else:
print('That name is not found.')
#delete an entry from the dictionary
def delete(friends):
#get a name to look up
name = input('Enter a name: ')
#if the name is found delete the entry
if name in friends:
del [name]
else:
print('That name is not found.')
#call the main function
main()
