Limited looping - Dev Shed

Dev Shed Forums Sponsor:

January 29th, 2012, 03:44 PM

jnever1

Registered User

Join Date: Jan 2012

Location: MI

Posts: 6

Time spent in forums: 3 h 33 m 12 sec
Reputation Power: 0

Limited looping

I'm afraid I don't know how to phrase my topic any more clearly. As a result, my searching efforts have remained fruitless. If this topic has been addressed elsewhere, please simply post a link and consider the topic resolved.

What I would like to do is provide a fail-out for a password input much like any decent web site does. The user has X chances to input a correct password or else the system kicks the user.

My scenario is a mock bank transaction. User names and ID numbers have been pre-defined as globals with lists.

main() makes the following function call:
validate_customer_ID(customer_ID, customer_index, customer_name)

which refers to the following definition:

Code:

# Verify ID entered is correct for the given customer name
##
## This function compares int ID against ID in CUSTOMER_ID at an index retrieved
## previously from CUSTOMER_NAMES (see main()). If ID at index does not match,
## user is prompted with invalid statement and to re-enter.
##
def validate_customer_ID(ID_number, index, name):
    print('\nValidating '+name+"'s Customer ID...")
    i = 3
    while ID_number != CUSTOMER_ID[index]:
        if i == 0:
            ID_number = False
            return ID_number
        else:
            print('\n'+name+"'s Customer ID number is not \""+str(ID_number)+
                '\".\n',sep='')
            print(i,'entries remaining')
            ID_number = get_an_int('Please enter '+name+
                "'s Customer ID number: ")
            print('\nValidating '+name+"'s Customer ID...")
            i -= 1
    print('Success!\n')
    return ID_number

Which is followed by...

Code:

    if not customer_ID:
        print('Maximum attempts exceeded.')
        break

...in main() that is meant to jump out of main() and end the program.

The problem is that returning ID_number = False from the if statement doesn't seem to change customer_ID. Returning ID_number = input(...) from the while statement does...
(i've tested using various print statements that have proven that the "= False" isn't sticking)

If I force customer_ID = validate_customer_ID(customer_ID, customer_name, customer_index) then everything is good.

I am happy that it seems to work, but unhappy that I had to explicitly state what I understand to be implied.

Can someone walk me through the logic that is the root of this distinction?

I can post the whole program if necessary, but I have 250 lines of code, fully commented.

Last edited by jnever1 : January 31st, 2012 at 07:03 PM. Reason: figured out how to properly format code in forum ^^

January 29th, 2012, 09:13 PM

b49P23TIvg

Contributing User

Join Date: Aug 2011

Posts: 1,075

Time spent in forums: 4 Weeks 1 Day 4 h 41 m 27 sec
Reputation Power: 98

Two ways to get information directly from a python function:
* Use the function return value.
* Pass a mutable object in the argument list.

(Indirectly your function can change global data, class attributes,
files, and all sorts of bits about the external state of the world.)

Code:

# using the function return value

def f():
    return 'The alphabet'

alph = f()

# then everything is good.
customer_ID = validate_customer_ID(customer_ID, customer_name, customer_index)



# using a mutable object passed as an argument

def g(d):
    d['Frozen food'] = 'Peas'

grocery_trip = dict()
g(grocery_trip)
print(grocery_trip)

January 31st, 2012, 09:00 AM

jnever1

Registered User

Join Date: Jan 2012

Location: MI

Posts: 6

Time spent in forums: 3 h 33 m 12 sec
Reputation Power: 0

Quote:

Originally Posted by b49P23TIvg

Two ways to get information directly from a python function:

I appreciate your reply, b49P23TIvg.

However something remains unclear to me:

Before I attempted to implement a countdown, I was able to pass ID_result as an INT out of the function and into main() without explicitly stating customer_ID = f(stuff). As a matter of debugging, I found that the last INT value of ID_result was passed to main() regardless of establishing ID_result = False, in the case of i == 0.

Is the True/False logic repressed by literals/numericals?

Am I correct in my understanding that literals may be passed but not logicals? In question form: Why can I not pass ID_result = False from the function to main() once a numerical value has been established?

Thank you for your time.

January 31st, 2012, 10:19 AM

b49P23TIvg

Contributing User

Join Date: Aug 2011

Posts: 1,075

Time spent in forums: 4 Weeks 1 Day 4 h 41 m 27 sec
Reputation Power: 98

Quote:

Am I correct in my understanding that literals may be passed but not logicals?

Incorrect.

Numbers, strings, lists, dictionaries, ... These are all objects. Python passes objects.

I think that you're victim to a mistaken, inconclusive, or incomplete observation. Please post a complete program demonstrating the situation and tell your expectation. You might need to post 2 codes.

Meanwhile---see if this helps your understanding:

Code:

>>> d={}      # create a mutable object
>>> print(id(d))
41125472
>>> d[8]=32   # change the state of the object.
>>> print(id(d))  # id is unchanged.  d refers to the same object
41125472
>>> 
>>> 
>>> s='a non-mutable object'
>>> print(id(s))
42924720
>>> s[2]='X'   # illegal
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'str' object does not support item assignment
>>> print(id(s))
42924720
>>> s='another string'
>>> print(id(s))  # s refers to a different object
42925000
>>> def f(variable_local_to_f):
...     print(id(variable_local_to_f))
...     variable_local_to_f = 7
...     print(id(variable_local_to_f))
... 

>>> f(s)
42925000
9427936
>>> print(id(s)) # changing the function scope variable did not affect the module scope variable
42925000
>>>

Experiment: if the name variable_local_to_f were changed to s would the result differ? Answer: no. The scope matters. You can declare a variable to have global scope. I forgot about that in my first post. But anyway, you don't have a global statement, nor do I.

January 31st, 2012, 02:11 PM

jnever1

Registered User

Join Date: Jan 2012

Location: MI

Posts: 6

Time spent in forums: 3 h 33 m 12 sec
Reputation Power: 0

b49P23TIvg,
As before, your guidance is rich and much appreciated. I wish to spend more time than I have at the moment ( break between classes) digesting your example and performing, as you mentioned, experiments of my own.

The following is the code I had originally written:

Code:

#===============================================================================
# Program:	Bank of Python
# Programmer:	xxxxxx
# Date:         2012-01-28
# Abstract:		This program accepts a user name and ID (verified
#               against predefined and corresponding lists) and performs
#               withdrawal or deposit transactions against a predefined starting
#               balance. Invalid inputs are rejected then user is prompted for
#               correction.
#===============================================================================

# Initialize CONSTANTS
##
## LISTS, index is primary key to customer.
## CUSTOMER_NAMES provides a list of customers, identified only by first name, 
## who have been granted transaction access.
## CUSTOMER_ID provides a corresponding list of passcode-like IDs that confirm
## an individual customer.
## INITIAL_BALANCE, for lack of database access, contains a corresponding list
## of customer starting balances. These are never modified in the sense of a
## transaction but are used to populate appropriate mutables.
## ACCEPTABLE_ACTION contains a list of potential user input
##
CUSTOMER_NAME = ('sally','bill','linda','tom','rich','chris')
CUSTOMER_ID = (10,23,42,31,75,40) ## not the most 'secure' of methods
INITIAL_BALANCE = (10.00,10.00,10.50,25.72,1234.56,10.00)
ACCEPTABLE_ACTION = ('w','W','d','D') ## case difficult to enforce at input

# MESSAGES
##
## Used with message() function, meant to reduce the clutter in main()
##
## The opening comment
##
WELCOME = '''
   **********************************
   ***         Welcome to         ***
   ***     The Bank of Python     ***
   **********************************
'''

## Transaction options message
##
OPTIONS = '''Please select from the following transactions:

Make a (W)ithdrawal
Make a (D)eposit
<enter \"W\" or \"D\"> : '''

## The exit comment
##
GOODBYE = '''\nThank you for banking with us!
Goodbye!'''

## No express reason to pull this out of main() other than to clearly establish
## the an exit point/method early in the code. A second exit point exists where
## the user fails to enter a valid ID within a certain amount of attempts
##
NAME_PROMPT = '''Please enter your name: <press enter to quit> '''

# define main()
def main():
    while True: ## infinite loop
        ## get customer name, format to proper name convention
        customer_name = input(NAME_PROMPT).title()
        
        ## Pass customer input to name validation function
        customer_name = validate_name(customer_name)
        
        ## If no input is received, exit main()
        if not customer_name:
            break
        
        ## Say hello
        print('Hello, '+customer_name+'!\n')
        
        ## Get list index, used as primary key for further list data retrieval
        customer_index = CUSTOMER_NAME.index(customer_name.lower())        
        
        ## Name is verified, get user ID
        customer_ID = get_an_int('Please enter '+customer_name+
                                 "'s Customer ID: ")
        ## Validate ID
        validate_customer_ID(customer_ID, customer_index,
                                           customer_name)
        
        if not customer_ID:
            print('\nMaximum attempts exceeded.')
            print('Please try again later.')
            break
        
        ## ID validated. Get customer balance.
        print('One moment while we retrieve your information...')
        current_balance = float(INITIAL_BALANCE[customer_index])
        print('Retrieval complete.\n')        
        
        ## Call transaction_decision function to get user choice of transaction
        transaction_decision = get_transaction_choice(OPTIONS)
        
        ## Test user input for decision, call user indicated function
        if transaction_decision == "w":
            new_balance = withdrawal(current_balance)
        else:
            new_balance = deposit(current_balance)
        
        ## Print balance reflecting user transaction
        print('\nYour new balance is: $\t'+format(new_balance, '8,.2f')+'\n')

# Display various multi-line messages
def message(comment):
    print(comment)

# Verify name entered is in the list of current customers, ensure name is a
# customer name before return
##
## This function forces lower status on user name input and checks it against
## CUSTOMER_NAMES list. Where no match is found, user is prompted to re-enter or
## exit. Case formatting is performed to enable exact list matching and proper 
## address.
## 
def validate_name(name_result):
    print('\nOne moment while we validate '+name_result+' as a customer...')
    name_result = name_result.lower() 
    while name_result not in CUSTOMER_NAME:
        if not name_result:
            return name_result
        else:
            name_result = name_result.title()
            print("We're sorry, but "+name_result+' is not currently a'
                 ' customer.')
            name_result = input('\nPlease enter your name: <press enter to'
                               ' quit> ')
            if not name_result:
                return name_result
            else:
                print('\nOne moment while we validate '+name_result+' as a'
                     ' customer...')
                name_result = name_result.lower()
    name_result = name_result.title()
    print('Confirmed! '+name_result+' is a valued customer!\n')
    return name_result 

# Test ID input for type = int, ensure int is entered before return
##
## This function ensures that the ID input provided by the user is int-able.
## Where translation is not possible, user is prompted to enter a numerical.
##
def get_an_int(ID_prompt):
    ID_result = input(ID_prompt) # get user ID
    while isinstance(ID_result, str):
        try:
            ID_result = int(ID_result)
        except:
            print('\nThe Customer ID is a number.\n')
            ID_result = input(ID_prompt)
    ID_result = int(ID_result)
    return ID_result

# Verify ID entered is correct for the given customer name
##
## This function compares int ID against ID in CUSTOMER_ID at an index retrieved
## previously from CUSTOMER_NAMES (see main()). If ID at index does not match,
## user is prompted with invalid statement and to re-enter.
##
def validate_customer_ID(ID_number, index, name):
    print('\nValidating '+name+"'s Customer ID...")
    i = 3
    while ID_number != CUSTOMER_ID[index]:
        if i == 0:
            ID_number = False
            return ID_number
        else:
            print('\n'+name+"'s Customer ID number is not \""+str(ID_number)+
                '\".\n',sep='')
            print(i,'entries remaining')
            ID_number = get_an_int('Please enter '+name+
                "'s Customer ID number: ")
            print('\nValidating '+name+"'s Customer ID...")
            i -= 1
    print('Success!\n')
    return ID_number

# Prompt customer to choose a transaction operation, ensure a valid entry is
# received before return
##
## Would have been nice to be able to force lowercase on trans_result in order
## to reduce the valid response list. However, retaining original case for 
## invalid input rejection statement proved too problematic.
##
## This function is obtaining withdrawal or deposit decision from user. If the
## user does not provide proper input, user is prompted to re-enter.
##
def get_transaction_choice(trans_prompt):
    trans_result = input(trans_prompt) 
    while trans_result not in ACCEPTABLE_ACTION: 
        print('\n\"'+trans_result+'\" is not an available option.\n')
        trans_result = input(trans_prompt)
    return trans_result.lower()

# Test for numerical transaction 
##
## This function retrieves input from user regarding the desired amount of
## transaction, tests for float-ability and returns user input as float to
## main() if translatable. Otherwise, user is prompted to input numericals until
## a numerical is received.
##
def get_a_float(amount_prompt):
    trans_amount = input(amount_prompt)
    while isinstance(trans_amount, str):
        try:
            trans_amount = float(trans_amount) 
        except:
            print('\nPlease enter a numerical value.')
            trans_amount = input(amount_prompt)
    trans_result = float(trans_amount)
    return trans_amount

# Process a withdrawal
##
## This function forces float on balance retrieved from INITIAL_BALANCE list
## then calls the get_a_float function to retrieve numerical input from user. A
## test is then run to ensure that the user is not withdrawing more funds than 
## are available. If all OK, withdrawal is processed, new_balance is calculated
## and returned.
##
def withdrawal(balance):
    withdrawal_amount = get_a_float('\nHow much would you like to withdraw? ')
    while withdrawal_amount > balance:
        print('\nAmount exceeds available balance.')
        withdrawal_amount = get_a_float('\nHow much would you '
                                        'like to withdraw? ')
    new_balance = balance - withdrawal_amount
    return new_balance

# Process a deposit
##
## This function accepts balance from main() and forces float status.
## get_a_float() is called to retrieve numerical input from user. Since deposits
## are addition operations by nature, no validation is required. Deposit is
## processed, new_balance is calculated and returned.
##
def deposit(balance):
    deposit_amount = get_a_float('\nEnter the amount to deposit: ')
    new_balance = balance + deposit_amount
    return new_balance
    
## Begin program
message(WELCOME)
main()
message(GOODBYE)

My understanding, and expectation, was that setting ID_result = False and returning ID_result, which was associated with customer_ID through argument/parameter relationship, would reassign customer_ID on the way out to customer_ID = False.
Instead, the last numerical value for ID_result is passed to customer_ID. This behavior was proven with a print(customer_ID) statement immediately following the validate_customer_ID function call.

Replacing the naked function call with customer_ID = validate_customer_ID(...) makes everything work the way I intended.

I understand why/how the explicit statement works. I do not understand why/how the implicit statement only works with numbers.

As you said though, False is not an object...

#############################
Upon a more rigorous investigation, I now realize that ID_number was never "passing" its value back to customer_ID. customer_ID remained as first defined by user input. I failed to notice this because I was not varying my fail input (I kept using "99" as a fail ID). The logic worked because, regardless on custom_ID value, it was the function that would not exit until a valid response was entered (or time-out). Sound logic - faulty application.

Apparently, I need to go back and review the argument/parameter relationship again, as well as variable scope.

Thank you for your time.

Last edited by jnever1 : January 31st, 2012 at 06:58 PM. Reason: Underdeveloped debugging skills

January 31st, 2012, 05:05 PM

b49P23TIvg

Contributing User

Join Date: Aug 2011

Posts: 1,075

Time spent in forums: 4 Weeks 1 Day 4 h 41 m 27 sec
Reputation Power: 98

You're using python3. In python3 True False are both reserved words and objects. In earlier pythons you can reassign True and False .

For you followers who cannot read the code, I *think* it boils down to this, and if you can explain better than I have please go for it.

Code:

#! python3

# HERE  HERE  HERE  HERE  HERE  HERE  HERE  HERE  HERE  HERE  HERE  HERE  HERE  HERE  HERE 
DEMONSTRATE_ISSUE = False  # run the program with False and with True

CUSTOMER_NAME = ('sally','bill',)
CUSTOMER_ID = (1,2,)

def main():
    customer_name = CUSTOMER_NAME[0].title()
    customer_index = CUSTOMER_NAME.index(customer_name.lower())
    customer_ID = get_an_int('Please enter '+customer_name+"'s Customer ID: ")
    if DEMONSTRATE_ISSUE:
        # HERE  HERE  HERE  HERE  HERE  HERE  HERE  HERE  HERE  HERE  HERE  HERE  HERE  HERE  HERE 
        # explain why this version does not change customer_ID
        validate_customer_ID(customer_ID, customer_index,customer_name)
    else:
        customer_ID = validate_customer_ID(customer_ID, customer_index,customer_name)
    if not customer_ID:
        print('\nMaximum attempts exceeded.')
        print('Please try again later.')
        break
    print('One moment while we retrieve your information...')

def get_an_int(ID_prompt):
    ID_result = input(ID_prompt)
    while isinstance(ID_result, str):
        try:
            ID_result = int(ID_result)
        except:
            print('\nThe Customer ID is a number.\n')
            ID_result = input(ID_prompt)
    return ID_result

def validate_customer_ID(ID_number, index, name):
    print('\nValidating '+name+"'s Customer ID...")
    i = 3
    while ID_number != CUSTOMER_ID[index]:
        if i == 0:
            ID_number = False
            return ID_number
        else:
            print('\n'+name+"'s Customer ID number is not \""+str(ID_number)+'\".\n',sep='')
            print(i,'entries remaining')
            ID_number = get_an_int('Please enter '+name+"'s Customer ID number: ")
            print('\nValidating '+name+"'s Customer ID...")
            i -= 1
    print('Success!\n')
    return ID_number

main()

Comments on this post

jnever1 agrees!

January 31st, 2012, 06:46 PM

jnever1

Registered User

Join Date: Jan 2012

Location: MI

Posts: 6

Time spent in forums: 3 h 33 m 12 sec
Reputation Power: 0

Quote:

Originally Posted by b49P23TIvg

I *think* it boils down to this

Code:

## Simpler still..  
def main():
    A = 6
    B = 7
    C = 8
    print(A,B,C)
    pass_int_return_false(A,B,C)
    if not A:
        print('A was returned False without explicit assignment')
        print(A,B,C)
    else:
        print('A was not returned false without explicit assignment')
        print(A,B,C)
    print()

    A,B,C = pass_int_return_false(A,B,C)
    if not A:
        print('A was returned False with explicit assignment')
        print(A,B,C)
    else:
        print('A was not returned false with explicit assignment')
        print(A,B,C)

def pass_int_return_false(d,e,f):
    d = False
    e += 1
    f -= 1
    return d,e,f

main()

So, my question boiled down to this: Why can I pass arguments into a function but not parameters out of a function?

The misunderstanding has to do with the difference between call-by-reference and call-by-value evaluation strategies. When I did not explicitly assign the customer_ID = (function returning "False") I was assuming that renaming passed variables did only that, renamed for clarity sake. I did not understand that I was in fact making copies instead...

My professor did a poor job of expressing this feature of Python3.

Again, thank you for your examples and input. I would not have been able to phrase my (re)search to a conclusive result without it!

Rep to you.