Basic question from pure beginner

[sato.photo]

I have been going through some Python Programming exercises while
following the MIT OpenCourseWare Intro to CS syllabus and am having
some trouble with the first "If" exercise listed on this page:

http://en.wikibooks.org/wiki/Python_Programming/Conditional_Statements#If_Exercises

I have been able to make the module quit after entering a password
three times, but can't get it to quit right away after the correct one
is entered. I know this is really basic, please forgive me. I have
no programming experience and have just started going through these
tutorials.

My code is here:

http://python.pastebin.com/m6036b52e

-daniel

 

[Peter Otten]

I have been going through some Python Programming exercises while
following the MIT OpenCourseWare Intro to CS syllabus and am having
some trouble with the first "If" exercise listed on this page:

http://en.wikibooks.org/wiki/Python_Programming/Conditional_Statements#If_Exercises

I have been able to make the module quit after entering a password
three times, but can't get it to quit right away after the correct one
is entered. I know this is really basic, please forgive me. I have
no programming experience and have just started going through these
tutorials.

My code is here:

http://python.pastebin.com/m6036b52e

-daniel

Some hints:

(1) Have the while loop check for both the correct password and the number
of tries. Pseudocode:

while <more tries allowed> and <password not entered correctly>:
...

(2) After the loop has terminated you can check whether the password was
entered correctly and print the appropriate message.

Aside: you can't mix if...else with other statements

correct:

if cond:
print "sunny"
else:
print "cloudy"
print "weather"

wrong:

if cond:
print "norwegian"
print "blue"
else: # syntax error, else must immediately follow the indented if-block:
print "moon"

you could fix it like so:

if cond:
print "norwegian"
print "blue"
if not cond:
print "moon"

Peter

 

[alex23]

I have been able to make the module quit after entering a password
three times, but can't get it to quit right away after the correct one
is entered.  

Not with the code you pasted, you haven't. There's a missing colon on
line 7 & line 9 isn't indented properly. It's always best if we're
referring to the same source when trying to help with a problem.

The problem you're having, though, has to do with the while loop and
the fact that it's only checking one condition: has the 'count'
counter been incremented to 3. What you need to do is _also_ check for
the success condition:

is_confirmed = False
while count != 3 or is_confirmed:
guess = raw_input("Enter your password: ")
guess = str(guess)
if guess != password:
print "Access Denied"
count = count + 1
else:
print "Password Confirmed"
is_confirmed = True

This also provides you with a nice boolean that shows whether or not
the password was confirmed, which may be useful for latter code.

However, whenever you want to loop a set number of times, it's usually
better to use a 'for' loop instead:

PASSWORD = 'qwerty'
MAXRETRY = 3
for attempt in xrange(MAXRETRY):
guess = str(raw_input('Enter your password: '))
is_complete = guess == PASSWORD
if is_complete:
print 'Password confirmed'
break # this exits the for loop
else:
print 'Access denied: attempt %s of %s' % (attempt+1, MAXRETRY)

This saves you from the burden of creating, incrementing & testing
your own counter.

If there's anything here that isn't clear, please don't hesitate to
ask.

 

[sato.photo]

Thank you for all of the help. With your assistance and help from the
Python Tutor mailing list I was able to come up with the following
code:

password = "qwerty"
correct_password_given = False
guess = "0"
count = 0
while count != 3 and not correct_password_given :
guess = raw_input("Enter your password: ")
guess = str(guess)
if guess != password:
print "Access Denied"
count = count + 1
else:
print "Password Confirmed"
correct_password_given = True


If I understand it correctly, the function will continue to loop until
either the count == 3 or until correct_password_give = True,
satisfying the two conditions of the assignment, which were to lock a
user out after three failed password attempts and to print "Access
Granted" and end the module if the correct password is given. Am I
understanding this correctly?

Thanks!

 

[Bruno Desthuilliers]

(e-mail address removed) a écrit :

Thank you for all of the help. With your assistance and help from the
Python Tutor mailing list I was able to come up with the following
code:

password = "qwerty"
correct_password_given = False
guess = "0"

You could just use None here:

guess=None

count = 0
while count != 3 and not correct_password_given :
guess = raw_input("Enter your password: ")
guess = str(guess)

IIRC, raw_input() already returns a string !-)

if guess != password:
print "Access Denied"
count = count + 1

Or:
count += 1

else:
print "Password Confirmed"
correct_password_given = True


If I understand it correctly, the function will continue to loop until
either the count == 3 or until correct_password_give = True,

There are many ways to write a same boolean test, and some are easier to
understand. Your test could be written:

while not (count == 3 or correct_password_given) :
# proceed

Does it answer your question ?-)

As a general rule:

* not A and not B <=> not (A or B)

satisfying the two conditions of the assignment, which were to lock a
user out after three failed password attempts and to print "Access
Granted" and end the module if the correct password is given.

Well, given your snippet, execution indeed terminates after either
successful 'login' or 3 failed attempts, but that's mostly because
there's no code to execute after the while loop...

 

[MRAB]

I have been going through some Python Programming exercises while
following the MIT OpenCourseWare Intro to CS syllabus and am having
some trouble with the first "If" exercise listed on this page:

http://en.wikibooks.org/wiki/Python_Programming/Conditional_Statements#If_Exercises

I have been able to make the module quit after entering a password
three times, but can't get it to quit right away after the correct one
is entered. I know this is really basic, please forgive me. I have
no programming experience and have just started going through these
tutorials.

My code is here:

http://python.pastebin.com/m6036b52e

raw_input() returns a string, so you don't need "guess = str(guess)".

The 'if' line should end with a colon.

Incrementing the count should probably be after the 'if' statement.

You can leave a loop prematurely with the 'break' statement.

 

[Charles Yeomans]

Let me offer a bit of editing.

First, using the condition count != 3 is perhaps risky. A mistake or
a change in logic in the loop body might result in an infinite loop.
So instead I suggest

while count < 3...

Second, I'd suggest storing the value 3 in a variable with a name that
describes it.

MaxAttempts = 3
while count < MaxAttempts

This suggests that you change the name 'count' to 'attemptcount'.

The variable 'guess' is used only inside the loop, so I suggest
removing the assignment outside the loop.

Finally, I'd remove correct_password_given from the loop test, and
replace it with a break statement when the correct password is entered.

password = "qwerty"
correct_password_given = False
attemptcount = 0
MaxAttempts = 3
while attemptcount < MaxAttempts:
guess = raw_input("Enter your password: ")
guess = str(guess)
if guess != password:
print "Access Denied"
attemptcount = attemptcount + 1
else:
print "Password Confirmed"
correct_password_given = True
break


Charles Yeomans

 

[Bruno Desthuilliers]

Charles Yeomans a écrit :

Let me offer a bit of editing.

First, using the condition count != 3 is perhaps risky. A mistake or a
change in logic in the loop body might result in an infinite loop. So
instead I suggest

while count < 3...

Second, I'd suggest storing the value 3 in a variable with a name that
describes it.

Pretty sensible suggestion, but then:

MaxAttempts = 3

follow conventions instead :

MAX_ATTEMPS = 3


(snip)

Finally, I'd remove correct_password_given from the loop test, and
replace it with a break statement when the correct password is entered.

Then using a while loop and manually incrementing a counter is a waste
of time - using a for loop would be simplier and less error-prone.

 

[Bruno Desthuilliers]

Scott David Daniels a écrit :
(snip)

And even simpler:
PASSWORD = "qwerty"
MAXRETRY = 3
for attempt in range(MAXRETRY):
if raw_input('Enter your password: ') == PASSWORD:
print 'Password confirmed'
break # this exits the for loop
print 'Access denied: attempt %s of %s' % (attempt+1, MAXRETRY)
else:
# The else for a for statement is not executed for breaks,
# So indicates the end of testing without a match
raise SystemExit # Or whatever you'd rather do.

Which is probably the most pythonic implementation.

 

[alex23]

And even simpler:
     PASSWORD = "qwerty"
     MAXRETRY = 3
     for attempt in range(MAXRETRY):
         if raw_input('Enter your password: ') == PASSWORD:
             print 'Password confirmed'
             break # this exits the for loop
         print 'Access denied: attempt %s of %s' % (attempt+1, MAXRETRY)
     else:
         # The else for a for statement is not executed for breaks,
         # So indicates the end of testing without a match
         raise SystemExit # Or whatever you'd rather do.

Nice one, I always forget about for-else

 

[Dennis Lee Bieber]

Nice one, I always forget about for-else

There is also the getpass module to play with!
--
Wulfraed Dennis Lee Bieber KD6MOG
(e-mail address removed) (e-mail address removed)
HTTP://wlfraed.home.netcom.com/
(Bestiaria Support Staff: (e-mail address removed))
HTTP://www.bestiaria.com/

 

[alex23]

        There is also the getpass module to play with!

I don't think I've ever seen getpass, so thanks for pointing that out.
Unfortunately, it wouldn't have helped the OP understand why his
original code wasn't working

 

[Dennis Lee Bieber]

I don't think I've ever seen getpass, so thanks for pointing that out.
Unfortunately, it wouldn't have helped the OP understand why his
original code wasn't working

Concur... But I was too lazy to work up an example incorporating
getpass along with an encrypted password <G>
--
Wulfraed Dennis Lee Bieber KD6MOG
(e-mail address removed) (e-mail address removed)
HTTP://wlfraed.home.netcom.com/
(Bestiaria Support Staff: (e-mail address removed))
HTTP://www.bestiaria.com/