python code problem

Question

i have this code:

class Check(webapp.RequestHandler):
  def get(self):
    user = users.get_current_user()

    be = "SELECT * FROM Benutzer ORDER BY date "
    c = db.GqlQuery(be)

    for x in c:
      if x.benutzer == user:
        s=1
        break
      else:
        s=2
    if s is 0:
      self.redirect('/')

to check whether the user is registered or not. but it gives me an error:

Traceback (most recent call last):
  File "/Applications/GoogleAppEngineLauncher.app/Contents/Resources/GoogleAppEngine-default.bundle/Contents/Resources/google_appengine/google/appengine/ext/webapp/__init__.py", line 511, in __call__
    handler.get(*groups)
  File "/Users/zainab_alhaidary/Desktop/الحمد لله/check.py", line 23, in get
    if s is 0:
UnboundLocalError: local variable 's' referenced before assignment

what should i do???

Why are you fetching all users in the table if you only need information about one specific user (the current one)? And your variable naming scheme has some room for improvement, good names are important. — Mad Scientist, Aug 11 '10 at 13:54

Studer · Answer 1 · 2010-08-11T14:03:04.567

6

Define s before to assign it a value (also, change the test on s):

user = users.get_current_user()

be = "SELECT * FROM Benutzer ORDER BY date "

c = db.GqlQuery(be)

s=0    # <- init s here

for x in c:
  if x.benutzer == user:
    s=1
    break
  else:
    s=2
if s == 0:    # <- change test on s
  self.redirect('/')

edited Aug 11 '10 at 14:03

answered Aug 11 '10 at 13:49

Studer

611
2
8
21

How do I correctly format the code block (with colour, ...) ? – Studer Aug 11 '10 at 13:51

Don't use `

`, just indent 4 space.  The syntax highlighting is automatic.

– Fred Larson Aug 11 '10 at 13:53

Indent it by 4 spaces, or mark it and use the code button to do it. – chryss Aug 11 '10 at 13:54

Jochen Ritzel · Answer 2 · 2010-08-11T15:04:16.517

4

Why exactly are you loading all users, then looping through them, just to find one? Use a where clause:

be = "SELECT * FROM Benutzer WHERE benutzer=:1"
c = db.GqlQuery(be, user)
user_from_db = c.get()
if user_from_db is not None: # found someone
    dostuff()
else:
    self.redirect('/')

edited Aug 11 '10 at 15:04

answered Aug 11 '10 at 13:55

Jochen Ritzel

104,512
31
200
194

I'd vote you up twice if I could, this wasn't his question but it should be his answer. – reconbot Aug 11 '10 at 14:06
Wooble: Ok, I've never used AppEngine so I didn't know. – Jochen Ritzel Aug 11 '10 at 15:03

score 2 · Answer 3 · edited May 23 '17 at 12:07

2

You want to set s to 0 before the for loop starts. If the query returns zero items, your for loop doesn't loop even once, so s is undefined.

Also, you should use if s == 0: instead of if s is 0:. In CPython, they are both equivalent, but you shouldn't rely on the fact. See: the documentation for PyInt_FromLong and "is" operator behaves unexpectedly with integers.

edited May 23 '17 at 12:07

Community

1
1

answered Aug 11 '10 at 13:50

Alok Singhal

93,253
21
125
158

Why shouldn't he rely on that fact? – reconbot Aug 11 '10 at 13:55
`is` tests that the variables refer to the same object in memory, `==` tests that their values are equal. CPython caches small `int`s (<265 I think) for optimisation, so all references to them point to the same place in memory. This is not true of large `int`s, or guaranteed by the specification. – Katriel Aug 11 '10 at 14:00
2

@wizard, because it's an implementation detail of one implementation of python. there is jython, pypy, ironpython, unladenswallow... any of those might decide not to cache the small ints. besides the code should **say what you mean** – John La Rooy Aug 11 '10 at 14:06
I understand what you mean now, I've spent 30 minutes trying to find the docs on "is". – reconbot Aug 12 '10 at 00:10

score 2 · Answer 4 · answered Aug 11 '10 at 13:51

2

You're using 's' before you assign something to it. Add an 's = 0' in the appropriate location.

answered Aug 11 '10 at 13:51

signine

2,985
1
18
9

David Webb · Answer 5 · 2010-08-11T14:00:07.177

1

Your problem is that if c is an empty list then the code in the for loop is never run and s never gets set, hence the error:

UnboundLocalError: local variable 's' referenced before assignment

What the error is telling you that you're referencing - i.e. using - s before it has any value - i.e. before a value has been assigned to it.

To fix this you just ensure s always is assigned a value:

s = 0

for x in c:
    if x.benutzer == user:
        s = 1
        break
    else:
        s = 2

edited Aug 11 '10 at 14:00

answered Aug 11 '10 at 13:53

David Webb

190,537
57
313
299

the `else` clause of the `for` loop gets executed if the for loop isn't terminated by a `break`, so this won't work – John La Rooy Aug 11 '10 at 13:56
@gnibbler - you're right; shouldn't have suggested something I don't use myself. Have changed to suggested solution. – David Webb Aug 11 '10 at 13:59

score 0 · Answer 6 · answered Aug 11 '10 at 13:52

0

In the case that c is empty the if statement in the loop never gets executed

you should set s=0 before the for loop

answered Aug 11 '10 at 13:52

John La Rooy

295,403
53
369
502

score 0 · Answer 7 · answered Aug 11 '10 at 13:57

I don't know why you are doing this, but if I understand your code correctly, you have s=1 when x.benutzer == user, and s=2 otherwise (shouldn't this be s=0 if you are going to check against 0?).

for x in c:
   if x.benutzer == user:
      s=1
      break
   else:
      s=2
if s is 0:
   self.redirect('/')

Anyway, here's my solution:

if not any(x.benutzer == user for x in c):
   self.redirect('/')

python code problem

7 Answers7