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
– Mad Scientist, Commented Aug 11, 2010 at 13:54

Studer · Accepted Answer · 2010-08-11 14:03:04Z

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, 2010 at 14:03

answered Aug 11, 2010 at 13:49

Studer

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3 Comments

Studer Over a year ago

How do I correctly format the code block (with colour, ...) ?

Fred Larson Over a year ago

Don't use <pre>, just indent 4 space. The syntax highlighting is automatic.

chryss Over a year ago

Indent it by 4 spaces, or mark it and use the code button to do it.

Jochen Ritzel · Accepted Answer · 2010-08-11 15:04:16Z

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, 2010 at 15:04

answered Aug 11, 2010 at 13:55

Jochen Ritzel

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2 Comments

reconbot Over a year ago

I'd vote you up twice if I could, this wasn't his question but it should be his answer.

Jochen Ritzel Over a year ago

Wooble: Ok, I've never used AppEngine so I didn't know.

signine · Accepted Answer · 2010-08-11 13:51:42Z

2

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

answered Aug 11, 2010 at 13:51

signine

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Comments

Community · Accepted Answer · 2017-05-23 12:07:08Z

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, 2017 at 12:07

CommunityBot

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answered Aug 11, 2010 at 13:50

Alok Singhal

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4 Comments

reconbot Over a year ago

Why shouldn't he rely on that fact?

Katriel Over a year ago

is tests that the variables refer to the same object in memory, == tests that their values are equal. CPython caches small ints (<265 I think) for optimisation, so all references to them point to the same place in memory. This is not true of large ints, or guaranteed by the specification.

John La Rooy Over a year ago

@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

reconbot Over a year ago

I understand what you mean now, I've spent 30 minutes trying to find the docs on "is".

David Webb · Accepted Answer · 2010-08-11 14:00:07Z

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, 2010 at 14:00

answered Aug 11, 2010 at 13:53

David Webb

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2 Comments

John La Rooy Over a year ago

the else clause of the for loop gets executed if the for loop isn't terminated by a break, so this won't work

David Webb Over a year ago

@gnibbler - you're right; shouldn't have suggested something I don't use myself. Have changed to suggested solution.

John La Rooy · Accepted Answer · 2010-08-11 13:52:47Z

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, 2010 at 13:52

John La Rooy

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Comments

satoru · Accepted Answer · 2010-08-11 13:57:02Z

0

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('/')

answered Aug 11, 2010 at 13:57

satoru

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