Changes between Version 22 and Version 23 of NewbieMistakes

Timestamp:

Jul 7, 2006, 9:35:41 AM (18 years ago)

Author:

Paul Childs

Comment:

--

NewbieMistakes

@@ -169 +169 @@
 
 Make sure Apache can also write to the parent directory of the database. SQLite needs to be able to write to this directory.
+
+== QuerySets aren't Lists ==
+
+==== Problem ====
+
+You have been playing with the database API and noticed that a returned query set looks a lot like a list:
+
+{{{
+#!python
+>>> from mysite.polls.models import Poll,Choice
+>>> Poll.objects.all()
+[<Poll: What is up?>, <Poll: What is your favourite colour?>]  ## looks a lot like a list to me
+}}}
+
+But, it doesn't behave like a list in some cases.
+
+==== Solution ====
+
+Here are a couple of cases where the behaviour is not list-like and their solution.
+
+In Python this is how we can test for an empty list:
+
+{{{
+#!python
+>>> b=[]
+>>> b==[]
+True
+}}}
+
+This doesn't work with a QuerySet. You might try the following but it will fail:
+
+{{{
+#!python
+>>> from mysite.polls.models import Poll,Choice
+>>> p = Poll.objects.filter(question__startswith='ZZZZZZZZZZZZ')
+>>> p
+[]
+>>> p==[]
+False
+}}}
+
+The way to do it is test for the length of p: 
+
+{{{
+#!python
+>>> from mysite.polls.models import Poll,Choice
+>>> p = Poll.objects.filter(question__startswith='ZZZZZZZZZZZZ')
+>>> p
+[]
+>>> len(p) == 0
+True
+}}}
+
+Another case is when you want to retrieve the last member of a QuerySet:
+
+{{{
+#!python
+>>> from mysite.polls.models import Poll,Choice
+>>> p = Poll.objects.all()
+>>> p
+[<Poll: What is up?>, <Poll: What is your favourite colour?>]
+>>> p[-1]
+Traceback (most recent call last):
+  File "<console>", line 1, in ?
+  File "c:\python24\lib\site-packages\django-0.95-py2.4.egg\django\db\models\query.py", line 98, in
+__getitem__
+    assert (not isinstance(k, slice) and (k >= 0)) \
+AssertionError: Negative indexing is not supported.
+}}}
+
+The way I get the last member is: 
+
+{{{
+#!python
+>>> p[len(p)-1]
+<Poll: What is your favourite colour?>
+}}}