Is it ever the case that __nonzero__ would return a value inconstant
from using __len__ so using it instead of len would cause a bug?
John
On Jun 5, 2007, at 10:00 PM, Heikki Toivonen wrote:
There are lots and lots of cases in our code where we use len() to
determine if a collection, list, tuple, string etc. is empty or not.
This is wasteful; it is much better to check if the item in
question is
empty. Python even has a special method for it, __nonzero__.
Also, doing a len() on some objects causes side effects to happen that
you may not want, like creating repository indexes (of course avoiding
index creation in one spot will likely just move it to another spot
which you may not want).
Any time you see one of these used to get a truth value, you may be
looking at a bug:
len(...)
len(...) == 0
len(...) != 0
len(...) > 0
What you may want to do instead is just check if the thing marked with
... evaluates to True. For example:
if mylist:
In some cases you have a function/method returning a truth value:
def isEmpty(self):
return bool(self.myList)
although if you see something like above you might want to do some
refactoring as well to get rid of that method.
This holds at least for builtin lists, tuples, dicts and strings. It
should also hold for Chandler Collection Items.
Below are some timeit runs to compare list, string, dict and tuple
performance:
[EMAIL PROTECTED]:~$ python -m timeit "a=[]" "if len(a)>0:pass"
10000000 loops, best of 3: 0.163 usec per loop
[EMAIL PROTECTED]:~$ python -m timeit "a=[]" "if len(a):pass"
10000000 loops, best of 3: 0.164 usec per loop
[EMAIL PROTECTED]:~$ python -m timeit "a=[]" "if a:pass"
10000000 loops, best of 3: 0.0892 usec per loop
[EMAIL PROTECTED]:~$ python -m timeit "a=[1,2,3,4]" "if len(a)>0:pass"
1000000 loops, best of 3: 0.328 usec per loop
[EMAIL PROTECTED]:~$ python -m timeit "a=[1,2,3,4]" "if len(a):pass"
1000000 loops, best of 3: 0.3 usec per loop
[EMAIL PROTECTED]:~$ python -m timeit "a=[1,2,3,4]" "if a:pass"
1000000 loops, best of 3: 0.242 usec per loop
[EMAIL PROTECTED]:~$ python -m timeit "a=''" "if len(a)>0:pass"
10000000 loops, best of 3: 0.18 usec per loop
[EMAIL PROTECTED]:~$ python -m timeit "a=''" "if len(a):pass"
10000000 loops, best of 3: 0.13 usec per loop
[EMAIL PROTECTED]:~$ python -m timeit "a=''" "if a:pass"
10000000 loops, best of 3: 0.0635 usec per loop
[EMAIL PROTECTED]:~$ python -m timeit "a='abcd'" "if len(a)>0:pass"
10000000 loops, best of 3: 0.136 usec per loop
[EMAIL PROTECTED]:~$ python -m timeit "a='abcd'" "if len(a):pass"
10000000 loops, best of 3: 0.129 usec per loop
[EMAIL PROTECTED]:~$ python -m timeit "a='abcd'" "if a:pass"
10000000 loops, best of 3: 0.0623 usec per loop
[EMAIL PROTECTED]:~$ python -m timeit "a={}" "if len(a)>0:pass"
10000000 loops, best of 3: 0.163 usec per loop
[EMAIL PROTECTED]:~$ python -m timeit "a={}" "if a:pass"
10000000 loops, best of 3: 0.0903 usec per loop
[EMAIL PROTECTED]:~$ python -m timeit "a=tuple()" "if len(a)>0:pass"
1000000 loops, best of 3: 0.253 usec per loop
[EMAIL PROTECTED]:~$ python -m timeit "a=tuple()" "if a:pass"
10000000 loops, best of 3: 0.177 usec per loop
--
Heikki Toivonen
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