[Erp5-dev] Why are some class instances false?

[Yoshinori Okuji]

In ERP5, sometimes class instances (such as a Folder) can be false, if
you do this:

obj = kwargs['context']
if obj:
blah blah


Short answer:
If obj has no subobjects, it is false. Do not use this test but use "obj
is not None".


Long answer:

Python checks if an object is false or true in this way:

1. If it is None, false.
2. If it has the attribute __int__, call int(obj). If this result is
zero, false.
3. If it has the attribute __len__, call len(obj). If this result is
zero, false.
4. Otherwise, true.

Note that if an object is an instance of BTreeFolder2 or a subclass of
it, the object has __len__. In this case, if this object has no
content, this object is false.

So it is always better to use more explicit comparison tests, like
these:

if obj is not None:
blah
if obj == 0:
blah
if len(obj) == 0:
blah

YO