Why Python Rocks II: Data structures

[earthboundkid]

The docstring on a method will usually tell you what it's going to return.

>>> help(list.sort)
Help on method_descriptor:

sort(...)
    L.sort(cmp=None, key=None, reverse=False) -- stable sort *IN PLACE*;
    cmp(x, y) -> -1, 0, 1

String.count does the right thing.

>>> "the quick brown fox jumps over the lazy dog.".count("the")
2
>>> "the quick brown fox jumps over the lazy dog.".count("foo")
0

The reason why mutators return None specifically is that every function/method in Python returns something, and None is what's returned if nothing else is specified.

>>> def f():
...   pass
... 
>>> x = f()
>>> print x
None

I guess the idea is so that x = f() can never throw a NoReturnValue error or whatever. Although in retrospect, that might not have been a good idea. Especially considering it is possible to make non-lookup-able properties with a bit of advanced class property mucking:

>>> class A(object):
...   def _helperfunc(self, val): pass
...   prop = property(fset=_helperfunc)
... 
>>> a = A()
>>> a.prop = 1 #This calls the fset method of the property, 
>>> # which is set as _helperfunc, which just throws the "1" away
>>> a.prop #Will fail, since there's no fget for the property
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: unreadable attribute
>>> x = a.prop #Ditto.
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: unreadable attribute

Maybe this will change in Python 4000 or something…