[Tutor] Java style assignments in Python

[Danny Yoo]

Hi Tpc,


> tpc at csua.berkeley.edu wrote:
>
> > hi Zak, well I was more familiar with Java's strong typing that I
> > assumed Python allowed assignment of a type to three variables.

(Small rant: I feel that Java's type system is a lot more trouble than
it's worth.  OCaml, now that's a language that has strong typing.  Mark
Jason Dominus has written a nice article that talks about type systems
here: http://perl.plover.com/yak/typing/typing.html.)



> Python has fairly strong typing too, but the type belongs to the object,
> *not* to the variable name that is bound to it.


yes.  Whenever we're saying the expression:

    []

we're actually constructing an empty list --- in Java, we might say:

    new ArrayList()

for a similar effect.



If we try something like this:

    a = b = c = []


Then the analogous Java code looks like this:

    Object a, b, c;
    a = b = c = new ArrayList();

The important part to see in this Java pseudocode is that each variable
name is of type Object --- and this is pretty much the situation in
Python!  Every name in Python is of a base type "PyObject", and the values
themselves contain runtime type information.


In fact, every method call we make on a value is looked up at runtime; we
can see this by co-opting the __getattr__() method:


###
>>> class WrapObject:
...     def __init__(self, obj):
...         self.obj = obj
...     def __getattr__(self, attr):
...         print "Intercepted call for attribute", attr
...         return getattr(self.obj, attr)
...
>>> s = WrapObject([])
>>> s.append
Intercepted call for attribute append
<built-in method append of list object at 0x8157c8c>
>>> s.extend([4, 5, 6])
Intercepted call for attribute extend
>>> s
Intercepted call for attribute __repr__
[4, 5, 6]
>>> print s
 Intercepted call for attribute __str__
[4, 5, 6]
>>>
>>>
>>> a, b = WrapObject(5), WrapObject(6)
>>> a + b
Intercepted call for attribute __coerce__
Intercepted call for attribute __add__
Intercepted call for attribute __coerce__
Intercepted call for attribute __radd__
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: unsupported operand types for +: 'instance' and 'instance'
###


(Hmmm!  I wonder why there's a leading space when I did 'print s'.  I'll
have to look into that sometime... *grin*)


Hope this helps!