Lab 1 Material / August 28th, 2008

This material is all in doctest format; to run the doctest, do

import doctest

in Python.

That will run all of the code following '>>>' and '...' and compare the actual output to the expected in-line output. Neat, eh?

Goals of this lab: get you used to basic Python syntax and some moderately complicated constructs; introduce introspection, print, and "duck typing".

Using "complex" data types in Python

Apart from the usual (int, string, etc.) there are some very nice complex data types in Python. The first two that I'd like to draw your attention to are lists and dictionaries.

Lists are just... lists of stuff. You can create empty ones, append to them, etc.

>>> x = []
>>> x.append(5)
>>> x.append('a')
>>> x.append(0.1)
>>> x
[5, 'a', 0.10000000000000001]

As you can see, there's no requirement to make all of the entries in a list the same type of object -- they can be whatever you want.

You can, of course, index lists by integers; 0 is the starting index, just as in C and Java.

>>> x[0]
>>> x[1]
>>> x[2]

Note that you can also create empty lists by explicitly calling the list constructor:

>>> x = list()

and you can also initialize them in place both ways:

>>> x = [5, 'a', 0.1]


>>> x = list((5, 'a', 0.1))

You can also assign to list elements by index:

>>> x[2] = 'z'
>>> x
[5, 'a', 'z']

Dictionaries are even more useful than lists, and they're really at the heart of Python. Dictionaries are basically just hashes, or maps, and they store key, value pairs:

>>> y = {}
>>> y['a'] = 1
>>> y[2] = 0.2
>>> y[0.3] = 5
>>> y
{'a': 1, 2: 0.20000000000000001, 0.29999999999999999: 5}

As with lists, there's no requirement that you store the same types of things in keys or values. Values can be whatever you want, while keys must be hashable (more on that later).

Instead of indexing dicts by integers, as with lists, you index them by key:

>>> y['a']
>>> y[2]
>>> y[0.3]

You can delete and overwrite as well:

>>> del y[0.3]
>>> y[2] = 'nada'
>>> y
{'a': 1, 2: 'nada'}

You can call 'len' on both lists and dicts to get their length:

>>> len(x)
>>> len(y)

and dictionaries support retrieval of keys, values, and key-value pairs ("items"):

>>> y.keys()
['a', 2]
>>> y.values()
[1, 'nada']
>>> y.items()
[('a', 1), (2, 'nada')]

Note that all of these are lists; in the last case, they are lists of two-tuples (see below), with the first element of each tuple being the key and the second the value. You can access things as you might expect:

>>> y.items()[0][1]

One type of Python object that you might confuse with a list is a tuple. tuples are just immutable lists, and you use '()' to define them rather than '[]'.

>>> z = (5, 'a', 'z')
>>> z[0]
>>> z[0] = 6
Traceback (most recent call last):
TypeError: 'tuple' object does not support item assignment

One useful difference between lists and tuples is that because tuples are immutable, you can use them as keys to dictionaries:

>>> y[z] = 'pony'
>>> y
{'a': 1, 2: 'nada', (5, 'a', 'z'): 'pony'}

Lists and tuples are interconvertible, too:

>>> x_tup = tuple(x)
>>> z_list = list(z)

and you can convert the keys in a dictionary into a list too:

>>> y_keys = list(y)

Technically, anything that is iterable can be passed into both the tuple and list constructors; see iterators, below.

There are a bunch of other Python types that we'll run across. The only really important one to mention at this point is 'object', which is the base of all (well, most) Python objects. To define a new class (type of object) you can just do this:

>>> class MyNewClass(object):
...    def __init__(self, value):
...       self.v = value
...    def some_method(self):
...       print 'my value is', self.v

To create objects of this type, just call the constructor like so:

>>> a = MyNewClass(5)

and then call methods or access attributes as you might expect:

>>> a.v
>>> a.some_method()
my value is 5

We'll talk more about classes down the road.

Iterating over lists and dicts

As you saw above, you can convert lists and dicts into, well, lists. How would you use this in a loop?

Let's start with a list:

>>> x = ['a', 'b', 'c']

To iterate over this in a loop, you could imagine doing this:

>>> i = 0
>>> while i < len(x):
...   print x[i]
...   i += 1

but that's grotesque! How about:

>>> for i in range(0, len(x)):
...   print x[i]

but it turns out you can actually do something even cleaner:

>>> for item in x:
...   print item

What if you want the item indices? Use 'enumerate':

>>> for n, item in enumerate(x):
...   print n, item
0 a
1 b
2 c

A couple notes on these last two examples. First, 'enumerate' wraps any iterable -- literally, any object which can be iterated over -- and simply returns each item count along with the item in a tuple. Second, we're actually doing something called "tuple unpacking" in the last example: you could rewrite it like this, to be more explicit:

>>> for (n, item) in enumerate(x):
...   print n, item
0 a
1 b
2 c

(but there's no need to put the parantheses in). Literally this is just unpacking the result of the enumerate, which we can see in list form as:

>>> list(enumerate(x))
[(0, 'a'), (1, 'b'), (2, 'c')]

We can do the same sort of thing (iterate with a for loop) to dictionaries, except now we're iterating (in no guaranteed order, note!) over the keys:

>>> y = dict(x=1, y=2, z=3)
>>> for key in y:
...   print key, y[key]
y 2
x 1
z 3

The sequence protocol: making things look like lists

Before we move on to the iterator protocol, let me show you a really neat feature of Python: making your own objects look like lists (technically "sequences").

Let's write a class that squares a list handed to it, but only when the elements are actually requested. By defining one special method, and '__getitem__', we can make this class behave like a list:

>>> class SquareMe(object):
...   def __init__(self, some_numbers):
...      self.n = some_numbers
...   def __getitem__(self, index):
...      print 'ASKING FOR ITEM', index, 'VALUE', self.n[index]
...      return self.n[index]**2

You can now access this object just as if it's a list:

>>> x = SquareMe([5, 6, 7])
>>> x[2]

and of course you can also iterate over it:

>>> for i in x:
...   print i

Note that here Python understands the kind of error returned by accessing an invalid index in a list, and the 'for' loops knows to quit at that point. That's why you get the 'ASKING FOR ITEM 3 VALUE' message at the end there.

Doing this kind of thing with your own objects is a powerful way to extend Python, and we'll make use of it in this class occasionally.

Here's the documentation on what you can or should extend to implement a full "container-like" type:

The mapping protocol: making things look like dicts

You can also make your own objects look like dictionaries. This is a bit more complicated, because dictionaries have a lot of different methods on them, but there is a nice trick to use...

Let's define a dictionary that, if any number under 100 is used as a key, returns the square of that number:

>>> import UserDict
>>> class MySquareDict(UserDict.DictMixin, object):
...   def keys(self):
...     return range(0, 100)
...   def __getitem__(self, key):
...     print 'ASKED FOR', key
...     return key**2
>>> x = MySquareDict()
>>> x[0]
>>> x[5]

OK, so what's really going on here!? Well, we're using DictMixin, which is a mixin class that provides a full read-only dictionary interface for classes that already possess __getitem__ and keys(). (If you provide __delitem__ and __setitem__, you get a full dictionary interface.) Here, keys() just returns the list of numbers between 0 and 100 (that's what range returns).

This means that not only can you call __getitem__ and keys(), but you can ask for other dictionary methods like has_key() that you didn't explicitly define:

>>> x.has_key(20)
>>> x.has_key(99)

Unfortunately in this case we didn't do a great job of defining this class, because keys() only returns a subset (0..99) of the actual legal keys:

>>> x.has_key(101)

As you can see, when you call has_key(), all it's doing is trying to ask for key 101 -- and if __getitem__ doesn't kick up a fuss, well, then it obviously has that key! We can redefine __getitem__ appropriately:

>>> class MySquareDict(UserDict.DictMixin, object):
...   def keys(self):
...     return range(0, 100)
...   def __getitem__(self, key):
...     if key >= 0 and key < 100:
...        return key**2
...     raise KeyError
>>> x = MySquareDict()
>>> x.has_key(101)

and get the right answer.

One last note -- if we want to pass in a max number through the constructor, we can do that, too:

>>> class MySquareDict(UserDict.DictMixin, object):
...   def __init__(self, max_num):
...     self.max_num = max_num
...   def keys(self):
...     return range(0, self.max_num)
...   def __getitem__(self, key):
...     if key >= 0 and key < self.max_num:
...        return key**2
...     raise KeyError
>>> x = MySquareDict(500)
>>> x.has_key(499)

Iterators and the iterator protocol

In programming, you frequently want to loop over things -- things in lists, or dictionaries, or on disk, in databases, over the Web, whatever. So Python has formalized this in an 'iterator' protocol, a way for objects to let Python know how to iterate over them.

The iterator protocol is pretty simple, but has a fair amount of syntax associated with it. Your class has to define an __iter__ method, and that method has to return an object with a 'next' function on it; then to iterate, Python calls the 'next' function until it raises a StopIteration exception. (More on exceptions much later...)

Here's a really simple and not so useful iterator:

>>> class NullThing(object):
...   def __iter__(self):
...     return self
...   def next(self):
...     raise StopIteration

(Note that here __iter__ is returning the same object, which is a convenient way of not having to define two classes...)

>>> x = NullThing()
>>> for y in x:
...   print 'HELLO, WORLD'

Yep, the for loop exits immediately because the 'next' method immediately raises StopIteration!

One thing to keep in mind about about iterators is that they do not necessarily know anything in advance. This means that you can't call 'len' on an iterator, for example...

Any finite iterator can be converted into a list like so:

>>> list(NullThing())

Generators and maintaining implicit state

One of the frustrating things about iterators is that with simple iterators, you spend a lot of time maintaining state -- that is, if you want to just do something to a list, you have to retain the index you're using. For example, consider the following ugly implementation of 'enumerate' as an iterator:

>>> class Enumerator(object):
...   def __init__(self, thing):
...     self.thing = iter(thing)
...     self.count = -1
...   def __iter__(self):
...     return self
...   def next(self):
...     val =
...     self.count += 1
...     return self.count, val

Sure, it works:

>>> for n, item in Enumerator(['a', 'b', 'c']):
...   print n, item
0 a
1 b
2 c

but that's a lot of code for something so simple!

It turns out, Python recently added generator functions. These are functions that generate re-entrant instances, by which I mean that they implicitly hold local state. Here's a generator version of 'enumerate':

>>> def enumerator(thing):
...   count = 0
...   for item in thing:
...     yield count, item
...     count += 1

Here, the 'yield' statement tells Python that the function 'enumerator' is actually a generator, and that when you call it, you should get back an iterator object that will return values from 'yield'.

See, it works!

>>> for n, item in enumerator(['a', 'b', 'c']):
...   print n, item
0 a
1 b
2 c

But how does the function know when to exit? The implicit exit condition is in the for loop, which will end when the 'thing' can no longer be iterated over.

Literally what is happening is that every time you call 'enumerator', Python is making a new generator (a.k.a. complicated iterator) out of it and storing the values of 'count' and the status of the 'thing' iteration in variables local to that instance of the function.

One of the fun things about generators and iterators, incidentally, is that they can both be infinite: consider this generator,

>>> def squares():
...   count = 0
...   while 1:
...     yield count**2
...     count += 1

Note the lack of exit condition... you can iterate over that indefinitely! You've got to put in your own exit condition:

>>> for x in squares():
...   print x
...   if x > 20:
...     break

Reimplementing 'range'

Just to drive the points home, here are two implementations of the same 'range'-like function.

>>> class RangeAsIterator(object):
...   def __init__(self, min_num, max_num):
...     self.max_num = max_num
...     self.curr = min_num - 1
...   def __iter__(self):
...     return self
...   def next(self):
...     if self.curr < self.max_num - 1:
...        self.curr += 1
...        return self.curr
...     raise StopIteration
>>> list(RangeAsIterator(0, 5))
[0, 1, 2, 3, 4]
>>> def range_as_generator(min_num, max_num):
...   while min_num < max_num:
...     yield min_num
...     min_num += 1
>>> list(range_as_generator(0, 5))
[0, 1, 2, 3, 4]

List and generator comprehensions, and scope

A few more cute Python tricks: you can generate a list in-place using a list comprehension, which returns a list:

>>> [ x**2 for x in range(1, 4) ]
[1, 4, 9]

or what's called a 'generator comprehension':

>>> y = ( x**2 for x in range(1, 4) )
>>> type(y)
<type 'generator'>
>>> for item in y:
...   print item

As above with lists and generators, the main difference here is that the generator values are not calculated until you ask for them. One minor difference is that of scope: variables used in list comprehensions "leak",

>>> z = [ x**2 for x in range(0, 100) ]
>>> x

while variables used in generator comprehensions do not:

>>> z = list( (x**2 for x in range(0, 500)) )
>>> x

Introspection and interactivity in Python

Python lets you figure out what methods and attributes objects have; consider the following way of figuring out what attributes dictionaries have:

>>> d = {}
>>> attributes = dir(d)
>>> print "\n".join(attributes)

Yikes, that's a lot! What do they all do?

>>> help(d.values)
Help on built-in function values:
    D.values() -> list of D's values

You can generally do that sort of thing (call 'dir', and/or 'help') to any Python object. In fact, if you define your own functions properly, you can ask for help on them, too:

>>> def f():
...   """
...   f prints hello, world
...   """
...   print 'hello, world'
>>> help(f)
Help on function f:
    f prints hello, world

One more Python jujitsu move: you can (if you really want to) use 'type' to figure out what things are:

>>> type(d)
<type 'dict'>
>>> type(f)
<type 'function'>

but Python is all about duck typing so you should view this as a way to explore code and not use it to confirm types in your code. More on this later.