Software Design
Fall 2004

For next time:

1) finish hw03

2) read Chapter 9 of "How to think..."


An early bird special
---------------------

eval is a built-in function that takes a string and evaluates
it using (of all things) the Python interpreter

Try this:

>>> eval('2+2')
4

>>> eval('eval')
<built-in function eval>

>>> eval('eval("2+2")')
4

>>> eval('eval("eval")')
<built-in function eval>

Why doesn't this work?

>>> eval('print "hello"')


There is an example of eval in my hw01 solutions:

def keypress(event):
    # this function gets called when the user presses a key
    try:
        # figure out which function to call, and call it
        func = eval('draw_' + event.char)
        func(bob, size)
        skip(bob, size/2)
    except NameError:
        print "I don't know how to draw an", event.char
    except SyntaxError:
        # this happens when the user presses return
        teleport(bob, -180, bob.y-size*3)


Strings
-------

Strings are immutable

s = 'allen'
print capitalize(s)
print replace(s, 'n', 'n downey')

Note that these methods return new strings;
they don't modify strings, because...

     STRINGS ARE IMMUTABLE.

Strings are compound objects, which means that they contain items.

What is an index?

An integer expression used to identify an item in a compound object.

What is a slice?

A pair of indices that identify a subsequence of a compound object.

for i in range(len(s)):
    print i, s[i]

What's the difference between

a = b

and

a = b[:]



string.constants
----------------

the string module defines some constants, like

>>> print string.punctuation
!"#$%&'()*+,-./:;<=>?@[\]^_`{|}~

>>> print string.ascii_lowercase
abcdefghijklmnopqrstuvwxyz

>>>print string.lowercase

that are useful for testing characters


Exception Handling
------------------

run-time errors are also known as exceptions

normally when an exception occurs, Python:

1) prints a traceback

2) prints an error message

3) ends the program


If you do something that might cause an error, you can
"catch" the exception:

def singular(x, y):
    try:
        res = x / y
    except:
        res = 10000.0
    return res

In this example, there is no great advantage over using
an if statement.

What's the difference between string.find and string.index?

LOOK BEFORE YOU LEAP

res = string.find('apple', 'pl')
if res == -1:
    print "didn't find it"
else:
    print "found it at index", res


BETTER TO ASK FORGIVENESS THAN PERMISSION

def myfind(s, sub):
    try:
        res = string.index(s, sub)
        return res
    except:
        print "didn't find it"
        return -1

Once you add try statements to your toolbox, you
will start seeing uses for them.