1. COMP280:Introduction to Software Development, Week 9, Lecture 27
Bernhard Pfahringer

2. How about a general solution to caching?
def memo(f): memo = {} def helper(x): if x not in memo: memo[x] = f(x) return memo[x] return def fib(n): return n if n < 2 else fib(n-1) + fib(n-2)

3. An example of the Decorator pattern
@memo def fib(n): … Is equivalent to the following: fib = can be used/wrapped around any function More on “Patterns” later

4. What else is wrong with fib(n) ?
>>> fib( -10 ) -10 ??? >>> fib( 10.2 )

5. Checking input validity
def fib(n): assert type(n) is int assert n >= 0 return n if n < 2 else fib(n-1) + fib(n-2) if type(n) is not int: raise TypeError( "fib: n must be an int: ” + str(n)) if n < 0: raise ValueError( "fib: must be non-negative: %d” % n) More on “defensive programming” later

6. What is still wrong with fib(n) ?
>>> help( fib ) ??? def fib(n): """Compute Fibonacci numbers""" return n if n < 2 else fib(n-1) + fib(n-2) >>> help(fib) fib(n) Compute Fibonacci numbers More on ”documenting code” later

7. What else is missing: Testing
def fib(n): """Compute Fibonacci numbers >>> fib(0) 0 >>> fib(30) """ return n if n < 2 else fib(n-1) + fib(n-2) if __name__ == '__main__': import doctest doctest.testmod()

8. Putting it all together
def fib(n): """Compute Fibonacci numbers >>> fib(0) 0 >>> fib(30) """ if type(n) is not int: raise TypeError( "fib: n must be an int: {}”.format(n)) if n < 0: raise ValueError( "fib: n must be non-negative: {}”.format(n)) if n < 2: return n else: a, b = 0, 1 for i in range(n-1): a, b = b, a+b return b

9. Design Patterns
1994 book by GoF “Gang of Four” Gamma, Helm, Johnson, Vlissides
Template solution for common problem
Make up for a
deficiency/shortcoming of the language
lack of library
Also help documenting/understanding systems
Generally: Patterns use classes and instances to
explicitly represent actions

10. Groups of patterns Creational, e.g. Structural, e.g. Behavioural, e.g.
Prototype
Abstract Factory
Builder
Structural, e.g.
Adapter
Decorator
Proxy
Behavioural, e.g.
Iterator
Command
[Concurrency]
[Architecture]

11. Prototype [ plus shallow and deep copying ]
Creating an instance using another as a template
In Java or C/C++: copy-constructor
Python: only one __init__ (more or less)
but copy(), copy.deepcopy(),
__copy__, __deepcopy__
a = [ [ 0 ] ]
b = a.copy()
a[0][0] = 1
b ?
b[0] = 2
a ?
a = [ [ 0 ], 1]
a[1] = a[0]
a ?
a[0][0] = 1
b = copy.deepcopy(a)
b[0][0] = 2
b ?