Lecture Notes 18

timer as context manager

import time

class Timer:
    def __enter__(self):
        self.t1 = time.time()
        
    def __exit__(self, *args):
        self.t2 = time.time()
        print(f"Time used here: {self.t2 - self.t1}")

with Timer() as t:
    print("Hello")
    time.sleep(1)
    print("World")

Hello

World
Time used here: 1.000225305557251

Iterables

• an object that supports iteration

• allowed in the top a for loop

iter([])

<list_iterator at 0x7f085188df00>

• iter([...]) delegates to [...].__iter__()

'__iter__' in dir([])

True

• not all objectes are iterable

iter(1) 

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[6], line 1
----> 1 iter(1)

TypeError: 'int' object is not iterable

'__iter__' in dir(int)

False

iterators

Generate values of a sequence when called by next

'__next__' in dir(iter([]))  # True for an iterator

True

next(iter([1, 2, 3]))

1

it = iter([1, 2, 3])

next(it)

1

next(it)

2

next(it)

3

next(it)

---------------------------------------------------------------------------
StopIteration                             Traceback (most recent call last)
Cell In[28], line 1
----> 1 next(it)

StopIteration: 

for element in [1, 2, 3]:
    print(element)

1
2
3

classes supporting iteration

class Counter:
    def __init__(self, size):
        print("__init__:", size)
        self.size = size
        self.start = 0
    def __iter__(self):
        print("__iter__:", self.size)
        return CounterIter(self.start, self.size)
    
class CounterIter:
    def __init__(self, start, size):
        self.start = start
        self.size = size
    def __next__(self):
        if self.start < self.size:
            self.start = self.start + 1
            return self.start
        raise StopIteration

for n in Counter(3):
    print(n)

__init__: 3
__iter__: 3
1
2
3

a_iterable = Counter(3)

__init__: 3

a_iterator = iter(a_iterable)

__iter__: 3

next(a_iterator)

1

next(a_iterator)

2

next(a_iterator)

3

next(a_iterator)

---------------------------------------------------------------------------
StopIteration                             Traceback (most recent call last)
Cell In[66], line 1
----> 1 next(a_iterator)

Cell In[59], line 18, in CounterIter.__next__(self)
     16     self.start = self.start + 1
     17     return self.start
---> 18 raise StopIteration

StopIteration: 

generators

• look like function

• has yield statement instead of return

def g(n):
    print('enter g with',n)
    yield n
    yield n + 1
    print('after yield')

g(2)

<generator object g at 0x7f8e701093c0>

'__iter__' in dir(g(2))

True

'__next__' in dir(g(2))

True

list(g(2))

enter g with 2
after yield

[2, 3]

for n in g(2):
    print(n)

enter g with 2
2
3
after yield

Example

• a given number of Fibonacci sequence nubmers

def f(n):
    """
    Return the n first numbers in the Fibonacci sequence
    """
    count = 0
    a = 0
    b = 1
    while count < n:
        yield a
        a, b = b, a+b
        count += 1

list(f(5))

[0, 1, 1, 2, 3]

list(f(10))

[0, 1, 1, 2, 3, 5, 8, 13, 21, 34]