iterator

iterators vs. iterables

i. iterables

• An object capable of returning its members one at a time is iterable:
  • All sequence types: list, str, tuple, bytes, etc.
  • Some non-sequence types: dict, file objects, etc.
  • Objects of any classes you define with an __iter__() method or with a __getitem__() method that implements Sequence semantics.
    • Sequence is an iterable which:
      • supports efficient element access using integer indices via the __getitem__() method.
      • defines a __len__() method that returns the length of the sequence.
• An infinite series of numbers cannot be stored in a collection.
• Every collection in Python is iterable.

ii. iterators

• It produces the next value with next().
• An iterator is an object representing a stream of data.
• Iteration is often implicit.
• It returns the data one element at a time.
• __next__() returns the next element of the stream.
• If there are no more elements in the stream, __next__() must raise the StopIteration exception.
• An iterator doesn’t have to be finite.
• An important foundation for writing functional-style programs.
• It raises TypeError if the object doesn’t support iteration.
• max(), min(), in, and not in support iterators. (max() and min() never end for infinite iterators.)
• Two common operations on an iterator’s output:
  1. Performing some operation for every element.
  2. Selecting a subset of elements that meet some condition.

Iterating over iterables: next()

word = 'Da'
it = iter(word)
next(it) # D
next(it) # a
next(it) # No values to return StopIteration error is thrown

Iterating at once with * (Splat operator)

• This star operator unpacks all elements of an iterator or an iterable.
• Once you do so, you cannot do it again.

word = 'Data'
it = iter(word)
print(*it) # D a t a

Iterating over

i. for loop

• We can iterate over a list using a for loop.

employees = [ 'Alice', 'Bob' ]
for employee in employees:
    print(employee)

• We can iterate over a range object using a for loop

for i in range(4):
    print(i)

ii. dictionaries

pythonistas = {'k1': 'v1', 'k2':'v2'}
for k, v in pythonistas.items():
    print(f"{k}:{v} ")

iii. file connections

file = open('file.txt')
it = iter(file)
print(next(it)) # first line of `file.txt`
print(next(it)) # second line of `file.txt`

L = [1, 2, 3]
it = iter(L)
it.__next()__ # 1
next(it) # 2
next(it) # 3
next(it) # StopIteration error
###
for i in iter(<object>):
    print(i)
# equal to
for i in <object>:
    print(i)
###
L = [('Italy', 'Rome'), ('France', 'Paris'), ('US', 'Washington DC')]
dict(iter(L)) # {'Italy': 'Rome', 'France': 'Paris', 'US': 'Washington DC'}
###
# Parantheses are different. [] := list, () := iterator
line_list = ['  line 1\n', 'line 2  \n', ...]

# Generator expression -- returns iterator
stripped_iter = (line.strip() for line in line_list)

# List comprehension -- returns list
stripped_list = [line.strip() for line in line_list]###

seq1 = 'abc'
seq2 = (1, 2, 3)

# if an expression is creating a tuple, it must be surrounded by parentheses.
[(x, y) for x in seq1 for y in seq2] # [('a', 1), ('a', 2), ('a', 3), ('b', 1), ('b', 2), ('b', 3), ('c', 1), ('c', 2), ('c', 3)]
###