Iteration, Comprehensions, and Generators¶

Coverage Of This Topic ¶

List comprehensions
The range function
Writing generators using yield
Generator expressions

`for` Recap: Sequential Datatypes¶

code/10-for-recap.py¶

for i in [1,2,3,4]:
    print(i)

$ python code/10-for-recap.py
1
2
3
4

This is the point in sequential datatypes: iteration with for, mainly

Primitive Transformation `[1,2,3,4]` ⟶ `[1,4,9,16]`¶

code/20-squares-func.py¶

def squares(seq):
    ret = []
    for i in seq:
        ret.append(i**2)
    return ret

for i in squares([1,2,3,4]):
    print(i)

$ python code/20-squares-func.py
1
4
9
16

Define function to create squares list

List Comprehension ¶

Problem:

function used in one place
… but defined elsewhere

code/30-squares-comprehension.py¶

for i in [i**2 for i in [1,2,3,4]]:
    print(i)

$ python code/30-squares-comprehension.py
1
4
9
16

Same behavior as squares(), but more readable
Provided that your colleagues know what list comprehensions are

Where does readability end? ⟶ if clause

code/40-odd-squares-comprehension.py¶

for i in [i**2 for i in [1,2,3,4,5,6,7,8,9] if i%2 != 0]:
    print(i)

$ python code/40-odd-squares-comprehension.py
1
9
25
49
81

Somewhere a line has to be drawn
⟶ it’s a team decision, as everything

Larger Input List (Still Allocated)¶

Problem: what about large lists? Say, 10 million elements.

code/50-larger-list.py¶

inputlist = []

print('filling input list')

cnt = 0
while cnt < 10000000:
    inputlist.append(cnt)
    cnt += 1

print('calculating squares')

for i in [i**2 for i in inputlist]:
    print(i)

$ python code/50-larger-list.py
filling input list
calculating squares
... numbers ...

Start without the print() statements.
Wonder why it takes so long. * Composing two lists
Burden on OS

Input List ⟶ `range`¶

Problem: using lists primarily for iteration. Can this be done: better?

code/60-input-range.py¶

print('creating input range')

inputrange = range(10000000)

print('calculating squares')

for i in [i**2 for i in inputrange]:
    print(i)

$ python code/60-input-range.py
creating input range
calculating squares
... numbers ...

Morph inputlist into inputrange
eliminate loop altogether * range has been created in no time
Discuss range
⟶ Generator

Output List ⟶ `yield`¶

Problem: still using temporary list in squares()

code/70-output-yield.py¶

def squares(seq):
    for i in seq:
        yield i**2

for i in squares(range(1,5)):
    print(i)

$ python code/70-output-yield.py
1
4
9
16

Morph back to squares() function
What’s so disgusting?
- Temporary list in squares()
- Gently introduce yield: remove temporary list + its return
What’s still disgusting?
- ⟶ input list
- ⟶ range
Perfect!
Discuss yield
- Still def, but contains yield ⟶ generator
- Morphing temporary lists to yield is straightforward

Generator Function ⟶ Generator Expression¶

Problem (re-introduced): squares() definition and usage are: too far apart

code/80-generator-expression.py¶

for i in (i**2 for i in range(1,5)):
    print(i)

$ python code/80-generator-expression.py
1
4
9
16

Generator expression
⟶ just like list comprehension, only smarter