9.8. Generator Expression

9.8.1. Rationale

>>> _ = list(x for x in range(0,5))      # list comprehension
>>> _ = tuple(x for x in range(0,5))     # tuple comprehension
>>> _ = set(x for x in range(0,5))       # set comprehension
>>> _ = dict((x,x) for x in range(0,5))  # dict comprehension
>>> _ = [x for x in range(0,5)]          # list comprehension
>>> _ = (x for x in range(0,5))          # generator expression
>>> _ = {x for x in range(0,5)}          # set comprehension
>>> _ = {x:x for x in range(0,5)}        # dict comprehension

9.8.2. Syntax

>>> (x for x in range(0,5))  
<generator object <genexpr> at 0x...>

9.8.3. Why Round Brackets?

>>> 1+2 * 3
7
>>>
>>> (1+2) * 3
9
>>> 1
1
>>>
>>> (1)
1
>>> 1, 2
(1, 2)
>>>
>>> (1, 2)
(1, 2)

9.8.4. Lazy vs Greedy

  • Comprehensions executes instantly

  • Generator expression executes lazily

>>> data = [x for x in range(0,10)]
>>>
>>> print(data)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> data = (x for x in range(0,10))
>>>
>>> print(data)  
<generator object <genexpr> at 0x...>

9.8.5. Generate

>>> result = (x for x in range(0,5))
>>>
>>> next(result)
0
>>> next(result)
1
>>> next(result)
2
>>> next(result)
3
>>> next(result)
4
>>> next(result)
Traceback (most recent call last):
StopIteration

9.8.6. Evaluate

>>> result = (x for x in range(0,5))
>>>
>>> list(result)
[0, 1, 2, 3, 4]
>>> list(x for x in range(0,5))
[0, 1, 2, 3, 4]

9.8.7. Comparison

Comprehension:

>>> data = [x for x in range(0,10)]
>>>
>>> for x in data:  
...     print(x, end=' ')
...     if x == 3:
...         break
0 1 2 3
>>>
>>> for x in data:  
...     print(x, end=' ')
...     if x == 6:
...         break
0 1 2 3 4 5 6
>>>
>>> print(list(data))
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>>
>>> print(list(data))
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Generator Expressions:

>>> data = (x for x in range(0,10))
>>>
>>> for x in data:  
...     print(x, end=' ')
...     if x == 3:
...         break
0 1 2 3
>>>
>>> for x in data:  
...     print(x, end=' ')
...     if x == 6:
...         break
4 5 6
>>>
>>> print(list(data))
[7, 8, 9]
>>>
>>> print(list(data))
[]

9.8.8. Use Case - 0x01

  • Comparison

>>> [x for x in range(0,100)]  
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20,
 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60,
 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80,
 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99]
>>> list(x for x in range(0,100))  
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20,
 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60,
 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80,
 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99]
>>> (x for x in range(0,100))  
<generator object <genexpr> at 0x...>
>>> result = (x for x in range(0,100))
>>>
>>> next(result)
0
>>> next(result)
1
>>> next(result)
2
>>> del result

9.8.9. Use Case - 0x01

  • Sum

>>> sum([1,2,3])
6
>>> sum([x for x in range(0,5)])
10
>>> sum(x for x in range(0,5))
10

9.8.10. Assignments