# 2.3. Generator Expression¶

## 2.3.1. Rationale¶

Comprehensions:

• Executes instantly

• Stored in the memory until end of a program

• When using values more than one

Generator Expressions:

• Lazy evaluated

• Cleared once they are executed

• When using value once (for example in the loop iterator)

## 2.3.2. List Comprehension¶

• Comprehensions executes instantly

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


## 2.3.3. Generator Expression¶

• Generators are lazy evaluated

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


## 2.3.4. Example¶

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

>>> data = list(x for x in range(0,5))      # list comprehension
>>> data = tuple(x for x in range(0,5))     # tuple comprehension
>>> data = set(x for x in range(0,5))       # set comprehension
>>> data = dict((x,x) for x in range(0,5))  # dict comprehension

>>> data = all(x for x in range(0,5))
>>> data = any(x for x in range(0,5))
>>> data = bool(x for x in range(0,5))
>>> data = sum(x for x in range(0,5))
>>> data = min(x for x in range(0,5))
>>> data = max(x for x in range(0,5))
>>> data = hash(x for x in range(0,5))
>>> data = callable(x for x in range(0,5))

>>> data = len(x for x in range(0,5))
Traceback (most recent call last):
TypeError: object of type 'generator' has no len()


## 2.3.5. Comprehensions or Generator Expression¶

Generator Expressions:

• Creates generator object and assign reference

• Code is not executed instantly

• Sometimes code is not executed at all!

• Are cleared once they are executed

• Generator will calculate next number for every loop iteration

• Generator forgets previous number

• Generator doesn't know the next number

• It is used for one-time access to values (for example in the loop iterator)

Comprehensions:

• Comprehensions will be in the memory until end of a program

• Comprehensions - Using values more than one

Summary:

• If you need values evaluated instantly, there is no point in using generators

Comprehensions vs Generator Expression:

>>> 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...>


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))
[]


## 2.3.6. Why Round Brackets?¶

• Round brackets does not produce tuples (commas does)

• Round brackets bounds context

>>> data = [x for x in range(0,5)]  # list comprehension
>>> data = (x for x in range(0,5))  # generator expression

>>> data = [1, 2, 3]
>>> type(data)
<class 'list'>
>>>
>>> data = (1, 2, 3)
>>> type(data)
<class 'tuple'>
>>>
>>> data = 1, 2, 3
>>> type(data)
<class 'tuple'>

>>> data = 1 + 2
>>> type(data)
<class 'int'>
>>>
>>> data = (1 + 2)
>>> type(data)
<class 'int'>

>>> data = (1, 2, 3)
>>> type(data)
<class 'tuple'>
>>>
>>> data = (1, 2)
>>> type(data)
<class 'tuple'>
>>>
>>> data = (1,)
>>> type(data)
<class 'tuple'>
>>>
>>> data = (1)
>>> type(data)
<class 'int'>

>>> data = 1, 2, 3
>>> type(data)
<class 'tuple'>
>>>
>>> data = 1, 2
>>> type(data)
<class 'tuple'>
>>>
>>> data = 1,
>>> type(data)
<class 'tuple'>
>>>
>>> data = 1
>>> type(data)
<class 'int'>