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Generator comprehension

As you may know, Python has list comprehension syntax.

>>> [i for i in range(1, 10)]
[1, 2, 3, 4, 5, 6, 7, 8, 9]

The expression is not only applied to list but also dictionary and even generator!

In case of dictionary, we can use like this.

>>> {str(v): v * v  for v in range(1, 4)}
{'2': 4, '1': 1, '3': 9}

We get the map from the str of number to squaring of the one.

Generator is used in for syntax look as range function.

>>> for i in range(1, 3):
...     print(i)
...
1
2

The same thing can be done by using list or dictionary. But if that data structure has a lot of data, it can have memory pressure. Generator does not keep whole sequence on memory.

>>> for i in (v * v for v in range(1, 5)):
...     print(i)
...
1
4
9
16
>>> g = (v * v for v in range(1, 5))
>>> type(g)
<class 'generator'>

So you find that it’s not list.