yield with list comprehension

之前無意間發現可在 list comprehension 裡面使用 yield 和 yield from ， 這是一個 tricky 的用法 :P

Example

list :

>>> list((yield x) or x for x in [1, 2, 3])
[1, 1, 2, 2, 3, 3]

>>> list((yield from range(x)) for x in [1, 2, 3, 4])
[0, None, 0, 1, None, 0, 1, 2, None, 0, 1, 2, 3, None]

>>> list((yield x) or x if not x % 2 else x for x in [1, 2, 3])
[1, 2, 2, 3]

>>> def f(val):
...     return "Hi"
>>> x = [1, 2, 3]
>>> list(f((yield a)) for a in x)
[1, 'Hi', 2, 'Hi', 3, 'Hi']

>>> x = [1, 2, 3]
>>> f = lambda x : x**2
>>> list(f((yield a) or a) for a in x)
[1, 1, 2, 4, 3, 9]

>>> x = (lambda : ((yield 666),(yield 777),(yield 888)))()
>>> next(x)
666
>>> next(x)
777
>>> next(x)
888
>>> next(x)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration

>>> (lambda x: 999)((yield 3))
  File "<stdin>", line 1
SyntaxError: 'yield' outside function
>>> list((lambda x: 999)((yield 3)) for i in [2])
[3, 999]

dictionary :

# In Python 2
>>> myset = set(['a', 'b', 'c', 'd'])
>>> mydict = {item: (yield ''.join([item, 's'])) for item in myset}
>>> mydict
<generator object <dictcomp> at 0x7f479b246eb0>
>>> dict(mydict)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: dictionary update sequence element #4 has length 4; 2 is required
>>> mydict = {item: (yield ''.join([item, 's'])) for item in myset}
>>> list(mydict)
['as', 'cs', 'bs', 'ds', {'a': None, 'c': None, 'b': None, 'd': None}]

# In Python 3
>>> myset = set(['a', 'b', 'c', 'd'])
>>> mydict = {item: (yield ''.join([item, 's'])) for item in myset}
>>> mydict
<generator object <dictcomp> at 0x7f85655eb410>
>>> dict(mydict)
{'d': 's', 'a': 's', 'c': 's', 'b': 's'}
>>> mydict = {item: (yield ''.join([item, 's'])) for item in myset}
>>> list(mydict)
['cs', 'ds', 'bs', 'as']

flat_list

把傳入的巢狀 list 壓平

# assume there is no "None" in real data
>>> flat_list = lambda l: list(filter(lambda x: x is not None, ((yield from flat_list(i)) if isinstance(i, list) else i for i in l)))
>>> flat_list([1, 2, 3])
[1, 2, 3]
>>> flat_list([1, [2, 2, 2], 4])
[1, 2, 2, 2, 4]
>>> flat_list([[[2]], [4, [5, 6, [6], 6, 6, 6], 7]])
[2, 4, 5, 6, 6, 6, 6, 6, 7]
>>> flat_list([-1, [1, [-2], 1], -1])
[-1, 1, -2, 1, -1]