Python: Generator Expressions / joblib.Parallel

其实 Digest of Fluent Python 里有讲，只是我实在是太不习惯它的用法了：

Generator expressions are syntactic sugar: they can always be replaced by generator functions, but sometimes are more convenient.

Syntax Tip: When a generator expression is passed as the single argument to a function or constructor, you don’t need to write its parentheses.

1. Generator Expression and Parentheses

绝大多数情况下，generator expression 是要带括号的，比如 (i * 5 for i in range(1, 5))，它返回一个 generator，相当于 yield from range(1, 5)。

当 generator expression 用作 function 的 唯一 argument 时，可以不带括号。Guido van Rossum: Disallow ambiguous syntax f(x for x in [1],) 有曰：

Initially generator expressions always had to be written inside parentheses, as documented in PEP 289. The additional parenthesis could be omitted on calls with only one argument, because in this case the generator expression already is written inside parentheses.

所以看到 func(i for i in xxx) 这种形式要注意：

• 这个函数其实只有一个参数
• 这个参数其实是个 generator
• 不要想到 unpack 那边去了

比如 PEP 289 – Generator Expressions 上的例子：

# For instance, the following summation code will build a full list of squares in memory, iterate over those values, and, when the reference is no longer needed, delete the list:
sum([x*x for x in range(10)])  # OK

# Memory is conserved by using a generator expression instead:
sum(x*x for x in range(10))  # OK

这也可以帮助记忆 “Python 不存在 tuple comprehension 这种东西”。下面这个 workaround 其实是调用 tuple 构造器，传了一个 generator expression 给它：

>>> tuple(i for i in [1, 2, 3])
(1, 2, 3)

2. SyntaxError: Generator expression must be parenthesized

但当 generator expression 不是 function 的唯一 argument 时，它必须带括号，否则会报 SyntaxError: Generator expression must be parenthesized。比如：

sum([x for x in range(10)], default = 0)  # OK

sum(x for x in range(10), default = 0)  # SyntaxError

题外话：这个 default 还是很有用的，它的作用是当前面的 generator 为空时让 sum 能返回一个默认值

3. joblib.Parallel

joblib.Parallel 里的一个用法是：

scores = parallel(
    delayed(_fit_and_score)(
        clone(estimator), X, y, scorers, train, test, verbose, None,
        fit_params, return_train_score=return_train_score,
        return_times=True, return_estimator=True, error_score=-1)
    for train, test in cv.split(X, y, groups))

这里 delayed 其实是个 decorator，整体上看来，它的作用相当于：

def delayed(func)(*args, **kwargs):
    return func, args, kwargs

所以 delayed(func)(*args, **kwargs) for train, test in cv.split() 本质是一个 generator expression，它相当于：

def gen():
    for train, test in cv.split():
        yield func, args, kwargs

然后 parallel.__call__(iterable) 来接收这个 generator，把每一组 (func, args, kwargs) dispatch 出去，让子进程或者子线程去处理。