python函数参数执行

Question

伙计们，什么是蟒蛇不执行语句，直到必要的正确方法。

可以说我有一个执行指数api补偿的函数：

def exponential_backoff_task(config, task, description):
    retry_count = config.get( 'retry_count', 5 )
    api_backoff_initial_msec = config.get( 'api_backoff_initial_msec', 200)
    print 'Running api-backoff task: %s, retry_count: %d, initial_backoff: %d' % (description, retry_count, api_backoff_initial_msec)
    result = None
    for r in range(retry_count):
        try:
            result = task
        except boto.exception.BotoServerError:
            delay_msec = (2 ** r) * api_backoff_initial_msec
            print 'Exponential backoff, retry %d for %d msecs...' % (r, delay_msec)
            time.sleep(delay_msec/1000)
            continue
        except:
            raise
    return result


def foo():
    all_instances = exponential_backoff_task(config, conn.get_all_dbinstances(), 'foo'))

在这种情况下， conn.get_all_instances()在调用函数时执行，而不是在exponential_backup函数内部执行

谢谢！

Answer 1

好吧，不要在传入时调用它，而仅在需要时才调用它：

from functools import partial

def exponential_backoff_task(config, task_fn, description):
    retry_count = config.get('retry_count', 5)
    api_backoff_initial_msec = config.get('api_backoff_initial_msec', 200)
    print 'Running api-backoff task: %s, retry_count: %d, initial_backoff: %d' % (description, retry_count, api_backoff_initial_msec)
    result = None
    for r in range(retry_count):
        try:
            # Call the function that got passed in
            result = task_fn()
        except boto.exception.BotoServerError:
            delay_msec = (2 ** r) * api_backoff_initial_msec
            print 'Exponential backoff, retry %d for %d msecs...' % (r, delay_msec)
            time.sleep(delay_msec / 1000)
            continue
        except:
            raise
    return result


def foo():
    # Note the missing parens: here you're just passing in the function
    all_instances = exponential_backoff_task(config, conn.get_all_dbinstances, 'foo')

编辑 ：要在函数中预定义一些参数，可以使用partial ，可以接受函数将参数应用到该函数，然后使用已应用的那些参数返回一个新函数，这是一个示例：

from functools import partial

def f(a, b):
    print a
    print b

g = partial(f, a=1, b=2)

g()

此打印

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