如何提高嵌套 if 和 for 语句的可读性答案

【问题标题】：How can i improve the readablility of nested if and for statements如何提高嵌套 if 和 for 语句的可读性
【发布时间】：2012-04-10 07:34:12
【问题描述】：

我有一段非常讨厌的代码我想重构，但因为它变得完全不可读。

 for region in feed['config']['regions']:
            if region['region'] == region_name:
                for instance_type in region['instanceTypes']:
                    if instance_type['type'] == instance_type_name:
                        for instance_size in instance_type['sizes']:
                            if instance_size['size'] == instance_size_name:
                                for platform in instance_size['valueColumns']:
                                    if platform['name'] == platform_name:
                                        prices = platform['prices']
                                        assert prices.keys() == ['USD']
                                        return decimal.Decimal(prices['USD'])
        assert False, "Failed to determine price for instance with region=%r, type=%r, size=%r, platform=%r" % \
                (region_name, instance_type_name, instance_size_name, platform_name)

我一直在为每个循环或 if 语句使用函数，但这会给我带来很多函数。有更好的解决方案吗？

【问题讨论】：

使用函数，所以你没有超过2个嵌套的for循环通常被认为是好的样式。 “大量函数”是一件好事（tm），特别是如果您使用文档字符串来记录它们:-)
@thebjorn 你的评论应该是一个答案

标签： python for-loop refactoring

【解决方案1】：

try:
    region = [r for r in feed['config']['regions'] if region['region'] == region_name][0]
    instance_type = [t for t in region['instanceTypes'] if i['type'] == instance_type_name][0]
    # ...
    return decimal.Decimal(prices['USD'])
except IndexError:
    raise Exception("Failed to determine price for instance with region=%r, type=%r, size=%r, platform=%r" % 
                (region_name, instance_type_name, instance_size_name, platform_name))

或者更进一步：

def filter_by_key(key, value, objects):
    return [o for o in objects if region[key] == value][0]

try:
    region = filter_by_key('region', region_name, feed['config']['regions'])
    instance_type = filter_by_key('type', instace_type_name, region['instanceTypes'])
    # ...
    return decimal.Decimal(prices['USD'])
except IndexError:
    raise Exception("Failed to determine price for instance with region=%r, type=%r, size=%r, platform=%r" % 
                (region_name, instance_type_name, instance_size_name, platform_name))

【讨论】：

【解决方案2】：

如果你想保持循环，你可以使用continue 来保存缩进：

for region in feed['config']['regions']:
    if region['region'] != region_name: continue
    for instance_type in region['instanceTypes']:
        if instance_type['type'] != instance_type_name: continue
        for instance_size in instance_type['sizes']:
            if instance_size['size'] != instance_size_name: continue
            for platform in instance_size['valueColumns']:
                if platform['name'] != platform_name: continue
                prices = platform['prices']
                assert prices.keys() == ['USD']
                return decimal.Decimal(prices['USD'])
assert False, "Failed to determine price for instance with region=%r, type=%r, size=%r, platform=%r" % \
                (region_name, instance_type_name, instance_size_name, platform_name)

或者使用列表推导：

regions = (r for r in feed['config']['regions'] if r['region'] == region_name)
instance_types = (i for i in r['instanceTypes'] for r in regions if i['type'] == instance_type_name)
instance_sizes = (i for i in t['sizes'] for t in instance_types if i['size'] == instance_size_name)
platforms = (p for p in s['valueColumns'] for s in instance_sizes if p['name'] == platform_name)
for platform in platforms:
    prices = platform['prices']
    assert prices.keys() == ['USD']
    return decimal.Decimal(prices['USD'])
assert False, "Failed to determine price for instance with region=%r, type=%r, size=%r, platform=%r" % \
                (region_name, instance_type_name, instance_size_name, platform_name)

【讨论】：