effective python 读书笔记:第20条-用None和文档字符串来描述具有动态默认值的参数

第20条-用None和文档字符串来描述具有动态默认值的参数

关键:
1 参数的默认值
特点: 包含代码的模块一旦加载进来，参数的默认值就不变了
2 动态设置默认值
把默认值设置为None,如果发现参数的值为None,就设置为默认值

先看一个示例

def log1(content, when=datetime.utcnow()):
    print("content: {content}, when: {when}".format(content=content, when=when))


def log2(content, when=None):
    """
    :param content:
    :param when:
    :return:
    """
    when = datetime.utcnow() if when is None else when
    print("content: {}, when: {}".format(content, when))


def process():
    log1('test1')
    time.sleep(1)
    log1('test2')
    print('=====')
    log2('test1')
    time.sleep(1)
    log2('test2')


if __name__ == "__main__":
    process()

#输出
content: test1, when: 2019-12-08 14:47:52.839435
content: test2, when: 2019-12-08 14:47:52.839435
=====
content: test1, when: 2019-12-08 14:47:53.841040
content: test2, when: 2019-12-08 14:47:54.842983

接着再看下一个示例：

import json


# global var default of dict type which is mutable
def decode_data(data, default={}):
    try:
        return json.loads(data)
    except Exception as e:
        # print(e)
        return default


foo = decode_data('name')
foo['name'] = 's'

bar = decode_data('age')
bar['age'] = 18

print('foo:', foo)
print('bar:', bar)
print('foo is bar:', foo is bar)
print(id(foo) == id(bar))
# foo: {'name': 's', 'age': 18}
# bar: {'name': 's', 'age': 18}
# foo is bar: True
# True


def decode_data_test(data):
    print('\n====decode_data_test')

    def decode_data(data, default={}):
        try:
            return json.loads(data)
        except Exception as e:
            # print(e)
            return default
    return decode_data(data)


foo = decode_data_test('name')
foo['name'] = 's'

bar = decode_data_test('age')
bar['age'] = 18

print('foo:', foo)
print('bar:', bar)
print('foo is bar:', foo is bar)
print(id(foo) == id(bar))

# ====decode_data_test
#
# ====decode_data_test
# foo: {'name': 's'}
# bar: {'age': 18}
# foo is bar: False
# False