标签：rip tle opera present 方法 fetchall 遍历 width ror
defaultdict()和namedtuple()是collections模块里面2个很实用的扩展类型。一个继承自dict系统内置类型，一个继承自tuple系统内置类型。在扩展的同时都添加了额外的很酷的特性，而且在特定的场合都很实用。
返回一个和dictionary类似的对象，和dict不同主要体现在2个方面：
from collections import defaultdict
s = [(‘yellow‘, 1), (‘blue‘, 2), (‘yellow‘, 3), (‘blue‘, 4), (‘red‘, 1)]
d = defaultdict(list)
for k, v in s:
d[k].append(v)
print(list(d.items()))
d_2 = {}
for k, v in s:
d_2.setdefault(k, []).append(v)
print(list(d_2.items()))
d_3 = {}
for k, v in s:
d_3[k].append(v)
print(d_3.items())
输出：
[(‘red‘, [1]), (‘blue‘, [2, 4]), (‘yellow‘, [1, 3])]
[(‘red‘, [1]), (‘blue‘, [2, 4]), (‘yellow‘, [1, 3])]
Traceback (most recent call last):
File "C:/Users/Administrator/Desktop/Python Scripts/collection_eg.py", line 22, in <module>
d_3[k].append(v)
KeyError: ‘yellow‘
d = defaultdict(list)，该语句创建一个defaultdict类型（你可以想象为dict类型），value的类型是list。通过对d_3的对比就能看到，defaultdict是可以直接就进行d[k]的操作，即使d此时还是一个空的字典。实际过程就是示例里d_2的处理过程。
主要使用根据数据创建字典时。当你需要为一些数据生成字典，而且对值的类型进行限定的时候，考虑defaultdict。
namedtuple是继承自tuple的子类。namedtuple和tuple比，有更多更酷的特性。namedtuple创建一个和tuple类似的对象，而且对象拥有可以访问的属性。这对象更像带有数据属性的类，不过数据属性是只读的。
>>> from collections import namedtuple
>>> TPoint = namedtuple(‘TPoint‘, [‘x‘, ‘y‘])
>>> p = TPoint(x=10, y=10)
>>> p
TPoint(x=10, y=10)
>>> p.x
10
>>> p.y
10
>>> p[0]
10
>>> type(p)
<class ‘__main__.TPoint‘>
>>> for i in p:
print(i)
10
10
>>>
TPoint = namedtuple(‘TPoint‘, [‘x‘, ‘y‘]) 创建一个TPoint类型，而且带有属性x, y.
通过上面的示例，可以看出不仅可以通过p.x， p.y的方式访问p的属性，而且还可以使用for来进行遍历。这些就和tuple是一样的。
还可以通过设置参数来看namedtuple的全貌。
TPoint = namedtuple(‘TPoint‘, [‘x‘, ‘y‘], verbose=True)
from builtins import property as _property, tuple as _tuple
from operator import itemgetter as _itemgetter
from collections import OrderedDict
class TPoint(tuple):
‘TPoint(x, y)‘
__slots__ = ()
_fields = (‘x‘, ‘y‘)
def __new__(_cls, x, y):
‘Create new instance of TPoint(x, y)‘
return _tuple.__new__(_cls, (x, y))
@classmethod
def _make(cls, iterable, new=tuple.__new__, len=len):
‘Make a new TPoint object from a sequence or iterable‘
result = new(cls, iterable)
if len(result) != 2:
raise TypeError(‘Expected 2 arguments, got %d‘ % len(result))
return result
def __repr__(self):
‘Return a nicely formatted representation string‘
return self.__class__.__name__ + ‘(x=%r, y=%r)‘ % self
def _asdict(self):
‘Return a new OrderedDict which maps field names to their values‘
return OrderedDict(zip(self._fields, self))
__dict__ = property(_asdict)
def _replace(_self, **kwds):
‘Return a new TPoint object replacing specified fields with new values‘
result = _self._make(map(kwds.pop, (‘x‘, ‘y‘), _self))
if kwds:
raise ValueError(‘Got unexpected field names: %r‘ % list(kwds))
return result
def __getnewargs__(self):
‘Return self as a plain tuple. Used by copy and pickle.‘
return tuple(self)
x = _property(_itemgetter(0), doc=‘Alias for field number 0‘)
y = _property(_itemgetter(1), doc=‘Alias for field number 1‘)
这里就显示出了namedtuple的一些方法。很明显的看到namedtuple是直接继承自tuple的。
几个重要的方法：
1.把数据变成namedtuple类：
TPoint = namedtuple(‘TPoint‘, [‘x‘, ‘y‘])
>>> t = [11, 22]
>>> p = TPoint._make(t)
>>> p
TPoint(x=11, y=22)
>>>
2. 根据namedtuple创建的类生成的类示例，其数据是只读的，如果要进行更新需要调用方法_replace.
>>> p
TPoint(x=11, y=22)
>>> p.y
22
>>> p.y = 33
Traceback (most recent call last):
File "<pyshell#18>", line 1, in <module>
p.y = 33
AttributeError: can‘t set attribute
>>> p._replace(y=33)
TPoint(x=11, y=33)
3.将字典数据转换成namedtuple类型。
>>> d = {‘x‘: 44, ‘y‘: 55}
>>> dp = TPoint(**d)
>>> dp
TPoint(x=44, y=55)
>>>
namedtuple最常用还是出现在处理来csv或者数据库返回的数据上。利用map()函数和namedtuple建立类型的_make（）方法。
EmployeeRecord = namedtuple(‘EmployeeRecord‘, ‘name, age, title, department, paygrade‘)
import csv
for emp in map(EmployeeRecord._make, csv.reader(open("employees.csv", "rb"))):
print(emp.name, emp.title)
# sqlite数据库
import sqlite3
conn = sqlite3.connect(‘/companydata‘)
cursor = conn.cursor()
cursor.execute(‘SELECT name, age, title, department, paygrade FROM employees‘)
for emp in map(EmployeeRecord._make, cursor.fetchall()):
print(emp.name, emp.title)
# MySQL 数据库
import mysql
from mysql import connector
from collections import namedtuple
user = ‘herbert‘
pwd = ‘######‘
host = ‘127.0.0.1‘
db = ‘world‘
cnx = mysql.connector.connect(user=user, password=pwd, host=host,database=db)
cur.execute("SELECT Name, CountryCode, District, Population FROM CITY where CountryCode = ‘CHN‘ AND Population > 500000")
CityRecord = namedtuple(‘City‘, ‘Name, Country, Dsitrict, Population‘)
for city in map(CityRecord._make, cur.fetchall()):
print(city.Name, city.Population)
再谈collections模块defaultdict()和namedtuple()
标签：rip tle opera present 方法 fetchall 遍历 width ror
原文地址：https://www.cnblogs.com/zknublx/p/9729503.html
