Python 基本特性
本文的Python知识基于 Python3
list的切片操作
list[x:y]
- 取list的从x到y下标, 不包括y
- 如果x是0, 可以省略
- 如果y是结尾, 可以省略
- 可以为负值, 表示从后向前
list[x:y:k]
- 取list的从x到y下标, 步长为k
- 如果x是0, 可以省略
- 如果y是结尾, 可以省略
- 可以为负值, 表示从后向前
>>> mylist = list(range(100))
>>> mylist
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99]
>>> other = [mylist[0], mylist[1]] # 取list的前两个
>>> other
[0, 1]
>>> other.clear()
>>> for i in range(2): # 取list的前两个
... other.append(mylist[i])
...
>>> other
[0, 1]
>>> other = mylist[0:2] # 切片操作 取list的前两个
>>> other
[0, 1]
>>> other = mylist[:2] # 切片操作 取list的前两个
>>> other
[0, 1]
>>> other = mylist[2:4] # 切片操作 取list的[2][3]两个
>>> other
[2, 3]
>>> other = mylist[::3] # 切片操作 取list从头到位, 每3个取一个
>>> other
[0, 3, 6, 9, 12, 15, 18, 21, 24, 27, 30, 33, 36, 39, 42, 45, 48, 51, 54, 57, 60, 63, 66, 69, 72, 75, 78, 81, 84, 87, 90, 93, 96, 99]
>>> other = mylist[40:60:2] # 切片操作 取list的从[40]到[59]范围内的 每2个取一个
>>> other
[40, 42, 44, 46, 48, 50, 52, 54, 56, 58]
>>> other = mylist[-1:0:2] # 切片操作 取list的从末尾到第一个, 每2个取一个 取不到, 因为开始位置大于结束位置
>>> other
[]
>>> other = mylist[-1:-10:2] # 切片操作 取list的从末尾到倒数第9个的范围内, 每2个取一个 取不到 因为开始位置大于结束位置
>>> other
[]
>>> other = mylist[-1:-10:-2] # 切片操作 取list的从末尾到倒数第9个的范围内, 每2个取一个 倒着取
>>> other
[99, 97, 95, 93, 91]
>>> other = mylist[-10::2] # 切片操作 取list的从倒数第10个到第一个, 每2个取一个
>>> other
[90, 92, 94, 96, 98]
>>> other = mylist[-1:0:-2] # 切片操作 取list的从最后一个到第一个, 倒着每2个取一个
>>> other
[99, 97, 95, 93, 91, 89, 87, 85, 83, 81, 79, 77, 75, 73, 71, 69, 67, 65, 63, 61, 59, 57, 55, 53, 51, 49, 47, 45, 43, 41, 39, 37, 35, 33, 31, 29, 27, 25, 23, 21, 19, 17, 15, 13, 11, 9, 7, 5, 3, 1]
>>> other = mylist[::-1] # 切片操作 取list的从头到尾, 倒着取
>>> other
[99, 98, 97, 96, 95, 94, 93, 92, 91, 90, 89, 88, 87, 86, 85, 84, 83, 82, 81, 80, 79, 78, 77, 76, 75, 74, 73, 72, 71, 70, 69, 68, 67, 66, 65, 64, 63, 62, 61, 60, 59, 58, 57, 56, 55, 54, 53, 52, 51, 50, 49, 48, 47, 46, 45, 44, 43, 42, 41, 40, 39, 38, 37, 36, 35, 34, 33, 32, 31, 30, 29, 28, 27, 26, 25, 24, 23, 22, 21, 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0]
>>> list的遍历
两种方法
>>> mylist = range(0, 10)
>>> for x in mylist:
... print(x)
...
0
1
2
3
4
5
6
7
8
9
>>> for x in range(len(mylist)):
... print(mylist[x])
...
0
1
2
3
4
5
6
7
8
9
>>> 字典的遍历
>>> mydict = {1:'a', 2:'b', 3:'c'}
>>> for x in mydict:
... print(x) # 遍历key
... print(mydict[x]) # 遍历value
...
1
a
2
b
3
c
>>> for x in mydict.keys():
... print(x) # 遍历key
... print(mydict[x]) # 遍历value
...
1
a
2
b
3
c
>>> for x in mydict.values(): # 遍历value
... print(x)
...
a
b
c
>>> for x in mydict.items(): # 遍历项
... print(x)
...
(1, 'a')
(2, 'b')
(3, 'c')
>>> 迭代
在Python中, for是很强大的
通常, for会和in配合进行遍历
它就是一个 迭代器
判断是否可迭代
>>> mydict = {1:'a', 2:'b', 3:'c'}
>>> import collections
>>> isinstance(mydict, collections.Iterable) # 是否可迭代
True
>>> list高级生成操作
>>> mylist = []
>>> for x in range(1, 11):
... mylist.append(x*x)
...
>>> mylist
[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
>>> mylist = [ x * x for x in range(2, 8)] # 列表生成器(列表生成式) 方式进行生成
>>> mylist
[4, 9, 16, 25, 36, 49]
>>> listgen = lambda x: x*x # lambda表达式配合map生成
>>> mylist = list(map(listgen, range(2, 8)))
>>> mylist
[4, 9, 16, 25, 36, 49]
>>> 生成器
- 语法和列表生成器一样
- 只不过是用
()包含 - 它具有
滞后性, 使用next来取值 - 每一个元素只能取一次, 没有下标
>>> gen = (x * x for x in range(2, 11))
>>> type(gen)
<class 'generator'>
>>> next(gen)
4
>>> next(gen)
9
>>> next(gen)
16
>>> next(gen)
25
>>> for x in gen:
... print(x)
...
36
49
64
81
100
>>> for x in gen:
... print(x)
...
... # 不能向前, 只能向后, 用完就没了
...
>>> next(gen)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
StopIteration
>>> 未完待续...
如有错误,请提出指正!谢谢.
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最后编辑时间为: 2017-07-16 at 04:05 am