切片:
>>> L = list(range(100))
>>> L
[0, 1, 2, 3, ..., 99]
>>> L[:10]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> L[-10:]
[90, 91, 92, 93, 94, 95, 96, 97, 98, 99]
>>> L[10:20]
[10, 11, 12, 13, 14, 15, 16, 17, 18, 19]
#每隔2个取
>>> L[:10:2]
[0, 2, 4, 6, 8]
#每隔5个取
>>> L[::5]
[0, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95]
#复制一份
>>> L[:]
[0, 1, 2, 3, ..., 99]
迭代:
使用for循环可以快速进行迭代
>>> for x, y in [(1, 1), (2, 4), (3, 9)]:
... print(x, y)
def findMinAndMax(L):
if len(L) == 0:
return (None, None)
min = L[0]
max = L[0]
for x in L:
if x > max:
max = x
if x < min:
min = x
return (min, max)
列表生成器list
>>> list(range(1, 11))
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
>>> [x * x for x in range(1, 11)]
[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
>>> [x * x for x in range(1, 11) if x % 2 == 0]
[4, 16, 36, 64, 100]
>>> [m + n for m in 'ABC' for n in 'XYZ']
['AX', 'AY', 'AZ', 'BX', 'BY', 'BZ', 'CX', 'CY', 'CZ']
#迭代key value
>>> d = {'x': 'A', 'y': 'B', 'z': 'C' }
>>> for k, v in d.items():
... print(k, '=', v)
...
y = B
x = A
z = C
#for 和 if 结合,注意没有else
>>> [x for x in range(1, 11) if x % 2 == 0]
[2, 4, 6, 8, 10]
或者这样 必须有else
>>> [x if x % 2 == 0 else -x for x in range(1, 11)]
[-1, 2, -3, 4, -5, 6, -7, 8, -9, 10]
L1 = ['Hello', 'World', 18, 'Apple', None]
L2 = []
for x in L1:
if isinstance(x, str) and x!= None:
L2.append(x.lower())
生成器generator
generator保存的是算法,每次调用next(g)
,就计算出g
的下一个元素的值,直到计算到最后一个元素,没有更多的元素时,抛出StopIteration
的错误。
通过for
循环来迭代
#list
>>> L = [x * x for x in range(10)]
#generator
>>> g = (x * x for x in range(10))
>>> for n in g:
... print(n)
#也可以用函数来生成
def fib(max):
n, a, b = 0, 0, 1
while n < max:
yield b
a, b = b, a + b
n = n + 1
return 'done'
#每次调用函数 执行到yield结束 下次调用直接从这里开始执行
>>> g = fib(6)
>>> while True:
... try:
... x = next(g)
... print('g:', x)
... except StopIteration as e:
... print('Generator return value:', e.value)
... break
生成杨辉三角形
def triangles():
n = 1
L = [1]
while n < 11:
yield L
L2 = L.copy()
L2.append(1)
if len(L2) <= 2:
pass
else:
for x in range(len(L2)):
if x ==0 or x == len(L2)-1:
pass
else:
L2[x]=L[x-1]+L[x]
L = L2
n = n + 1
return 'done'
results = []
for t in triangles():
results.append(t)
n = n + 1
if n == 10:
break
for t in results:
print(t)
if results == [
[1],
[1, 1],
[1, 2, 1],
[1, 3, 3, 1],
[1, 4, 6, 4, 1],
[1, 5, 10, 10, 5, 1],
[1, 6, 15, 20, 15, 6, 1],
[1, 7, 21, 35, 35, 21, 7, 1],
[1, 8, 28, 56, 70, 56, 28, 8, 1],
[1, 9, 36, 84, 126, 126, 84, 36, 9, 1]
]:
print('测试通过!')
else:
print('测试失败!')
可迭代对象Iterable和迭代器Iterator
可迭代对象不一定是迭代器
>>> from collections.abc import Iterable
>>> isinstance([], Iterable)
True
>>> isinstance({}, Iterable)
True
>>> isinstance('abc', Iterable)
True
>>> isinstance((x for x in range(10)), Iterable)
True
>>> isinstance(100, Iterable)
False
>>> from collections.abc import Iterator
>>> isinstance((x for x in range(10)), Iterator)
True
>>> isinstance([], Iterator)
False
>>> isinstance({}, Iterator)
False
>>> isinstance('abc', Iterator)
False
>>> isinstance(iter([]), Iterator)
True
>>> isinstance(iter('abc'), Iterator)
True
生成器都是Iterator
对象,但list
、dict
、str
虽然是Iterable
,却不是Iterator
。
把list
、dict
、str
等Iterable
变成Iterator
可以使用iter()
函数
参考来源:https://www.liaoxuefeng.com/wiki/1016959663602400/1017323698112640