python cookbook 迭代器與生成器

**迭代

a = [1, 2, 3]
fori in iter(a):
print(i)
fori in a.__iter__():
print(i)

這裡的兩個方法是一樣的，呼叫iter()其實就是簡單的呼叫了物件的__iter__()方法。

使用生成器建立新的迭代器

def frange(start, stop, increment):
x =start
while x yield x
x +=increment
for n in frange(0, 4, 0.5):
print(n)
print(list(frange(0, 4, 0.5)))

看下面這個

>>>def countdown (n):
... print ('starting to count from', n)
... 
while n > 0:
... yield n
... n -= 1... print ('done!')
...>>> c = countdown(3)
>>>c
>>>next(c)
starting to count from 3
3>>>next(c)
2>>>next(c)
1>>>next(c)
done!traceback (most recent call last):
file "", line 1, in < module >stopiteration

乙個生成器函式主要特徵是它只會回應在迭代中使用到的 next 操作。一旦生成器函式返回退出,迭代終止。我們在迭代中通常使用的for語句會自動處理這些細節,所以你無需擔心。

def frange(start, stop, increment):
x =start
while x yield
x x +=increment
for n in frange(0, 4, 0.5
): print(n)
print(list(frange(
0, 4, 0.5)))

反向迭代

a = [1, 2, 3, 4]
for x in
reversed(a):
print(x, end='
')

主要想說的是，物件重寫__reversed__()方法即可呼叫reversed()進行反向迭代

itertools的一些使用

def
count(n):
while
true:
yield
n n += 1c =count(0)
import
itertools
for x in itertools.islice(c, 10, 20):
print(x, end="")
with open(
'test.txt
') as f:
for line in
f: 
print(line, end='') 
print(format('
開始部分的注釋不顯示
', '
*>30'))
with open(
'test.txt
') as f:
for line in itertools.dropwhile(lambda line: line.startswith('#'
), f):
print(line, end=''
) 
items = ['
a', '
b', 'c'
] #items的所有可能組合，不包含相同元素
for p in
itertools.permutations(items):
print(p, end='')
print
() 
#指定長度的所有排序
for p in itertools.permutations(items, 2):
print(p, end='
')

如果遇到一些複雜的迭代器的使用，可以先看看itertools裡有沒有可用的方法。

同時迭代多個序列

a = [1, 2, 3]
b = ['
w', '
x', '
y', 'z'
] for i in
zip(a,b):
print
(i)輸出：
(1, 'w')
(2, 'x')
(3, 'y')
for i in
itertools.zip_longest(a, b):
print(i)
輸出：(1, 'w')
(2, 'x')
(3, 'y')
(none, 'z')

不同序列上的迭代

import
itertools
a = [1, 2, 3, 4]
b = ['
x', '
y', 'z'
] for x in
itertools.chain(a, b):
print(x)

這樣比 a + b 在進行迭代要好很多

展開巢狀的序列

from collections import
iterable
def flattern(items, ignore_types=(str, bytes)):
for x in
items:
if isinstance(x, iterable) and
notisinstance(x, ignore_types):
yield
from
flattern(x)
else
: 
yield
x items = [1, 2, [3, 4, [5, '
hello world
'], 7], 8]
for x in
flattern(items):
print(x, end='
')

def count(n): while true: yield n n += 1 c = count(0) import itertools for x in itertools.islice(c, 10, 20): print(x, end=" ") with open('test.txt') as f: for line in f: print(line, end='') print(format('開始部分的注釋不顯示', '*>30')) with open('test.txt') as f: for line in itertools.dropwhile(lambda line: line.startswith('#'), f): print(line, end='') items = ['a', 'b', 'c'] #items的所有可能組合，不包含相同元素 for p in itertools.permutations(items): print(p, end=' ') print() #指定長度的所有排序 for p in itertools.permutations(items, 2): print(p, end=' ')

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