双for循环使用2个列表，而双for循环使用2个不同的Outcom文本文件

Question

所以我有两个小脚本。1产生我期望的输出，而另一个不产生。产生我期望的输出的第一个代码：

with open('cities.txt', 'r') as cities, \
    open('test_file.txt', 'r') as test:
    space = " "
    city_lst = []
    test_lst = []
    for c in cities:
        city_lst.append(c)
    for t in test:
        test_lst.append(t)
    for city in city_lst:
        for tes in test_lst:
            print city.rstrip(),space,tes.rstrip() 

输出（如我所料）：

san diego   san diego is the best place
san diego   Then there is new york state
san diego   And now we have tuscon in arizona
san francisco   san diego is the best place
san francisco   Then there is new york state
san francisco   And now we have tuscon in arizona
tuscon   san diego is the best place
tuscon   Then there is new york state
tuscon   And now we have tuscon in arizona
pheonix   san diego is the best place
pheonix   Then there is new york state
pheonix   And now we have tuscon in arizona
sedona   san diego is the best place
sedona   Then there is new york state
sedona   And now we have tuscon in arizona
baton rouge   san diego is the best place
baton rouge   Then there is new york state
baton rouge   And now we have tuscon in arizona

在下一段代码中，我没有得到我认为会得到的输出。它基本上与上面的代码相同，只是我直接处理文本文件，而不是首先将它们转换为列表。然而，这让我困惑，为什么我没有得到完全相同的输出

代码：

with open('cities.txt', 'r') as cities, \
    open('test_file.txt', 'r') as test:
    space = " "
    for c in cities:
        for t in test:
            print c.rstrip(), space, t.rstrip()

输出：

san diego   san diego is the best place
san diego   Then there is new york state
san diego   And now we have tuscon in arizona

既然我在每段代码中使用相同的print语句执行相同的double for循环，为什么输出会有差异呢

以下是文本文件的内容： 城市.txt：

san diego
san francisco
tuscon
pheonix
sedona
baton rouge

测试文件.txt：

san diego is the best place
Then there is new york state
And now we have tuscon in arizona

Answer 1

因为文件是迭代器，而列表是列表

当你这么做的时候

for t in test:
    pass # do anything here

循环结束时，您已经耗尽了迭代器。里面什么都没有了！你自己试试！：

with open('testfile.txt') as inf:
    for line in inf:
        print("There's a line here, I'm reading!")
    for line in inf:
        print("Turn lead into gold")

你会注意到这里完全没有炼金术

您可以做的是在每次读取文件之前seek返回到文件的开头

for c in cities:
    test.seek(0)
    # place the pointer at the beginning of the file
    for t in test:
        frobnicate_stuff()

不过，我更喜欢一次读取每个文件并对列表进行操作，就像您在上面的示例中所做的那样。使用^{}可能会做得更好：

import itertools

with open('cities.txt') as cities, \
         open('test.txt') as test:
    city_lst = cities.readlines()
    test_lst = test.readlines()

for c, t in itertools.product(city_lst, test_lst):
    print(c.rstrip() + " " + t.rstrip())
    # or using string formatting:
    # # print("{} {}".format(c.rstrip(), t.rstrip()))

编辑

事实上，进一步的测试表明^{}在使用每个迭代器之前将其内部化！这意味着我们可以：

with open('cities.txt') as cities, \
        open('tests.txt') as tests:
    for c, t in itertools.product(cities, tests):
        print(c.rstrip() + " " + t.rstrip())

Answer 2

因为文件的对象是迭代器。要将其转换为列表，请使用.readlines()函数。你的代码应该是这样的：

with open('cities.txt') as cities, open('tests.txt') as tests:
    for c in cities.readlines()
        for t in tests.readlines():
            print(c.rstrip() + " " + t.rstrip())

或者，也可以使用itertools.product()来防止嵌套循环。在这种情况下，您的代码应该如下所示：

with open('cities.txt') as cities, open('tests.txt') as tests:
    for c, t in itertools.product(cities.readlines(), tests.readlines()):
        print("{city} {test}".format(city=c,test=t))

注意：不是使用+直接追加字符串。使用.format()方法是更好的方法