相关疑难解决方法(0)

我注意到在尝试p= p+i不同的列表时, p += i 例如:

test = [0, 1, 2, 3,]
p = test
test1 = [8]
p = p + test1
print test

在上面的代码中test打印出原始值[0, 1, 2, 3,]

但是如果我在下面p = p + test1用p += test1As 切换

test = [0, 1, 2, 3,]
p = test
test1 = [8]

p += test1

print test

test 现在等于 [0, 1, 2, 3, 8]

价值不同的原因是什么？

当我们使用时,我们的代码需要10分钟来虹吸68,000条记录:

new_file = new_file + line + string

但是,当我们执行以下操作时,只需1秒钟:

new_file += line + string

这是代码:

for line in content:
import time
import cmdbre

fname = "STAGE050.csv"
regions = cmdbre.regions
start_time = time.time()
with open(fname) as f:
        content = f.readlines()
        new_file_content = ""
        new_file = open("CMDB_STAGE060.csv", "w")
        row_region = ""
        i = 0
        for line in content:
                if (i==0):
                        new_file_content = line.strip() + "~region" + "\n"
                else:
                        country = line.split("~")[13]
                        try:
                                row_region = regions[country]
                        except KeyError:
                                row_region = "Undetermined"
                        new_file_content += …

考虑下面的代码，乍一看它做了同样的事情，但结果不同，有时列表似乎是按值传递，有时列表似乎是按引用传递：

lst = [1, 2]
def f(lst):
#     lst = lst + [3]  # seems pass by value
#     lst += [3]  # strange! same as above but seems pass by reference
    lst = lst.append(3)  # seems pass by reference
    return lst
f(lst)  
print(lst)

谁能告诉我发生了什么事吗？

>>> c = [1, 2, 3]
>>> print(c, id(c))
[1, 2, 3] 43955984
>>> c += c
>>> print(c, id(c))
[1, 2, 3, 1, 2, 3] 43955984
>>> del c
>>> c = [1, 2, 3]
>>> print(c, id(c))
[1, 2, 3] 44023976
>>> c = c + c
>>> print(c, id(c))
[1, 2, 3, 1, 2, 3] 26564048

有什么不同？是+ =和+不应该仅仅是语法糖？

我已经在Stack Exchange上阅读了很多像Python一样的答案- 为什么在课堂上使用"self"？ 阅读完这些内容之后,我了解到实例变量对于类的每个实例都是唯一的,而类变量是在所有实例之间共享的.在玩的时候我发现这个代码 -

class A:
    x = []
    def add(self):
        self.x.append(1)

x = A()
y = A()
x.add()
print "Y's x:",y.x

确实给出了输出[1].但是,这段代码 -

class A:
    x = 10
    def add(self):
        self.x += 1

x = A()
y = A()
x.add()

print "Y's x:",y.x

给出输出,就像10我认为的那样11.如果这是一个非常无聊的问题,请原谅我,但我在编程方面不是很有经验.

我总是这么认为

x += y

只是一个捷径

x = x + y

但似乎不是列表的情况:

x = []

x2 = x

print x is x2
True

x += [2]

print x is x2
True

x = x + [2]

print x is x2
False

我有一个如下所示的数据框：

df1

        A        Q
0   Apple    chair
1  orange     desk
2    pear  monitor
3   salad     room

df2 = df1[['Q']]

print(df2)

           Q
0      chair
1       desk
2    monitor
3       room


df2 = df2.insert(1, 'file_id_value', range(0, len(df2)))

print(df2)

不返回任何内容，特别是单词'None'。你知道我做错了什么吗？我没有收到错误消息。