python上的列表总是添加相同的值

Question

我在while循环中有以下代码.

if gender == 0 and len(men) < 51 :
    height = float((random.uniform(1.3, 1.9) + (random.randint(10, 20)/100.)).__format__('.2f'))
    weight = float((random.uniform(45, 100) * height).__format__('.2f'))
    attr['height'] = height 
    attr['weight'] = weight

    men.append(attr)

所以这段代码总是给出一些随机高度和随机权重.但是outsite de loop(当它完成时).如果我这样做print men,我得到以下结果:

[{'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}, {'weight': 76.64, 'height': 1.75}]

它总是一样的.但是,如果我不使用attr[height] = height; attr['weight] = weight和使用men.append(height); men.append(weight)我得到以下结果:

print men [1.91, 145.95, 1.64, 95.66, 2.0, 159.94, 1.74, 143.36, 1.68, 97.99, 1.6, 90.11, 1.63, 116.2, 1.56, 96.8, 2.04, 198.56, 1.56, 145.96, 1.44, 67.57, 1.83, 94.97, 1.85, 175.69, 1.84, 101.84, 1.54, 135.0, 1.41, 101.23, 1.92, 167.59, 1.74, 142.55, 1.49, 129.07, 1.83, 161.28, 1.59, 97.16, 1.46, 134.53, 2.03, 158.72, 2.05, 184.43, 1.97, 162.81]

如果我在循环中打印attr它总是有一个不同的值(它是我想要的).但是当我将它附加到我的列表时,我的列表的值总是相同的.我究竟做错了什么？

Answer 1

您的代码的简化示例,目前可以更全面地解释为什么您的结果是这样的:

all_items = []
new_item = {}
for i in range(0,5):
    new_item['a'] = i
    new_item['b'] = i

    all_items.append(new_item)
    print new_item
    print hex(id(new_item))  # print memory address of new_item

print all_items

请注意,每次循环时,对象的内存地址都是相同的.这意味着每次添加的对象都是相同的.因此,当您打印最终列表时,您将在循环中的每个位置打印同一对象的坐标.

每次进行循环时,值都会更新 - 假设您每天都在同一面墙上绘画.第一天,它可能是蓝色的.第二天,你重绘同一面墙(或物体),然后是绿色.最后一天你把它漆成橙色并且它是橙色的 - 同样的墙现在总是橙色.您对该attr对象的引用就像是说您拥有相同的墙.

即使你在画完后看着墙壁,颜色也会改变.但之后它是橙色的墙 - 即使你看了5次.

当我们在每次迭代中将对象作为新对象时,请注意两件事情发生:

1. 内存地址发生了变化
2. 值保持为唯一值

这类似于绘制不同的墙壁.完成最后一个画面后,每个先前的墙壁仍然会涂上您首次绘制的颜色.

您可以在下面看到这一点,每次迭代都会创建每个对象:

all_items = []
for i in range(0,5):
    new_item = {}
    new_item['a'] = i
    new_item['b'] = i

    all_items.append(new_item)
    print hex(id(new_item))


 print all_items

您也可以采用不同的方式,例如:

all_items = []
for i in range(0,5):
    new_item = {'a': i, 'b': i}
    all_items.append(new_item)
    print hex(id(new_item))    
print all_items

甚至一步到位:

all_items = []
for i in range(0,5):
    all_items.append({'a': i, 'b': i})

print all_items

因此,以下任何一个都可以工作:

attr = {}
attr['height'] = height 
attr['weight'] = weight

men.append(attr)

要么:

men.append({'height': height, 'weight': weight})

Answer 2

您要attr多次将相同的字典添加到列表中.以下行只会改变attr字典而不是创建新字典:

attr['height'] = height 
attr['weight'] = weight

您应该每次都创建一个新的dict,例如:

attr = {'height': height, 'weight': weight}