Python - map()和对象列表

Question

在变量游戏中有一个对象列表。

for g in games:
  if g.clam ==5: g.new_var=1
  if g.clam ==4: g.new_var=0

如何使用map()函数获得上述功能？我尝试了下面这样的方法，但我认为它不是正确的方法。

def assign_var(clam):
  if clam==5: return 1
  if clam==4: return 0

games.new_var = map(assign_var, games.clam)

Answer 1

在assign_var函数中创建新属性

>>> def assign_var(instance):
...     if instance.clam == 5:
...         instance.new_var = 1
...     elif instance.clam == 4:
...         instance.new_var = 0
... 
>>> map(assign_var, games)
[None, None, None] # Intentional. It will modify the list "games" in place.
>>> for ins in games:
...     print ins.new_var
... 
0
1
0

但实际上，这不是map()应该用到的。map()应该用于可以随更改的数据一起返回的列表，而您不能真正使用类的属性。

一个简单的for循环应该完全没问题：

for ins in games:
    if ins.clam == 5:
        instance.new_var = 1
    elif instance.clam == 4:
        instance.new_var = 0

请注意，请记住，稀疏比密集要好;)。

Answer 2

现在还不清楚你为什么要在这里使用map()。如果您要对games进行突变，那么一个简单的for-loop就足够了。

map()更适合于创建新的list (即使这样，列表理解也会更好)。

我还认为dict是定义映射的一种更简洁的方式，因为它维护起来不那么麻烦。这可以包含在您的Game类中。

下面是一个示例：

#!/usr/bin/env python


class Game(object):
    clam_to_new_var = {
        4: 0,
        5: 1,
    }

    def __init__(self, clam):
        self.clam = clam

    @property
    def new_var(self):
        return Game.clam_to_new_var.get(self.clam, None)

    def __str__(self):
        return 'Game with clam "{}" has new_var "{}"'.format(
            self.clam,
            self.new_var,
        )

if __name__ == '__main__':
    games = map(Game, xrange(10))
    for g in games:
        print g

示例输出：

Game with clam "0" has new_var "None"
Game with clam "1" has new_var "None"
Game with clam "2" has new_var "None"
Game with clam "3" has new_var "None"
Game with clam "4" has new_var "0"
Game with clam "5" has new_var "1"
Game with clam "6" has new_var "None"
Game with clam "7" has new_var "None"
Game with clam "8" has new_var "None"
Game with clam "9" has new_var "None"

我在该示例中保留了map()，但在生产代码中，我更倾向于：

games = [Game(clam) for clam in range(10)]

为什么？参见The fate of reduce() in Python 3000。