在Python中迭代列表时修改列表的值？

Question

例如：

def update_condition(self, type, params):
    for condition in self.conditions:
        condition_loaded = json.loads(condition)
        if condition_loaded['type'] == type:
            condition_loaded['params'] = params
            condition = json.dumps(condition_loaded)

上面的代码什么也不做，因为条件不是通过引用的。正确的方法是什么？

Answer 1

你可以使用列表理解：

def update_single_condition(self, condition, type, params):
    condition_loaded = json.loads(condition)
    if condition_loaded['type'] == type:
        condition_loaded['params'] = params
    return json.dumps(condition_loaded)

def update_conditions(self, type, params): 
    self.conditions = [self.update_single_condition(c, type, params) for c in self.conditions]

Answer 2

您可以使用enumerate：

def update_condition(self, type, params):
    for i,condition in enumerate(self.conditions):
        condition_loaded = json.loads(condition)
        if condition_loaded['type'] == type:
            condition_loaded['params'] = params
            self.conditions[i] = json.dumps(condition_loaded)

但是，一般来说，这些东西在helper函数和列表理解方面稍微干净一点：

def helper(condition,type,params)
    loaded = json.loads(condition)
    if loaded['type'] == type:
       loaded['params'] = params
       return json.dumps(loaded)
    return condition

...

def update_condition(self, type, params):
    self.conditions = [helper(c,type,params) for c in self.conditions]

应该注意的是，第二个解决方案并没有及时更新列表——换句话说，如果您有其他对该列表的引用，它们将不会受到影响。如果需要，可以使用slice赋值很容易地进行替换：

def update_condition(self, type, params):
    self.conditions[:] = [helper(c,type,params) for c in self.conditions]