Python:避免短路评估 [英] Python: Avoid short circuit evaluation

问题描述

这是在处理Django项目时出现的一个问题.这与表单验证有关.

This is a problem that occured to me while working on a Django project. It's about form validation.

在Django中，当您拥有提交的表单时，可以在相应的表单对象上调用is_valid()来触发验证并返回布尔值.因此，通常在视图函数中会包含类似的代码:

In Django, when you have a submitted form, you can call is_valid() on the corresponding form object to trigger the validation and return a Boolean value. So, usually you have code like that inside your view functions:

if form.is_valid():
    # code to save the form data

is_valid()不仅可以验证表单数据，还可以向表单对象添加错误消息，然后可以向用户显示错误消息.

is_valid() not only validates the form data but also adds error messages to the form object that can afterwards be displayed to the user.

在一页上，我同时使用两种形式，并且还希望仅当两种形式均包含有效数据时才保存数据.这意味着在执行代码保存数据之前，我必须在两种形式上都调用is_valid().最明显的方法:

On one page I use two forms together and also want the data to be saved only if both forms contain valid data. That means I have to call is_valid() on both forms before executing the code to save the data. The most obvious way:

if form1.is_valid() and form2.is_valid():
    # ...

由于逻辑运算符的短路评估而无法工作.如果form1无效，则将不评估form2，并且将丢失其错误消息.

won't work because of the short circuit evaluation of logical operators. If form1 is not valid, form2 will not be evaluated and its error messages would be missing.

那只是一个例子.据我所知，没有像其他语言(例如Smalltalk)那样贪婪地替代and/or.我可以想象这个问题会在不同的情况下发生(不仅在Python中).我能想到的解决方案都是笨拙的(嵌套的ifs，将返回值分配给局部变量并在if语句中使用它们).我想知道解决此类问题的Python方法.

That's only an example. As far as I know, there is no greedy alternative to and/or as in other languages (i.e. Smalltalk). I can imagine that problem occurring under different circumstances (and not only in Python). The solutions I could think of are all kind of clumsy (nested ifs, assigning the return values to local variables and using them in the if statement). I would like to know the pythonic way to solve this kind of problems.

提前谢谢！

推荐答案

诸如此类的事情

if all([form1.is_valid(), form2.is_valid()]):
   ...

在一般情况下，可以使用列表理解，以便预先计算结果(与在此上下文中通常使用的生成器表达式相反).例如:

In a general case, a list-comprehension could be used so the results are calculated up front (as opposed to a generator expression which is commonly used in this context). e.g.:

if all([ form.is_valid() for form in (form1,form2) ])  

这也可以很好地扩展到任意数量的条件...唯一的不足是，它们都需要通过"and"而不是if foo and bar or baz: ...进行连接.

This will scale up nicely to an arbitrary number of conditions as well ... The only catch is that they all need to be connected by "and" as opposed to if foo and bar or baz: ....

(对于非短路or，可以使用any代替all).

(for a non-short circuiting or, you could use any instead of all).

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