仅在满足条件时才添加到 dict [英] Only add to a dict if a condition is met

问题描述

我正在使用 urllib.urlencode 来构建 Web POST 参数，但是如果存在除 None 以外的值，我只想添加一些值.

I am using urllib.urlencode to build web POST parameters, however there are a few values I only want to be added if a value other than None exists for them.

apple = 'green'
orange = 'orange'
params = urllib.urlencode({
    'apple': apple,
    'orange': orange
})

这很好用，但是如果我将 orange 变量设为可选，如何防止它被添加到参数中?像这样(伪代码):

That works fine, however if I make the orange variable optional, how can I prevent it from being added to the parameters? Something like this (pseudocode):

apple = 'green'
orange = None
params = urllib.urlencode({
    'apple': apple,
    if orange: 'orange': orange
})

我希望这已经足够清楚了，有谁知道如何解决这个问题?

I hope this was clear enough, does anyone know how to solve this?

推荐答案

在创建初始 dict 后，您必须单独添加密钥:

You'll have to add the key separately, after the creating the initial dict:

params = {'apple': apple}
if orange is not None:
    params['orange'] = orange
params = urllib.urlencode(params)

Python 没有将键定义为条件的语法；如果您已经将所有内容按顺序排列，则可以使用字典理解:

Python has no syntax to define a key as conditional; you could use a dict comprehension if you already had everything in a sequence:

params = urllib.urlencode({k: v for k, v in (('orange', orange), ('apple', apple)) if v is not None})

但这不是很可读.

如果您使用 Python 3.9 或更新版本，您可以使用 新的 dict 合并运算符支持 和条件表达式:

If you are using Python 3.9 or newer, you could use the new dict merging operator support and a conditional expression:

params = urllib.urlencode(
    {'apple': apple} | 
    ({'orange': orange} if orange is not None else {})
)

但我发现可读性受到影响，因此可能仍会使用单独的 if 表达式:

but I find readability suffers, and so would probably still use a separate if expression:

params = {'apple': apple}
if orange is not None:
    params |= {'orange': orange}
params = urllib.urlencode(params)

另一种选择是使用字典解包，但对于单个密钥这不是更易读:

Another option is to use dictionary unpacking, but for a single key that's not all that more readable:

params = urllib.urlencode({
    'apple': apple,
    **({'orange': orange} if orange is not None else {})
})

我个人永远不会使用它，它太老套了，不像使用单独的 if 语句那样明确和清晰.正如 Python 之禅 所说:可读性很重要.

I personally would never use this, it's too hacky and is not nearly as explicit and clear as using a separate if statement. As the Zen of Python states: Readability counts.

这篇关于仅在满足条件时才添加到 dict的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！