如果未以str.format传递，则将值保留为空白 [英] Leaving values blank if not passed in str.format

问题描述

我遇到了一个非常简单的问题，我无法提出一个优雅的解决方案.

I've run into a fairly simple issue that I can't come up with an elegant solution for.

我正在使用str.format在一个函数中创建一个字符串，该函数以dict替代形式传递以用于格式.我想创建字符串并将其值格式化(如果已传递)，否则将其留空.

I'm creating a string using str.format in a function that is passed in a dict of substitutions to use for the format. I want to create the string and format it with the values if they're passed and leave them blank otherwise.

Ex

kwargs = {"name": "mark"}
"My name is {name} and I'm really {adjective}.".format(**kwargs)

应该返回

"My name is mark and I'm really ."

而不是抛出KeyError(如果我们什么都不做，将会发生什么情况).

instead of throwing a KeyError (Which is what would happen if we don't do anything).

令人尴尬的是，对于这个问题，我什至无法提出一个优雅的解决方案.我想我可以通过不使用str.format来解决此问题，但我宁愿使用内置的(主要是我想要的功能).

Embarrassingly, I can't even come up with an inelegant solution for this problem. I guess I could solve this by just not using str.format, but I'd rather use the built-in (which mostly does what I want) if possible.

注意:我事先不知道将使用什么键.如果有人包含密钥但没有将其放入kwargs字典中，我将尝试以失败告终.如果我以100％的准确度知道将查找哪些键，则只需填充所有键并完成操作即可.

Note: I don't know in advance what keys will be used. I'm trying to fail gracefully if someone includes a key but doesn't put it in the kwargs dict. If I knew with 100% accuracy what keys would be looked up, I'd just populate all of them and be done with it.

推荐答案

您可以按照

You can follow the recommendation in PEP 3101 and use a subclass Formatter:

import string

class BlankFormatter(string.Formatter):
    def __init__(self, default=''):
        self.default=default

    def get_value(self, key, args, kwds):
        if isinstance(key, str):
            return kwds.get(key, self.default)
        else:
            return string.Formatter.get_value(key, args, kwds)

kwargs = {"name": "mark", "adj": "mad"}     
fmt=BlankFormatter()
print fmt.format("My name is {name} and I'm really {adj}.", **kwargs)
# My name is mark and I'm really mad.
print fmt.format("My name is {name} and I'm really {adjective}.", **kwargs)
# My name is mark and I'm really .  

---

从Python 3.2开始，您可以使用 .format_map 作为替代:

---

As of Python 3.2, you can use .format_map as an alternative:

class Default(dict):
    def __missing__(self, key):
        return '{'+key+'}'

kwargs = {"name": "mark"}

print("My name is {name} and I'm really {adjective}.".format_map(Default(kwargs)))

打印:

My name is mark and I'm really {adjective}.

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