如果 foo 中不存在键，则忽略 str.format(**foo)

Question

我正在尝试将电子邮件模板放在一起。消息内容将取决于字典中的值。但是，字典可能不会每次都包含所有键。

目前这很好，因为所有值都在字典中（'Title'、'Surname'、'Additional Details'）：

practise_dict = {"Additional Details":"blah blah blah blah", "Title": "Mr", "Surname": "Smith", "URL": "/test/tester"}

msg = """From: John Smith <no-reply@somethingsomething.co.uk>
To: {Title} {Surname} <blah@blahblah.co.uk>
MIME-Version: 1.0
Content-type: text/html
Subject: New Website Enquiry
This is an e-mail message to be sent in HTML format
{Additional Details}
<b>This is HTML message.</b>
<h1>This is headline.</h1>
`""".format(**practise_dict)

print(msg)

在msg 变量中，我正在尝试创建我的“模板”。这意味着我需要拥有字典中所有可能的项目。

例如，下一段将失败，因为它正在查找此字典中不存在的 'Date'：

practise_dict = {"Additional Details":"blah blah blah blah", "Title": "Mr", "Surname": "Smith", "URL": "/test/tester"}

msg = """From: John Smith <no-reply@somethingsomething.co.uk>
To: {Title} {Surname} <blah@blahblah.co.uk>
MIME-Version: 1.0
Content-type: text/html
Subject: New Website Enquiry
This is an e-mail message to be sent in HTML format
{Additional Details}
{Date}
<b>This is HTML message.</b>
<h1>This is headline.</h1>
`""".format(**practise_dict)

print(msg)

如果查找字典中不存在作为键的字符串替换，有没有办法让它忽略它？

Answer 1

您可以为此使用Template 和safe_substitute：

from string import Template

practise_dict = {"Additional_Details":"blah blah blah blah", "Title": "Mr", "Surname": "Smith", "URL": "/test/tester"}

msg = """From: John Smith <no-reply@somethingsomething.co.uk>
To: $Title $Surname <blah@blahblah.co.uk>
MIME-Version: 1.0
Content-type: text/html
Subject: New Website Enquiry
This is an e-mail message to be sent in HTML format
$Additional_Details
$Date
<b>This is HTML message.</b>
<h1>This is headline.</h1>
`"""

s = Template(msg).safe_substitute(**practise_dict)
print(s)

输出

From: John Smith <no-reply@somethingsomething.co.uk>
To: Mr Smith <blah@blahblah.co.uk>
MIME-Version: 1.0
Content-type: text/html
Subject: New Website Enquiry
This is an e-mail message to be sent in HTML format
blah blah blah blah
$Date
<b>This is HTML message.</b>
<h1>This is headline.</h1>

Answer 2

我认为没有办法告诉format 某些替换是可选的。但是，这里有两种解决方法。

如果只缺少一个字段，请尝试以下操作：

msg = """...""".format(Date=practise_dict.pop("Date", ""), # handle optional field
                       **practise_dict) # pass the rest as before

但是，请注意，这会修改practise_dict，如果要处理多个值会很尴尬。

如果可能缺少多个字段，或者您不想修改字典，则以下方法可能更可取：

defaults = {"Date": "", "Additional Details": ""} # define optional fields
defaults.update(practise_dict) # override user data where available
msg = """...""".format(**defaults)

对于您没有明确提供替代方案的键，这两种方法仍然会失败，这允许您将某些字段设为必填。

Answer 3

如果您还不知道密钥：

你可以只写一个 dict 子类，如果它在字典中没有找到，它将返回键：

class FailsafeDict(dict):
    def __getitem__(self, item):
        try:
            return super().__getitem__(item)
        except KeyError:
            return "{" + str(item) + "}"
        # end try
    # end def
# end class

这通过覆盖处理dict[item] 调用的__getitem__(item) 函数来工作。

你可以这样使用它：

f = FailsafeDict({"yo": "lo", "foo": "bar"})
text = "{yo}! {lol}! {foo}!".format(**f)
print(text)  

打印lo! {lol}! bar!。

在你的情况下

msg = """...""".format(**FailsafeDict(practise_dict))