字符串格式:％vs.format vs.f字符串文字 [英] String formatting: % vs. .format vs. f-string literal

问题描述

Python 2.6引入了 str.format() 方法，其语法与现有的％运算符略有不同.哪种更好，什么情况下适合?

Python 2.6 introduced the str.format() method with a slightly different syntax from the existing % operator. Which is better and for what situations?

Python 3.6现在通过语法 f"my string" 引入了字符串文字(又称"f"字符串)的另一种字符串格式化格式.这个格式选项比其他格式更好吗?

Python 3.6 has now introduced another string formatting format of string literals (aka "f" strings) via the syntax f"my string". Is this formatting option better than the others?

1. 以下内容使用每种方法并具有相同的结果，所以有什么区别?

1. The following uses each method and has the same outcome, so what is the difference?

 #!/usr/bin/python
 sub1 = "python string!"
 sub2 = "an arg"

 sub_a = "i am a %s" % sub1
 sub_b = "i am a {0}".format(sub1)
 sub_c = f"i am a {sub1}"

 arg_a = "with %(kwarg)s!" % {'kwarg':sub2}
 arg_b = "with {kwarg}!".format(kwarg=sub2)
 arg_c = f"with {sub2}!"

 print(sub_a)    # "i am a python string!"
 print(sub_b)    # "i am a python string!"
 print(sub_c)    # "i am a python string!"

 print(arg_a)    # "with an arg!"
 print(arg_b)    # "with an arg!"
 print(arg_c)    # "with an arg!"

此外，何时在Python中进行字符串格式化?例如，如果我的日志记录级别设置为HIGH，那么执行以下％操作是否还会受到影响?如果是这样，有办法避免这种情况吗?

Furthermore when does string formatting occur in Python? For example, if my logging level is set to HIGH will I still take a hit for performing the following % operation? And if so, is there a way to avoid this?

 log.debug("some debug info: %s" % some_info)

推荐答案

要回答第一个问题... .format 在许多方面似乎都更加复杂.关于％的一个烦人的事情是它如何可以接受变量或元组.您会认为以下内容将始终有效:

To answer your first question... .format just seems more sophisticated in many ways. An annoying thing about % is also how it can either take a variable or a tuple. You'd think the following would always work:

"hi there %s" % name

但是，如果 name 恰好是(1、2、3)，它将抛出 TypeError .为了确保它始终打印，您需要这样做

yet, if name happens to be (1, 2, 3), it will throw a TypeError. To guarantee that it always prints, you'd need to do

"hi there %s" % (name,)   # supply the single argument as a single-item tuple

这很丑. .format 没有这些问题.同样在您提供的第二个示例中， .format 示例看起来更加简洁.

which is just ugly. .format doesn't have those issues. Also in the second example you gave, the .format example is much cleaner looking.

您为什么不使用它?

• 不知道(我在阅读本文之前)
• 必须与Python 2.5兼容

要回答您的第二个问题，字符串格式化与任何其他操作同时发生-在计算字符串格式化表达式时.而且Python不是一种惰性语言，它会在调用函数之前先对表达式求值，因此在您的 log.debug 示例中，表达式"some debug info:％s"％some_info 首先评估，例如一些调试信息:直升机处于活动状态" ，然后将该字符串传递给 log.debug().

To answer your second question, string formatting happens at the same time as any other operation - when the string formatting expression is evaluated. And Python, not being a lazy language, evaluates expressions before calling functions, so in your log.debug example, the expression "some debug info: %s"%some_infowill first evaluate to, e.g. "some debug info: roflcopters are active", then that string will be passed to log.debug().

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