字符串格式:％与.format [英] String formatting: % vs. .format
问题描述
Python 2.6引入了 str.format() 方法，与现有%运算符的语法略有不同.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?
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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?
The following uses each method and has the same outcome, so what is the difference?
#!/usr/bin/python
sub1 = "python string!"
sub2 = "an arg"
a = "i am a %s" % sub1
b = "i am a {0}".format(sub1)
c = "with %(kwarg)s!" % {'kwarg':sub2}
d = "with {kwarg}!".format(kwarg=sub2)
print a # "i am a python string!"
print b # "i am a python string!"
print c # "with an arg!"
print d # "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似乎在许多方面都更加复杂.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.同样在您给出的第二个示例中，.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评估表达式，因此在您的log.debug示例中，表达式"some debug info: %s"%some_info首先会评估为，例如"some debug info: roflcopters are active"，则该字符串将传递给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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