标签归档:printf

Python的字符串格式化的许多方式-较旧的(即将被淘汰)吗?

问题:Python的字符串格式化的许多方式-较旧的(即将被淘汰)吗?

Python至少有六种格式化字符串的方式:

In [1]: world = "Earth"

# method 1a
In [2]: "Hello, %s" % world
Out[2]: 'Hello, Earth'

# method 1b
In [3]: "Hello, %(planet)s" % {"planet": world}
Out[3]: 'Hello, Earth'

# method 2a
In [4]: "Hello, {0}".format(world)
Out[4]: 'Hello, Earth'

# method 2b
In [5]: "Hello, {planet}".format(planet=world)
Out[5]: 'Hello, Earth'

# method 2c
In [6]: f"Hello, {world}"
Out[6]: 'Hello, Earth'

In [7]: from string import Template

# method 3
In [8]: Template("Hello, $planet").substitute(planet=world)
Out[8]: 'Hello, Earth'

不同方法的简要历史:

  • printf自从Python诞生以来,样式样式格式化就已经存在
  • Template班是在Python 2.4中引入
  • format方法在Python 2.6中引入
  • f-strings是在Python 3.6中引入的

我的问题是:

  • 是否printf不赞成使用-style格式?
  • 在中Template class,该substitute方法是否已弃用或将要弃用?(我不是在谈论safe_substitute,据我所知它提供了独特的功能)

类似的问题以及为什么我认为它们不是重复的:

也可以看看

Python has at least six ways of formatting a string:

In [1]: world = "Earth"

# method 1a
In [2]: "Hello, %s" % world
Out[2]: 'Hello, Earth'

# method 1b
In [3]: "Hello, %(planet)s" % {"planet": world}
Out[3]: 'Hello, Earth'

# method 2a
In [4]: "Hello, {0}".format(world)
Out[4]: 'Hello, Earth'

# method 2b
In [5]: "Hello, {planet}".format(planet=world)
Out[5]: 'Hello, Earth'

# method 2c
In [6]: f"Hello, {world}"
Out[6]: 'Hello, Earth'

In [7]: from string import Template

# method 3
In [8]: Template("Hello, $planet").substitute(planet=world)
Out[8]: 'Hello, Earth'

A brief history of the different methods:

  • printf-style formatting has been around since Pythons infancy
  • The Template class was introduced in Python 2.4
  • The format method was introduced in Python 2.6
  • f-strings were introduced in Python 3.6

My questions are:

  • Is printf-style formatting deprecated or going to be deprecated?
  • In the Template class, is the substitute method deprecated or going to be deprecated? (I’m not talking about safe_substitute, which as I understand it offers unique capabilities)

Similar questions and why I think they’re not duplicates:

  • Python string formatting: % vs. .format — treats only methods 1 and 2, and asks which one is better; my question is explicitly about deprecation in the light of the Zen of Python

  • String formatting options: pros and cons — treats only methods 1a and 1b in the question, 1 and 2 in the answer, and also nothing about deprecation

  • advanced string formatting vs template strings — mostly about methods 1 and 3, and doesn’t address deprecation

  • String formatting expressions (Python) — answer mentions that the original ‘%’ approach is planned to be deprecated. But what’s the difference between planned to be deprecated, pending deprecation and actual deprecation? And the printf-style method doesn’t raise even a PendingDeprecationWarning, so is this really going to be deprecated? This post is also quite old, so the information may be outdated.

See also


回答 0

尽管在文档中有各种各样的迹象表明,.formatf字符串优于%字符串,但尚无可行的方案来弃用后者。

在提交的问题#14123中:明确提及旧样式%字符串格式有一些警告,但不会很快消失。,受问题启发,表明目前没有计划弃用printf样式格式,有关%-formatting 的文档已被编辑为包含以下短语:

由于新的字符串格式语法更加灵活并且可以自然地处理元组和字典,因此建议将其用于新代码。但是,目前没有废弃过printf样式格式的计划

(强调我的。)

此短语稍后在commit Close#4966中删除:修改序列文档,以更好地解释现代Python的状态。这看起来似乎是一个迹象,表明不再支持%格式化的计划已经重新出现在卡上了……但是,深入研究Bug跟踪程序后,发现其意图恰恰相反。在错误跟踪器上,提交的作者描述了更改的特征,如下所示

  • 更改了描述printf样式格式与str.format方法之间关系的散文(故意消除了前者可能会消失的真正危险的暗示-认真考虑将其销毁是不切实际的)

换句话说,我们对%-formatting文档进行了两次连续更改,旨在明确强调不会被弃用,更不用说删除了。这些文档仍然对不同类型的字符串格式的相对优点持保留意见,但他们也清楚%格式不会被弃用或删除。

更重要的是,该段落的最新更改是在2017年3月,对此进行了更改…

此处描述的格式化操作表现出各种古怪,这些古怪会导致许多常见错误(例如无法正确显示元组和字典)。使用较新的格式化字符串文字或str.format接口有助于避免这些错误。这些替代方法还提供了更强大,灵活和可扩展的文本格式设置方法。

…对此:

此处描述的格式化操作表现出各种古怪,这些古怪会导致许多常见错误(例如无法正确显示元组和字典)。使用更新的格式化字符串文字,str.format接口或模板字符串可能有助于避免这些错误。这些选择中的每一个都提供了自己的权衡,并带来了简单性,灵活性和/或可扩展性的好处。

请注意,从“避免使用帮助”到“可以避免使用”的变化,以及关于.formatf和弦的清晰建议如何被蓬松,模棱两可的散文所取代,有关每种样式如何“提供自己的取舍和好处”。也就是说,不仅不再正式弃用卡片,而且当前的文档公开承认%格式至少比其他方法具有一些“好处”。

从这一切中我可以推断出,弃用或删除%格式的运动不仅步履蹒跚,而且被彻底永久地击败。

While there are various indications in the docs that .format and f-strings are superior to % strings, there’s no surviving plan to ever deprecate the latter.

In commit Issue #14123: Explicitly mention that old style % string formatting has caveats but is not going away any time soon., inspired by issue Indicate that there are no current plans to deprecate printf-style formatting, the docs on %-formatting were edited to contain this phrase:

As the new string-formatting syntax is more flexible and handles tuples and dictionaries naturally, it is recommended for new code. However, there are no current plans to deprecate printf-style formatting.

(Emphasis mine.)

This phrase was removed later, in commit Close #4966: revamp the sequence docs in order to better explain the state of modern Python. This might seem like a sign that a plan to deprecate % formatting was back on the cards… but diving into the bug tracker reveals that the intent was the opposite. On the bug tracker, the author of the commit characterises the change like this:

  • changed the prose that describes the relationship between printf-style formatting and the str.format method (deliberately removing the implication that the former is any real danger of disappearing – it’s simply not practical for us to seriously contemplate killing it off)

In other words, we’ve had two consecutive changes to the %-formatting docs intended to explicitly emphasise that it will not be deprecated, let alone removed. The docs remain opinionated on the relative merits of different kinds of string formatting, but they’re also clear the %-formatting isn’t going to get deprecated or removed.

What’s more, the most recent change to that paragraph, in March 2017, changed it from this…

The formatting operations described here exhibit a variety of quirks that lead to a number of common errors (such as failing to display tuples and dictionaries correctly). Using the newer formatted string literals or the str.format interface helps avoid these errors. These alternatives also provide more powerful, flexible and extensible approaches to formatting text.

… to this:

The formatting operations described here exhibit a variety of quirks that lead to a number of common errors (such as failing to display tuples and dictionaries correctly). Using the newer formatted string literals, the str.format interface, or template strings may help avoid these errors. Each of these alternatives provides their own trade-offs and benefits of simplicity, flexibility, and/or extensibility.

Notice the change from “helps avoid” to “may help avoid”, and how the clear recommendation of .format and f-strings has been replaced by fluffy, equivocal prose about how each style “provides their own trade-offs and benefits”. That is, not only is a formal deprecation no longer on the cards, but the current docs are openly acknowledging that % formatting at least has some “benefits” over the other approaches.

I’d infer from all this that the movement to deprecate or remove % formatting has not only faltered, but been defeated thoroughly and permanently.


回答 1

.format()方法旨在替换旧的%格式语法。后者已经不再强调,(但没有正式弃用尚未)。方法文档指出:

字符串格式化的这种方法是在Python 3的新标准,并应首选%格式化中所描述的字符串的格式化操作在新的代码。

(强调我的)。

为了保持向后兼容性,并让您更容易过渡,旧格式已经被留在原地现在。根据最初的PEP 3101提案

向后兼容

可以通过保留现有机制来保持向后兼容性。新系统不会与现有字符串格式化技术的任何方法名称发生冲突,因此这两个系统可以共存,直到需要弃用旧系统为止。

请注意,直到该淘汰旧系统为止;它尚未被弃用,但是只要您编写新代码,就将使用新系统

新系统的一个优点是您可以结合使用旧%格式化程序的元组和字典方法:

"{greeting}, {0}".format(world, greeting='Hello')

并可以通过 object.__format__()用于处理各个值格式钩子进行。

请注意,旧系统具有%Template类,后者允许您创建添加或更改其行为的子类。新型系统具有Formatter一流填充相同细分市场。

Python 3进一步远离了弃用,而是在printf-style String Formatting部分中给您警告:

注意:此处描述的格式化操作表现出各种古怪,导致许多常见错误(例如未能正确显示元组和字典)。使用较新的格式化字符串文字str.format()接口有助于避免这些错误。这些替代方法还提供了更强大,灵活和可扩展的文本格式设置方法。

Python 3.6还添加了格式化的字符串文字,将表达式内联格式字符串中。这些是使用内插值创建字符串的最快方法,应使用它,而不是str.format()在可以使用文字的任何地方。

The new .format() method is meant to replace the old % formatting syntax. The latter has been de-emphasised, (but not officially deprecated yet). The method documentation states as much:

This method of string formatting is the new standard in Python 3, and should be preferred to the % formatting described in String Formatting Operations in new code.

(Emphasis mine).

To maintain backwards compatibility and to make transition easier, the old format has been left in place for now. From the original PEP 3101 proposal:

Backwards Compatibility

Backwards compatibility can be maintained by leaving the existing mechanisms in place. The new system does not collide with any of the method names of the existing string formatting techniques, so both systems can co-exist until it comes time to deprecate the older system.

Note the until it comes time to deprecate the older system; it hasn’t been deprecated, but the new system is to be used whenever you write new code.

The new system has as an advantage that you can combine the tuple and dictionary approach of the old % formatter:

"{greeting}, {0}".format(world, greeting='Hello')

and is extensible through the object.__format__() hook used to handle formatting of individual values.

Note that the old system had % and the Template class, where the latter allows you to create subclasses that add or alter its behaviour. The new-style system has the Formatter class to fill the same niche.

Python 3 has further stepped away from deprecation, instead giving you warning in the printf-style String Formatting section:

Note: The formatting operations described here exhibit a variety of quirks that lead to a number of common errors (such as failing to display tuples and dictionaries correctly). Using the newer formatted string literals or the str.format() interface helps avoid these errors. These alternatives also provide more powerful, flexible and extensible approaches to formatting text.

Python 3.6 also added formatted string literals, which in-line the expressions into the format strings. These are the fastest method of creating strings with interpolated values, and should be used instead of str.format() wherever you can use a literal.


回答 2

%尽管有其他答案,但不建议使用字符串格式的运算符,并且不会删除该运算符。
每次在Python开发列表中提出该主题时,都会有一个关于哪个更好的参数,但是对于是否要删除经典方法却没有争议-它会一直存在。尽管在PEP 3101上有说明,但Python 3.1来了又去了,%格式化仍然存在。

保持经典风格的说法很明确:它很简单,很快,可以快速完成简短的事情。使用该.format方法并不总是那么容易理解-几乎没有人-即使在核心开发人员中,也可以使用所提供的完整语法,.format而无需查看参考资料甚至在2009年,就有这样的消息:http:// mail。 python.org/pipermail/python-dev/2009-October/092529.html 几乎没有出现该主题。

2016年更新

在当前的Python开发版本(将成为Python 3.6)中,有第三种字符串内插方法,如PEP-0498所述。它定义了一个新的报价前缀f""(除了当前的u""b""r"")。

给字符串加上前缀f将在运行时在字符串对象上调用一个方法,该方法将自动将当前作用域中的变量插入到字符串中:

>>> value = 80
>>> f'The value is {value}.'
'The value is 80.'

The % operator for string formatting is not deprecated, and is not going to be removed – despite the other answers.
Every time the subject is raised on Python development list, there is strong controversy on which is better, but no controversy on whether to remove the classic way – it will stay. Despite being denoted on PEP 3101, Python 3.1 had come and gone, and % formatting is still around.

The statements for the keeping classic style are clear: it is simple, it is fast, it is quick to do for short things. Using the .format method is not always more readable – and barely anyone – even among the core developers, can use the full syntax provided by .format without having to look at the reference Even back in 2009, one had messages like this: http://mail.python.org/pipermail/python-dev/2009-October/092529.html – the subject had barely showed up in the lists since.

2016 update

In current Python development version (which will become Python 3.6) there is a third method of string interpolation, described on PEP-0498. It defines a new quote prefix f"" (besides the current u"", b"" and r"").

Prefixing a string by f will call a method on the string object at runtime, which will automatically interpolate variables from the current scope into the string:

>>> value = 80
>>> f'The value is {value}.'
'The value is 80.'

回答 3

圭多对此的最新立场似乎在这里指出:

Python 3.0的新增功能

PEP 3101:字符串格式化的新方法

用于内置字符串格式化操作的新系统取代了%字符串格式化运算符。(但是,仍然支持%运算符;它将在Python 3.1中弃用,并在以后的某个时间从语言中删除。)有关完整说明,请阅读PEP 3101。

PEP3101本身,它有最后的修改可以追溯到(周五,2011年9月30日),这样的晚的,一个没有进步,我想。

Guido’s latest position on this seems to be indicated here:

What’s New In Python 3.0

PEP 3101: A New Approach To String Formatting

A new system for built-in string formatting operations replaces the % string formatting operator. (However, the % operator is still supported; it will be deprecated in Python 3.1 and removed from the language at some later time.) Read PEP 3101 for the full scoop.

And the PEP3101 itself, which has the last modified dating back to (Fri, 30 Sep 2011), so no progress as of late on that one, I suppose.


回答 4

在查看较旧的Python文档和PEP 3101时,有一条语句表示将来将不推荐使用%运算符并将其从该语言中删除。在下面的语句是在Python文档的Python 3.0,3.1和3.2:

由于str.format()很新,因此许多Python代码仍然使用%运算符。但是,由于最终会从该语言中删除这种旧的格式设置样式,因此通常应使用str.format()。

如果转到Python 3.3和3.4文档中的同一部分,您将看到该语句已被删除。我也无法在文档中的任何其他地方找到任何其他声明,表明该运算符将不推荐使用或从该语言中删除。还需要注意的是,PEP3101两年半没有进行过修改(2011年9月30日,星期五)。

更新资料

PEP461接受将%格式添加到字节和字节数组中,并且应该是Python 3.5或3.6的一部分。这是%运算符还活着而且在踢的另一个迹象。

Looking at the older Python docs and PEP 3101 there was a statement that the % operator will be deprecated and removed from the language in the future. The following statement was in the Python docs for Python 3.0, 3.1, and 3.2:

Since str.format() is quite new, a lot of Python code still uses the % operator. However, because this old style of formatting will eventually be removed from the language, str.format() should generally be used.

If you go to the same section in Python 3.3 and 3.4 docs, you will see that statement has been removed. I also cannot find any other statement anywhere else in the documentation indicating that the operator will be deprecated or removed from the language. It’s also important to note that PEP3101 has not been modified in over two and a half years (Fri, 30 Sep 2011).

Update

PEP461 Adding % formatting to bytes and bytearray is accepted and should be part of Python 3.5 or 3.6. It’s another sign that the % operator is alive and kicking.