Python的内置打印为什么/如何允许“>>"操作员? [英] Why/how does Python's print built-in allow the ">>" operator?
问题描述
我到处搜索，但无论是在本网站还是在其他地方都找不到答案(总是很难找到涉及标点符号的主题).
I searched around and couldn't find an answer to this either on this site or elsewhere (always a challenge searching for topics involving punctuation chars).
我在Python标准库(此处)，其中一个示例是(摘录):
I was looking up the StringIO in the Python standard library (here) and one of the examples is this (excerpt):
import StringIO
output = StringIO.StringIO()
output.write('First line.\n')
print >>output, 'Second line.' # <-- This is the line I'm asking about
>>运算符在这里如何或为什么起作用?据我所知(而且我不是Python专家)，这是正确的shift运算符.我以为也许StringIO会覆盖__rshift__之类的东西，但是StringIO的来源并没有出卖任何此类东西.
How or why does the >> operator work here? As far as I can tell (and I'm no Python expert), this is the right shift operator. I thought perhaps StringIO overrides __rshift__ or something, but the source for StringIO does not betray any such thing.
我还没有四处看看如何实现内置的print，但是通过最初的搜索，我无法弄清楚它是如何工作的.有人吗?
I haven't poked around to see how the print built-in is implemented yet, but with initial searching I'm unable to figure out how this works. Anyone?
推荐答案
这实际上是几个问题.
首先，如何"问题:
>>令牌实际上不是此处的运算符；它是print语句语法的一部分，如此处所述.语法是:
The >> token is not actually an operator here; it's part of the syntax of the print statement, as documented here. The grammar is:
print_stmt ::= "print" ([expression ("," expression)* [","]]
| ">>" expression [("," expression)+ [","]])
...的语义是:
这种形式有时也称为印刷人字形".以这种形式，
>>之后的第一个表达式必须计算为文件状"对象，特别是具有如上所述的write()方法的对象.通过这种扩展形式，后续表达式将打印到该文件对象.
This form is sometimes referred to as "print chevron." In this form, the first expression after the
>>must evaluate to a "file-like" object, specifically an object that has awrite()method as described above. With this extended form, the subsequent expressions are printed to this file object.
第二，为什么"问题:
Second, the "why" question:
早期，Python开发人员认为这是一种方便的写入文件的方式.
Early on, the Python developers thought this was a convenient way to write to files.
从那时起，他们添加了 stdout重定向，以及更强大的文件和字符串格式API，因此它不再有用.而且，由于关键字参数等其他现代功能，以及几十年来的思考，他们提出了一种更灵活的方法来设计
Since then, they've added stdout redirection, and more powerful file and string-formatting APIs, so it's not as useful anymore. And, thanks to other modern features like keyword arguments, and a few decades to think about it, they've come up with a more flexible way to design print as a regular function that doesn't require any special syntax at all. But removing it would break backward compatibility, so it couldn't be removed until 3.0.
并且在3.0中将其删除；您仍然看到它，因为您使用的是旧版本.
And it was removed in 3.0; you're only still seeing it because you're using an older version.
If you want the new print function in 2.7, however, you can use a future statement: from __future__ import print_function. But that will of course break print >>foo, spam; you'll have to rewrite it as print(spam, file=foo).
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