覆盖Python的print语句的换行生成行为 [英] Overriding the newline generation behaviour of Python's print statement

问题描述

我有一堆用于编码原始电子邮件的旧代码，其中包含许多打印语句，例如

I have a bunch of legacy code for encoding raw emails that contains a lot of print statements such as

print >>f, "Content-Type: text/plain"

对于电子邮件来说，这一切都很好，但是我们现在利用相同的代码来输出HTTP请求。问题是Python打印语句输出'\n'，而HTTP要求'\r\n'。

This is all well and good for emails, but we're now leveraging the same code for outputting HTTP request. The problem is that the Python print statement outputs '\n' whilst HTTP requires '\r\n'.

看起来Python（至少为2.6.4）会生成尾随 PRINT_NEWLINE 字节代码进行打印语句实现为

It looks like Python (2.6.4 at least) generates a trailing PRINT_NEWLINE byte code for a print statement which is implemented as

ceval.c:1582: err = PyFile_WriteString("\n", w);

因此，似乎没有简单的方法可以覆盖默认的换行行为。我考虑过以下解决方案

Thus it appears there's no easy way to override the default newline behaviour of print. I have considered the following solutions

写完输出后，只需执行 .replace（'\n'，'\r \n'）。这会干扰使用分段编码的HTTP消息。

在目标文件对象周围创建包装器，并代理 .write 方法

After writing the output simply do a .replace('\n', '\r\n'). This will interfere with HTTP messages that use multipart encoding.

Create a wrapper around the destination file object and proxy the .write method

def write(self, data):
    if data == '\n':
        data = '\r\n'
    return self._file.write(data)

编写一个正则表达式，将 print>> f，文本转换为 f.write（text + line_end） ，其中 line_end 可以是'\n'或' lir\n'。

Write a regular expression that translates print >>f, text to f.write(text + line_end) where line_end can be '\n' or '\r\n'.

我相信第三个选择最合适。听说您使用Python方式解决问题会很有趣。

I believe the third option would be the most appropriate. It would be interesting to hear what your Pythonic approach to the problem would be.

推荐答案

您应该现在就永远解决问题通过定义一个新的输出函数。如果打印了一个函数，那会容易得多。

You should solve your problem now and for forever by defining a new output function. Were print a function, this would have been much easier.

我建议编写一个新的输出函数，尽可能地模仿现代打印函数的签名（因为重用了良好的界面是好的），例如：

I suggest writing a new output function, mimicing as much of the modern print function signature as possible (because reusing a good interface is good), for example:

def output(*items, end="\n", file=sys.stdout):
    pass

一旦您替换了所有有问题的打印件，就不再有这个问题-您可以随时更改函数的行为！这是在Python 3中将print设置为函数的一个重要原因-因为在Python 2.x中，所有项目总是经过所有 print 语句的阶段不再灵活，也没有简单的出路。

Once you have replaced all prints in question, you no longer have this problem -- you can always change the behavior of your function instead! This is a big reason why print was made a function in Python 3 -- because in Python 2.x, "all" projects invariably go through the stage where all the print statements are no longer flexible, and there is no easy way out.

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