与python 2 print语句相比，python 3.x中新的print函数有什么优势？


What is the advantage of the new print function in Python 3.x over the Python 2 print statement?

我听过很多次，打印是一个函数(3.x)比它是一个语句(2.x)要好。但是为什么呢？

我非常喜欢它是一个语句，主要是因为它允许我少键入两个字符(即括号)。

我有兴趣看到一些情况下，打印语句只是不削减它，一个函数是优越的。

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Rationale

The print statement has long appeared on lists of dubious language features that are to be removed in Python 3000, such as Guido's"Python Regrets" presentation [1]. As such, the objective of this PEP is not new, though it might become much disputed among Python developers.

The following arguments for a print() function are distilled from a python-3000 message by Guido himself [2]:

  • print is the only application-level functionality that has a statement dedicated to it. Within Python's world, syntax is generally used as a last resort, when something can't be done without help from the compiler. Print doesn't qualify for such an exception.
  • At some point in application development one quite often feels the need to replace print output by something more sophisticated, like logging calls or calls into some other I/O library. With a print() function, this is a straightforward string replacement, today it is a mess adding all those parentheses and possibly converting >>stream style syntax.
  • Having special syntax for print puts up a much larger barrier for evolution, e.g. a hypothetical new printf() function is not too far fetched when it will coexist with a print() function.
  • There's no easy way to convert print statements into another call if one needs a different separator, not spaces, or none at all. Also, there's no easy way at all to conveniently print objects with some other separator than a space.
  • If print() is a function, it would be much easier to replace it within one module (just def print(*args):...) or even throughout a program (e.g. by putting a different function in __builtin__.print). As it is, one can do this by writing a class with a write() method and assigning that to sys.stdout – that's not bad, but definitely a much larger conceptual leap, and it works at a different level than print.

— PEP 3105 – Make print a function

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从乔森的回答到斯文的回答，再加上：

您可以在不能使用print的地方使用print()，例如：

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[print(x) for x in range(10)]
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print作为函数的一个优点是一致性。它没有理由成为一个声明。比较这两行

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2.x: print >> my_file, x
3.x: print(x, file=my_file)

新版本看起来更像python，不是吗？

函数版本的另一个优点是灵活性。例如，如果您想捕获所有用于调试目的的print调用，现在只需重新定义print：

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def print(*args, **kwargs):
    # whatever
    __builtins__.print(*args, **kwargs)
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我考虑过这个问题，不知道Python3版本的优点。但当我需要打印pandas.DataFrame列(没有Index([...]))时，我发现

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print *df.columns

引发异常，而

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print(*df.columns)

工作很好！如果您想在多个调用打印中使用相同的(可配置的)打印选项，可以将它们保存到字典中并作为**print_options传递。

所以至少*args和**kw_args技巧是print发挥作用的一个很好的理由！

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您可以将内置的print替换为自定义的：

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import os
import sys

def print(s):
   sys.stderr.write('Will now print ' + str(s) + '.' + os.linesep)
   sys.stdout.write(str(s) + os.linesep)

print(['A', 'list'])
# Output:
# stderr: Will now print ['A', 'list'].
# stdout: ['A', 'list']

您可以在lambda或函数调用等内部使用print：

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example_timeout_function(call=lambda: print('Hello world'), timeout=5)
do_things(print_function=print)