python 中的print 是语句还是函数

在python 2.5 系列中，最简单的print语句，总是让初学者疑惑？到底是函数还是一个语句（语句的意思是和 if for等是一个级别的）。

 

看起来 print "hellow " 和 print("hellow") 都是正确的。

但是执行 type(print) 时确得到了一个异常，说明print 是语句而不是函数。

 

接下来让我们看看在 2.6版本里面。

同样得到 和 2.5版本一样的结果。但是从2.6里面 增加了一个新方法。

执行 

 

from
 __future__
 import
 print_function

 

那么此时你执行 type(print) 命令，就会发现 print是个函数了。原来的print "hellow"就不能使用了。

 

2.6的这个做法大概是为了3000吧。到了3000里面 print 就只能是函数了。避免歧义。

 

 

评论

1 楼 supermarshal 2009-03-19  

文档里有
6.6 The print statement

print_stmt ::= "print" ([expression ("," expression)* [","]
  | ">>" expression [("," expression)+ [","])

Download entire grammar as text.
print evaluates each expression in turn and writes the resulting object to standard output (see below). If an object is not a string, it is first converted to a string using the rules for string conversions. The (resulting or original) string is then written. A space is written before each object is (converted and) written, unless the output system believes it is positioned at the beginning of a line. This is the case (1) when no characters have yet been written to standard output, (2) when the last character written to standard output is "\n", or (3) when the last write operation on standard output was not a print statement. (In some cases it may be functional to write an empty string to standard output for this reason.) Note: Objects which act like file objects but which are not the built-in file objects often do not properly emulate this aspect of the file object's behavior, so it is best not to rely on this.

A "\n" character is written at the end, unless the print statement ends with a comma. This is the only action if the statement contains just the keyword print.

Standard output is defined as the file object named stdout in the built-in module sys. If no such object exists, or if it does not have a write() method, a RuntimeError exception is raised.

print also has an extended form, defined by the second portion of the syntax described above. 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 a write() method as described above. With this extended form, the subsequent expressions are printed to this file object. If the first expression evaluates to None, then sys.stdout is used as the file for output.




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