问题
I understand the difference between a statement and an expression, and I understand that Python3 turned print() into a function.
However I ran a print() statement surrounded with parenthesis on various Python2.x interpreters and it ran flawlessly, I didn't even have to import any module.
My question: Is the following code print("Hello SO!") evaluated as a statement or an expression in Python2.x?
回答1:
Consider the following expressions:
a = ("Hello SO!")
a = "Hello SO!"
They're equivalent. In the same way, with a statement:
statement_keyword("foo")
statement_keyword "foo"
are also equivalent.
Notice that if you change your print function to:
print("Hello","SO!")
You'll notice a difference between python 2 and python 3. With python 2, the (...,...) is interpteted as a tuple since print is a statement whereas in python 3, it's a function call with multiple arguments.
Therefore, to answer the question at hand, print is evaluated as a statement in python 2.x unless you from __future__ import print_function (introduced in python 2.6)
回答2:
print("Hello SO!") is evaluated as the statement print ("Hello SO!"), where the argument to the print statement is the expression ("Hello SO!").
This can make a difference if you are printing more than one value; for example print("Hello", "world") will print the 2-element tuple ('Hello', 'world') instead of the two strings "Hello" and "world".
For compatibility with Python 3 use from __future__ import print_function:
>>> print("Hello", "world")
('Hello', 'world')
>>> from __future__ import print_function
>>> print("Hello", "world")
Hello world
回答3:
It is still evaluated as a statement, you are simply printing ("Hello SO!"), which simply evaluates to "Hello SO!" since it is not a tuple (as mentioned by delnan).
来源：https://stackoverflow.com/questions/12162629/using-print-the-function-version-in-python2-x