为什么在 Python 2.7中印刷体中的括号是自愿的？

在 Python 2.7中，下面两个代码将执行相同的操作

print("Hello, World!") # Prints "Hello, World!"


print "Hello, World!" # Prints "Hello, World!"

然而以下不会吗

print("Hello,", "World!") # Prints the tuple: ("Hello,", "World!")


print "Hello,", "World!" # Prints the words "Hello, World!"

在 Python 3.x 中，print上的括号是强制性的，本质上使它成为一个函数，但在2.7中，两者都将处理不同的结果。关于 Python 2.7中的 print，我还应该知道什么？

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最佳答案

In Python 2.x print is actually a special statement and not a function*.

This is also why it can't be used like: lambda x: print x

Note that (expr) does not create a Tuple (it results in expr), but , does. This likely results in the confusion between print (x) and print (x, y) in Python 2.7

(1)   # 1 -- no tuple Mister!
(1,)  # (1,)
(1,2) # (1, 2)
1,2   # 1 2 -- no tuple and no parenthesis :) [See below for print caveat.]

However, since print is a special syntax statement/grammar construct in Python 2.x then, without the parenthesis, it treats the ,'s in a special manner - and does not create a Tuple. This special treatment of the print statement enables it to act differently if there is a trailing , or not.

Happy coding.

*This print behavior in Python 2 can be changed to that of Python 3:

from __future__ import print_function
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Basically in Python before Python 3, print was a special statement that printed all the strings if got as arguments. So print "foo","bar" simply meant "print 'foo' followed by 'bar'". The problem with that was it was tempting to act as if print were a function, and the Python grammar is ambiguous on that, since (a,b) is a tuple containing a and b but foo(a,b) is a call to a function of two arguments.

So they made the incompatible change for 3 to make programs less ambiguous and more regular.

(Actually, I think 2.7 behaves as 2.6 did on this, but I'm not certain.)

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It's all very simple and has nothing to do with forward or backward compatibility.

The general form for the print statement in all Python versions before version 3 is:

print expr1, expr2, ... exprn

(Each expression in turn is evaluated, converted to a string and displayed with a space between them.)

But remember that putting parentheses around an expression is still the same expression.

So you can also write this as:

print (expr1), (expr2), ... (expr3)

This has nothing to do with calling a function.

小开

Here we have interesting side effect when it comes to UTF-8.

>> greek = dict( dog="σκύλος", cat="γάτα" )
>> print greek['dog'], greek['cat']
σκύλος γάτα
>> print (greek['dog'], greek['cat'])
('\xcf\x83\xce\xba\xcf\x8d\xce\xbb\xce\xbf\xcf\x82', '\xce\xb3\xce\xac\xcf\x84\xce\xb1')

The last print is tuple with hexadecimal byte values.


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