Why is parenthesis in print voluntary in Python 2.

问题:

In Python 2.7 both the following will do the same

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

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

However the following will not

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

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

In Python 3.x parenthesis on print is mandatory, essentially making it a function, but in 2.7 both will work with differing results. What else should I know about print in Python 2.7?

回答1:

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

回答2:

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.

回答3:

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.

回答4:

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.)