Why is parenthesis in print voluntary in Python 2.7?

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.

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

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.