TypeError: unsupported operand type(s) in "print >> ..." statement

Question

When I try to run this code under Python 3.6:

import sys

print >>sys.stderr, 'waiting for a connection'

I get this TypeError:

Traceback (most recent call last):
  File "D:/Users/Chanhc1997/Desktop/test_c.py", line 8, in <module>
    print >>sys.stderr, 'waiting for a connection'
TypeError: unsupported operand type(s) for >>:
'builtin_function_or_method' and 'PseudoOutputFile'.
Did you mean "print(<message>, file=<output_stream>)"?

The code works fine in Python 2. What's going on?

Answer 1

In Python 2, this:

print >>sys.stderr, 'waiting for a connection'

means "print the string 'waiting for a connection' to the file-like object sys.stderr".

In Python 3, print becomes a function rather than a statement, and the syntax to redirect its output looks like this:

print('waiting for a connection', file=sys.stderr)

You get a TypeError (rather than, say, a SyntaxError) in Python 3 because, now that print is a function (and therefore an object), it can be part of an expression … and since >> is an operator, the expression fragment

print >>sys.stderr

is interpreted as "shift the print function right by sys.stderr bits" – which is syntactically valid, but doesn't make any sense for those objects.

If you need to write code which runs under both Python 2 and Python 3, you can import Python 3's behaviour into Python 2:

from __future__ import print_function  # works in Python 2.6 and onwards

print('whatever', file=some_file)

Note that this will disable the ability to treat print as a statement, so you'll have to update any code which uses that behaviour.