invalid Syntax Error in python print statement using File

It seems as if you are trying to use Python 3.x's print function in Python 2.x. In order to do so, you need to first import print_function from __future__:

Place the following line at the very top of your source file, after any comments and/or docstrings:

from __future__ import print_function

Below is a demonstration:

>>> # Python 2.x interpreter session
...
>>> print('a', 'b', sep=',')
  File "<stdin>", line 1
    print('a', 'b', sep=',')
                       ^
SyntaxError: invalid syntax
>>>

>>> # Another Python 2.x interpreter session
...
>>> from __future__ import print_function
>>> print('a', 'b', sep=',')
a,b
>>>

It looks like you are trying to use the Python 3 print syntax in Python 2.

Either use a Python 3 interpreter, or rewrite the print like so:

print >>sys.stderr, 'Group output sizes: |A| = {}, |B| = {}'.format(len(A),len(B))