Syntax warning from __init__.py: assert file != "" is not None

[juliangilbey]

Pweave/pweave/__init__.py

Line 47 in 45b56ec

assert file != "" is not None, "No input specified"

raises the followng warning:

SyntaxWarning: "is not" with a literal. Did you mean "!="?
  assert file != "" is not None, "No input specified"

The code as written means: assert (file != "") and ("" is not None), which is clearly not what is wanted; perhaps it should be assert (file != "") and (file is not None) instead?