Check source code with pyflakes
    Returns an empty list if pyflakes is not installed

def check_with_pyflakes(source_code, filename=None):
    """Check source code with pyflakes
    Returns an empty list if pyflakes is not installed"""
    try:
        if filename is None:
            filename = '<string>'
        try:
            source_code += '\n'
        except TypeError:
            # Python 3
            source_code += to_binary_string('\n')
            
        import _ast
        from pyflakes.checker import Checker
        # First, compile into an AST and handle syntax errors.
        try:
            tree = compile(source_code, filename, "exec", _ast.PyCF_ONLY_AST)
        except SyntaxError as value:
            # If there's an encoding problem with the file, the text is None.
            if value.text is None:
                results = []
            else:
                results = [(value.args[0], value.lineno)]
        except (ValueError, TypeError):
            # Example of ValueError: file contains invalid \x escape character
            # (see https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=674797)
            # Example of TypeError: file contains null character
            # (see https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=674796)
            results = []
        else:
            # Okay, it's syntactically valid.  Now check it.
            w = Checker(tree, filename)
            w.messages.sort(key=lambda x: x.lineno)
            results = []
            coding = encoding.get_coding(source_code)
            lines = source_code.splitlines()
            for warning in w.messages:
                if 'analysis:ignore' not in \
                   to_text_string(lines[warning.lineno-1], coding):
                    results.append((warning.message % warning.message_args,
                                    warning.lineno))
    except Exception:
        # Never return None to avoid lock in spyder/widgets/editor.py
        # See Issue 1547
        results = []
        if DEBUG_EDITOR:
            traceback.print_exc()  # Print exception in internal console
    return results