"""Updates all source files to import the same set of __future__ directives.
"""
from lib2to3 import fixer_base
from lib2to3 import pytree
from lib2to3.pgen2 import token
from lib2to3.fixer_util import FromImport, Name, Comma, Newline

# copied from fix_tuple_params.py
def is_docstring(stmt):
    return isinstance(stmt, pytree.Node) and \
           stmt.children[0].type == token.STRING

class FixFutureImports(fixer_base.BaseFix):
    BM_compatible = True

    PATTERN = """import_from< 'from' module_name="__future__" 'import' any >"""

    def start_tree(self, tree, filename):
        self.found_future_import = False

    def new_future_import(self, old):
        new = FromImport("__future__",
                         [Name("absolute_import", prefix=" "), Comma(),
                          Name("division", prefix=" "), Comma(),
                          Name("with_statement", prefix=" ")])
        if old is not None:
            new.prefix = old.prefix
        return new
       
    def transform(self, node, results):
        self.found_future_import = True
        return self.new_future_import(node)

    def finish_tree(self, tree, filename):
        if self.found_future_import:
            return
        if not isinstance(tree, pytree.Node):
            # Empty files (usually __init__.py) show up as a single Leaf
            # instead of a Node, so leave them alone
            return
        first_stmt = tree.children[0]
        if is_docstring(first_stmt):
            # Skip a line and add the import after the docstring
            tree.insert_child(1, Newline())
            pos = 2
        elif first_stmt.prefix:
            # No docstring, but an initial comment (perhaps a #! line).
            # Transfer the initial comment to a new blank line.
            newline = Newline()
            newline.prefix = first_stmt.prefix
            first_stmt.prefix = ""
            tree.insert_child(0, newline)
            pos = 1
        else:
            # No comments or docstring, just insert at the start
            pos = 0
        tree.insert_child(pos, self.new_future_import(None))
        tree.insert_child(pos+1, Newline())  # terminates the import stmt