import contextlib
import os
import sys
import random
import string

class ImportKiller(object):
    "Context manager to make an import of a given name or names fail."
    def __init__(self, *names):
        self.names = names
    def find_module(self, fullname, path=None):
        if fullname in self.names:
            return self
    def load_module(self, fullname):
        assert fullname in self.names
        raise ImportError(fullname)
    def __enter__(self):
        self.original = {}
        for name in self.names:
            self.original[name] = sys.modules.pop(name, None)
        sys.meta_path.insert(0, self)
    def __exit__(self, *args):
        sys.meta_path.remove(self)
        for key, value in self.original.items():
            if value is not None:
                sys.modules[key] = value

@contextlib.contextmanager
def NoNoneDictMutator(destination, **changes):
    """Helper context manager to make and unmake changes to a dict.

    A None is not a valid value for the destination, and so means that the
    associated name should be removed."""
    original = {}
    for key, value in changes.items():
        original[key] = destination.get(key)
        if value is None:
            if key in destination:
                del destination[key]
        else:
            destination[key] = value
    yield
    for key, value in original.items():
        if value is None:
            if key in destination:
                del destination[key]
        else:
            destination[key] = value


def Environ(**changes):
    """A context manager to temporarily change the os.environ"""
    return NoNoneDictMutator(os.environ, **changes)

ALPHABET = string.ascii_letters + string.digits

def random_string(k, source = ALPHABET):
    """Generate a random string with length <i>k</i>
    """
    result = ''
    for i in range(0, k):
        result += random.choice(source)
    return result
