util.py

#!/usr/bin/env python
#
# Copyright 2010 Google Inc.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
#

"""Common utility library."""

from __future__ import with_statement
import six

__author__ = ['rafek@google.com (Rafe Kaplan)',
              'guido@google.com (Guido van Rossum)',
]

import cgi
import datetime
import inspect
import os
import re
import sys

__all__ = ['AcceptItem',
           'AcceptError',
           'Error',
           'choose_content_type',
           'decode_datetime',
           'get_package_for_module',
           'pad_string',
           'parse_accept_header',
           'positional',
           'PROTORPC_PROJECT_URL',
           'TimeZoneOffset',
           'total_seconds',
]


class Error(Exception):
  """Base class for protorpc exceptions."""


class AcceptError(Error):
  """Raised when there is an error parsing the accept header."""


PROTORPC_PROJECT_URL = 'http://code.google.com/p/google-protorpc'

_TIME_ZONE_RE_STRING = r"""
  # Examples:
  #   +01:00
  #   -05:30
  #   Z12:00
  ((?P<z>Z) | (?P<sign>[-+])
   (?P<hours>\d\d) :
   (?P<minutes>\d\d))$
"""
_TIME_ZONE_RE = re.compile(_TIME_ZONE_RE_STRING, re.IGNORECASE | re.VERBOSE)


def pad_string(string):
  """Pad a string for safe HTTP error responses.

  Prevents Internet Explorer from displaying their own error messages
  when sent as the content of error responses.

  Args:
    string: A string.

  Returns:
    Formatted string left justified within a 512 byte field.
  """
  return string.ljust(512)


def positional(max_positional_args):
  """A decorator to declare that only the first N arguments may be positional.

  This decorator makes it easy to support Python 3 style keyword-only
  parameters. For example, in Python 3 it is possible to write:

    def fn(pos1, *, kwonly1=None, kwonly1=None):
      ...

  All named parameters after * must be a keyword:

    fn(10, 'kw1', 'kw2')  # Raises exception.
    fn(10, kwonly1='kw1')  # Ok.

  Example:
    To define a function like above, do:

      @positional(1)
      def fn(pos1, kwonly1=None, kwonly2=None):
        ...

    If no default value is provided to a keyword argument, it becomes a required
    keyword argument:

      @positional(0)
      def fn(required_kw):
        ...

    This must be called with the keyword parameter:

      fn()  # Raises exception.
      fn(10)  # Raises exception.
      fn(required_kw=10)  # Ok.

    When defining instance or class methods always remember to account for
    'self' and 'cls':

      class MyClass(object):

        @positional(2)
        def my_method(self, pos1, kwonly1=None):
          ...

        @classmethod
        @positional(2)
        def my_method(cls, pos1, kwonly1=None):
          ...

    One can omit the argument to 'positional' altogether, and then no
    arguments with default values may be passed positionally. This
    would be equivalent to placing a '*' before the first argument
    with a default value in Python 3. If there are no arguments with
    default values, and no argument is given to 'positional', an error
    is raised.

      @positional
      def fn(arg1, arg2, required_kw1=None, required_kw2=0):
        ...

      fn(1, 3, 5)  # Raises exception.
      fn(1, 3)  # Ok.
      fn(1, 3, required_kw1=5)  # Ok.

  Args:
    max_positional_arguments: Maximum number of positional arguments.  All
      parameters after the this index must be keyword only.

  Returns:
    A decorator that prevents using arguments after max_positional_args from
    being used as positional parameters.

  Raises:
    TypeError if a keyword-only argument is provided as a positional parameter.
    ValueError if no maximum number of arguments is provided and the function
      has no arguments with default values.
  """
  def positional_decorator(wrapped):
    def positional_wrapper(*args, **kwargs):
      if len(args) > max_positional_args:
        plural_s = ''
        if max_positional_args != 1:
          plural_s = 's'
        raise TypeError('%s() takes at most %d positional argument%s '
                        '(%d given)' % (wrapped.__name__,
                                        max_positional_args,
                                        plural_s, len(args)))
      return wrapped(*args, **kwargs)
    return positional_wrapper

  if isinstance(max_positional_args, six.integer_types):
    return positional_decorator
  else:
    args, _, _, defaults = inspect.getargspec(max_positional_args)
    if defaults is None:
      raise ValueError(
          'Functions with no keyword arguments must specify '
          'max_positional_args')
    return positional(len(args) - len(defaults))(max_positional_args)


# TODO(rafek): Support 'level' from the Accept header standard.
class AcceptItem(object):
  """Encapsulate a single entry of an Accept header.

  Parses and extracts relevent values from an Accept header and implements
  a sort order based on the priority of each requested type as defined
  here:

    http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html

  Accept headers are normally a list of comma separated items.  Each item
  has the format of a normal HTTP header.  For example:

    Accept: text/plain, text/html, text/*, */*

  This header means to prefer plain text over HTML, HTML over any other
  kind of text and text over any other kind of supported format.

  This class does not attempt to parse the list of items from the Accept header.
  The constructor expects the unparsed sub header and the index within the
  Accept header that the fragment was found.

  Properties:
    index: The index that this accept item was found in the Accept header.
    main_type: The main type of the content type.
    sub_type: The sub type of the content type.
    q: The q value extracted from the header as a float.  If there is no q
      value, defaults to 1.0.
    values: All header attributes parsed form the sub-header.
    sort_key: A tuple (no_main_type, no_sub_type, q, no_values, index):
        no_main_type: */* has the least priority.
        no_sub_type: Items with no sub-type have less priority.
        q: Items with lower q value have less priority.
        no_values: Items with no values have less priority.
        index: Index of item in accept header is the last priority.
  """

  __CONTENT_TYPE_REGEX = re.compile(r'^([^/]+)/([^/]+)$')

  def __init__(self, accept_header, index):
    """Parse component of an Accept header.

    Args:
      accept_header: Unparsed sub-expression of accept header.
      index: The index that this accept item was found in the Accept header.
    """
    accept_header = accept_header.lower()
    content_type, values = cgi.parse_header(accept_header)
    match = self.__CONTENT_TYPE_REGEX.match(content_type)
    if not match:
      raise AcceptError('Not valid Accept header: %s' % accept_header)
    self.__index = index
    self.__main_type = match.group(1)
    self.__sub_type = match.group(2)
    self.__q = float(values.get('q', 1))
    self.__values = values

    if self.__main_type == '*':
      self.__main_type = None

    if self.__sub_type == '*':
      self.__sub_type = None

    self.__sort_key = (not self.__main_type,
                       not self.__sub_type,
                       -self.__q,
                       not self.__values,
                       self.__index)

  @property
  def index(self):
    return self.__index

  @property
  def main_type(self):
    return self.__main_type

  @property
  def sub_type(self):
    return self.__sub_type

  @property
  def q(self):
    return self.__q

  @property
  def values(self):
    """Copy the dictionary of values parsed from the header fragment."""
    return dict(self.__values)

  @property
  def sort_key(self):
    return self.__sort_key

  def match(self, content_type):
    """Determine if the given accept header matches content type.

    Args:
      content_type: Unparsed content type string.

    Returns:
      True if accept header matches content type, else False.
    """
    content_type, _ = cgi.parse_header(content_type)
    match = self.__CONTENT_TYPE_REGEX.match(content_type.lower())
    if not match:
      return False

    main_type, sub_type = match.group(1), match.group(2)
    if not(main_type and sub_type):
      return False

    return ((self.__main_type is None or self.__main_type == main_type) and
            (self.__sub_type is None or self.__sub_type == sub_type))


  def __cmp__(self, other):
    """Comparison operator based on sort keys."""
    if not isinstance(other, AcceptItem):
      return NotImplemented
    return cmp(self.sort_key, other.sort_key)

  def __str__(self):
    """Rebuilds Accept header."""
    content_type = '%s/%s' % (self.__main_type or '*', self.__sub_type or '*')
    values = self.values

    if values:
      value_strings = ['%s=%s' % (i, v) for i, v in values.items()]
      return '%s; %s' % (content_type, '; '.join(value_strings))
    else:
      return content_type

  def __repr__(self):
    return 'AcceptItem(%r, %d)' % (str(self), self.__index)


def parse_accept_header(accept_header):
  """Parse accept header.

  Args:
    accept_header: Unparsed accept header.  Does not include name of header.

  Returns:
    List of AcceptItem instances sorted according to their priority.
  """
  accept_items = []
  for index, header in enumerate(accept_header.split(',')):
    accept_items.append(AcceptItem(header, index))
  return sorted(accept_items)


def choose_content_type(accept_header, supported_types):
  """Choose most appropriate supported type based on what client accepts.

  Args:
    accept_header: Unparsed accept header.  Does not include name of header.
    supported_types: List of content-types supported by the server.  The index
      of the supported types determines which supported type is prefered by
      the server should the accept header match more than one at the same
      priority.

  Returns:
    The preferred supported type if the accept header matches any, else None.
  """
  for accept_item in parse_accept_header(accept_header):
    for supported_type in supported_types:
      if accept_item.match(supported_type):
        return supported_type
  return None


@positional(1)
def get_package_for_module(module):
  """Get package name for a module.

  Helper calculates the package name of a module.

  Args:
    module: Module to get name for.  If module is a string, try to find
      module in sys.modules.

  Returns:
    If module contains 'package' attribute, uses that as package name.
    Else, if module is not the '__main__' module, the module __name__.
    Else, the base name of the module file name.  Else None.
  """
  if isinstance(module, six.string_types):
    try:
      module = sys.modules[module]
    except KeyError:
      return None

  try:
    return six.text_type(module.package)
  except AttributeError:
    if module.__name__ == '__main__':
      try:
        file_name = module.__file__
      except AttributeError:
        pass
      else:
        base_name = os.path.basename(file_name)
        split_name = os.path.splitext(base_name)
        if len(split_name) == 1:
          return six.text_type(base_name)
        else:
          return u'.'.join(split_name[:-1])

    return six.text_type(module.__name__)


def total_seconds(offset):
  """Backport of offset.total_seconds() from python 2.7+."""
  seconds = offset.days * 24 * 60 * 60 + offset.seconds
  microseconds = seconds * 10**6 + offset.microseconds
  return microseconds / (10**6 * 1.0)


class TimeZoneOffset(datetime.tzinfo):
  """Time zone information as encoded/decoded for DateTimeFields."""

  def __init__(self, offset):
    """Initialize a time zone offset.

    Args:
      offset: Integer or timedelta time zone offset, in minutes from UTC.  This
        can be negative.
    """
    super(TimeZoneOffset, self).__init__()
    if isinstance(offset, datetime.timedelta):
      offset = total_seconds(offset) / 60
    self.__offset = offset

  def utcoffset(self, dt):
    """Get the a timedelta with the time zone's offset from UTC.

    Returns:
      The time zone offset from UTC, as a timedelta.
    """
    return datetime.timedelta(minutes=self.__offset)

  def dst(self, dt):
    """Get the daylight savings time offset.

    The formats that ProtoRPC uses to encode/decode time zone information don't
    contain any information about daylight savings time.  So this always
    returns a timedelta of 0.

    Returns:
      A timedelta of 0.
    """
    return datetime.timedelta(0)


def decode_datetime(encoded_datetime):
  """Decode a DateTimeField parameter from a string to a python datetime.

  Args:
    encoded_datetime: A string in RFC 3339 format.

  Returns:
    A datetime object with the date and time specified in encoded_datetime.

  Raises:
    ValueError: If the string is not in a recognized format.
  """
  # Check if the string includes a time zone offset.  Break out the
  # part that doesn't include time zone info.  Convert to uppercase
  # because all our comparisons should be case-insensitive.
  time_zone_match = _TIME_ZONE_RE.search(encoded_datetime)
  if time_zone_match:
    time_string = encoded_datetime[:time_zone_match.start(1)].upper()
  else:
    time_string = encoded_datetime.upper()

  if '.' in time_string:
    format_string = '%Y-%m-%dT%H:%M:%S.%f'
  else:
    format_string = '%Y-%m-%dT%H:%M:%S'

  decoded_datetime = datetime.datetime.strptime(time_string, format_string)

  if not time_zone_match:
    return decoded_datetime

  # Time zone info was included in the parameter.  Add a tzinfo
  # object to the datetime.  Datetimes can't be changed after they're
  # created, so we'll need to create a new one.
  if time_zone_match.group('z'):
    offset_minutes = 0
  else:
    sign = time_zone_match.group('sign')
    hours, minutes = [int(value) for value in
                      time_zone_match.group('hours', 'minutes')]
    offset_minutes = hours * 60 + minutes
    if sign == '-':
      offset_minutes *= -1

  return datetime.datetime(decoded_datetime.year,
                           decoded_datetime.month,
                           decoded_datetime.day,
                           decoded_datetime.hour,
                           decoded_datetime.minute,
                           decoded_datetime.second,
                           decoded_datetime.microsecond,
                           TimeZoneOffset(offset_minutes))