1""" 

2The lazy loader is a replacement for the ``load`` template tag that allows template tags to be conditionally loaded and 

3used. It is inspired by `django-friendly-tag-loader`_, but is intentionally much less robust. 

4 

5.. _django-friendly-tag-loader: https://github.com/ataylor32/django-friendly-tag-loader 

6 

7See also: https://stackoverflow.com/q/7835155/241720 

8 

9Generic templates and templates provided by contrib apps utilize lazy loader to conditionally load and snippet tags. 

10 

11""" 

12from django.template.base import TokenType 

13from django.template.defaulttags import find_library, CommentNode, IfNode, LoadNode 

14from django.template.library import Library, TemplateSyntaxError 

15from django.template.smartif import Literal 

16import logging 

17 

18TOKEN_BLOCK = TokenType.BLOCK 

19 

20log = logging.getLogger(__name__) 

21register = Library() 

22 

23 

24@register.tag 

25def lazy_load(parser, token): 

26 """Load template tag library if it exists. 

27 

28 .. code-block:: 

29 

30 {% load lazy_loader %} 

31 {% lazy_load support_tags %} 

32 

33 """ 

34 library_name = token.split_contents()[-1] 

35 

36 try: 

37 library = find_library(parser, library_name) 

38 parser.add_library(library) 

39 except TemplateSyntaxError as e: 

40 log.debug("Lazy load failed: %s" % e) 

41 pass 

42 

43 return LoadNode() 

44 

45 

46@register.tag 

47def if_tag_exists(parser, token): 

48 """Determine if a given tag exists before calling it. 

49 

50 .. code-block:: 

51 

52 {% if_tag_exists sidebar_items %} 

53 {% sidebar_items page=page %} 

54 {% else %} 

55 No sidebar for you! 

56 {% endif_tag_exists %} 

57 

58 """ 

59 bits = list(token.split_contents()) 

60 if len(bits) < 2: 

61 raise TemplateSyntaxError("%r requires an argument." % bits[0]) 

62 

63 end_tag = "end%s" % bits[0] 

64 has_tag = all([tag in parser.tags for tag in bits[1:]]) 

65 nodelist_true = nodelist_false = CommentNode() 

66 if has_tag: 

67 nodelist_true = parser.parse(("else", end_tag)) 

68 token = parser.next_token() 

69 if token.contents == "else": 

70 parser.skip_past(end_tag) 

71 else: 

72 while parser.tokens: 

73 token = parser.next_token() 

74 if token.token_type == TOKEN_BLOCK and token.contents == end_tag: 

75 return IfNode([ 

76 (Literal(has_tag), nodelist_true), 

77 (None, nodelist_false) 

78 ]) 

79 elif token.token_type == TOKEN_BLOCK and token.contents == "else": 

80 break 

81 else: 

82 pass 

83 

84 nodelist_false = parser.parse((end_tag,)) 

85 parser.next_token() 

86 

87 return IfNode([ 

88 (Literal(has_tag), nodelist_true), 

89 (None, nodelist_false) 

90 ])