allow specify context using special header

doc/UltiSnips.txt

@@ -1387,6 +1387,11 @@ In that case snippet should be defined using this syntax: >
 
     snippet tab_trigger "description" "expression" options
 
+Context can be defined using special header using this syntax: >
+
+    context "expression"
+    snippet tab_trigger "description" options
+
 The 'expression' can be any python expression. If 'expression' evaluates to
 'True', then this snippet will be chosen for expansion. The 'expression' must
 be wrapped with double-quotes.

pythonx/UltiSnips/snippet/source/file/_common.py

@@ -19,3 +19,11 @@ def handle_action(head, tail, line_index):
     else:
         return 'error', ("'{}' without specified action".format(head),
             line_index)
+
+
+def handle_context(tail, line_index):
+    if tail:
+        return 'context', tail.strip('"').replace(r'\"', '"')\
+            .replace(r'\\\\', r'\\')
+    else:
+        return 'error', ("'context' without body", line_index)

pythonx/UltiSnips/snippet/source/file/ultisnips.py

@@ -11,7 +11,7 @@ from UltiSnips import _vim
 from UltiSnips.snippet.definition import UltiSnipsSnippetDefinition
 from UltiSnips.snippet.source.file._base import SnippetFileSource
 from UltiSnips.snippet.source.file._common import handle_extends, \
-    handle_action
+    handle_action, handle_context
 from UltiSnips.text import LineIterator, head_tail
 
 
@@ -55,7 +55,7 @@ def find_all_snippet_files(ft):
 
 
 def _handle_snippet_or_global(
-    filename, line, lines, python_globals, priority, pre_expand
+    filename, line, lines, python_globals, priority, pre_expand, context
 ):
     """Parses the snippet that begins at the current line."""
     start_line_index = lines.line_index
@@ -75,8 +75,7 @@ def _handle_snippet_or_global(
             opts = words[-1]
             remain = remain[:-len(opts) - 1].rstrip()
 
-    context = None
+    if 'e' in opts and not context:
-    if 'e' in opts:
         left = remain[:-1].rfind('"')
         if left != -1 and left != 0:
             context, remain = remain[left:].strip('"'), remain[:left]
@@ -134,6 +133,7 @@ def _parse_snippets_file(data, filename):
     lines = LineIterator(data)
     current_priority = 0
     actions = {}
+    context = None
     for line in lines:
         if not line.strip():
             continue
@@ -144,16 +144,22 @@ def _parse_snippets_file(data, filename):
                 filename, line, lines,
                 python_globals,
                 current_priority,
-                actions
+                actions,
+                context
             )
 
             actions = {}
+            context = None
             if snippet is not None:
                 yield snippet
         elif head == 'extends':
             yield handle_extends(tail, lines.line_index)
         elif head == 'clearsnippets':
             yield 'clearsnippets', (current_priority, tail.split())
+        elif head == 'context':
+            head, context, = handle_context(tail, lines.line_index)
+            if head == 'error':
+                yield (head, tail)
         elif head == 'priority':
             try:
                 current_priority = int(tail.split()[0])

syntax/snippets.vim

@@ -155,6 +155,12 @@ syn match snipPriority "^priority\%(\s.*\|$\)" contains=snipPriorityKeyword disp
 syn match snipPriorityKeyword "^priority" contained nextgroup=snipPriorityValue skipwhite display
 syn match snipPriorityValue "-\?\d\+" contained display
 
+" context {{{3
+
+syn match snipContext "^\context\%(\s.*\|$\)" contains=snipContextKeyword display
+syn match snipContextKeyword "^context" contained nextgroup=snipContextValue skipwhite display
+syn match snipContextValue '".*"' contained display
+
 " Actions {{{3
 
 syn match snipAction "^\%(pre_expand\|post_expand\|post_jump\).*$" contains=snipActionKeyword display skipwhite
@@ -212,6 +218,9 @@ hi def link snipTransformationReplace      String
 hi def link snipTransformationEscape       snipEscape
 hi def link snipTransformationOptions      Operator
 
+hi def link snipContextKeyword  Keyword
+hi def link snipContextValue    String
+
 hi def link snipPriorityKeyword  Keyword
 hi def link snipPriorityValue    Number
 

test/test_ContextSnippets.py

@@ -11,7 +11,6 @@ class ContextSnippets_SimpleSnippet(_VimTest):
     keys = 'a' + EX
     wanted = 'abc'
 
-
 class ContextSnippets_ExpandOnTrue(_VimTest):
     files = { 'us/all.snippets': r"""
         global !p
@@ -139,6 +138,7 @@ class ContextSnippets_CursorIsZeroBased(_VimTest):
     keys = "e" + EX
     wanted = "(2, 1)"
 
+
 class ContextSnippets_ContextIsClearedBeforeExpand(_VimTest):
     files = { 'us/all.snippets': r"""
         pre_expand "snip.context = 1 if snip.context is None else 2"
@@ -164,3 +164,35 @@ class ContextSnippets_ContextHasAccessToVisual(_VimTest):
     keys = "123" + ESC + "vhh" + EX + "test" + EX + " zzz" + ESC + \
         "vhh" + EX + "test" + EX
     wanted = "Yes No"
+
+
+class ContextSnippets_Header_ExpandOnTrue(_VimTest):
+    files = { 'us/all.snippets': r"""
+        global !p
+        def check_context():
+            return True
+        endglobal
+
+        context "check_context()"
+        snippet a "desc" e
+        abc
+        endsnippet
+        """}
+    keys = 'a' + EX
+    wanted = 'abc'
+
+
+class ContextSnippets_Header_DoNotExpandOnFalse(_VimTest):
+    files = { 'us/all.snippets': r"""
+        global !p
+        def check_context():
+            return False
+        endglobal
+
+        context "check_context()"
+        snippet a "desc" e
+        abc
+        endsnippet
+        """}
+    keys = 'a' + EX
+    wanted = keys