[Python] Exit contexts gracefully upon keywords

An unclosed function definition (or similar) will no longer cause the
rest of the file to become unreadable but rather only shows
"invalid.illegal" for the first keyword it encounters, upon which it
exits the context normally.

Python/Python.sublime-syntax

@@ -25,6 +25,7 @@ variables:
   identifier: '\b[[:alpha:]_]{{identifier_continue}}*\b'
   path: '({{identifier}} *\. *)*{{identifier}}'
   sql_indicator: \s*(?:SELECT|INSERT|UPDATE|DELETE|CREATE|REPLACE|ALTER|WITH)\b
+  illegal_names: (?:and|as|assert|break|class|continue|def|del|elif|else|except|finally|for|from|global|if|import|in|is|lambda|not|or|pass|raise|return|try|while|with|yield)
 
 contexts:
   main:
@@ -42,7 +43,7 @@ contexts:
     - include: assignments
     - match: ;
       scope: punctuation.terminator.statement.python
-    - include: expressions
+    - include: line-expressions
 
   inline-statements:
     - include: imports
@@ -209,7 +210,7 @@ contexts:
       set: with-body
     - include: name
 
-  expressions: # Always include this last!
+  expressions-common:
     - include: comments
     - include: constants
     - include: numbers
@@ -223,7 +224,6 @@ contexts:
     - include: function-calls
     - include: item-access
     - include: line-continuation
-    - include: dotted-name
     - match: \(
       scope: punctuation.section.group.begin.python
       push:
@@ -243,6 +243,19 @@ contexts:
       scope: invalid.illegal.stray.brace.curly.python
     - include: lists
     - include: dictionaries-and-sets
+
+  line-expressions: # Always include this last!
+    - include: expressions-common
+    - include: dotted-name
+    # For debugging
+    # - match: \S
+    #   scope: invalid.illegal.unmatched
+
+  expressions: # Always include this last!
+    # Differs from the line scope in that invalid-name matches will pop the current context
+    - include: expressions-common
+    - include: illegal-names-pop
+    - include: dotted-name
     - match: '(\.) *(?={{identifier}})'
       captures:
         1: punctuation.accessor.python
@@ -712,9 +725,14 @@ contexts:
     - match: "{{identifier}}"
 
   illegal-names:
-    - match: \b(and|as|assert|break|class|continue|def|del|elif|else|except|finally|for|from|global|if|import|in|is|lambda|not|or|pass|raise|return|try|while|with|yield)\b
+    - match: \b{{illegal_names}}\b
       scope: invalid.illegal.name.python
 
+  illegal-names-pop:
+    - match: \b{{illegal_names}}\b
+      scope: invalid.illegal.name.python
+      pop: true
+
   keyword-arguments:
     - match: '(?={{identifier}}\s*=(?!=))'
       push:

Python/syntax_test_python.py

@@ -613,3 +613,15 @@ def wrapper(self):
 #   ^ keyword.operator.assignment.python
 foo == bar()
 #   ^^ keyword.operator.comparison.python
+
+
+# Pop contexts gracefully
+def func(unclosed, parameters: if else
+    pass
+#   ^^^^ invalid.illegal.name
+
+# The following function should be matched as normal
+# despite the above definition not being closed correctly
+def another_func():
+#^^ -invalid
+    pass