add support for get_user_model(), fixes #16

mypy_django_plugin/helpers.py

@@ -1,7 +1,9 @@
 import typing
 from typing import Dict, Optional
 
-from mypy.nodes import Expression, ImportedName, MypyFile, NameExpr, SymbolNode, TypeInfo
+from mypy.checker import TypeChecker
+from mypy.nodes import AssignmentStmt, Expression, ImportedName, Lvalue, MypyFile, NameExpr, Statement, SymbolNode, TypeInfo, \
+    ClassDef
 from mypy.plugin import FunctionContext
 from mypy.types import AnyType, Instance, Type, TypeOfAny, TypeVarType
 
@@ -146,3 +148,40 @@ def get_argument_type_by_name(ctx: FunctionContext, name: str) -> Optional[Type]
         # Either an error or no value passed.
         return None
     return arg_types[0]
+
+
+def get_setting_expr(api: TypeChecker, setting_name: str) -> Optional[Expression]:
+    try:
+        settings_sym = api.modules['django.conf'].names['settings']
+    except KeyError:
+        return None
+
+    settings_type: TypeInfo = settings_sym.type.type
+    auth_user_model_sym = settings_type.get(setting_name)
+    if not auth_user_model_sym:
+        return None
+
+    module, _, name = auth_user_model_sym.fullname.rpartition('.')
+    if module not in api.modules:
+        return None
+
+    module_file = api.modules.get(module)
+    for name_expr, value_expr in iter_over_assignments(module_file):
+        if isinstance(name_expr, NameExpr) and name_expr.name == setting_name:
+            return value_expr
+    return None
+
+
+def iter_over_assignments(class_or_module: typing.Union[ClassDef, MypyFile]) -> typing.Iterator[typing.Tuple[Lvalue, Expression]]:
+    if isinstance(class_or_module, ClassDef):
+        statements = class_or_module.defs.body
+    else:
+        statements = class_or_module.defs
+
+    for stmt in statements:
+        if not isinstance(stmt, AssignmentStmt):
+            continue
+        if len(stmt.lvalues) > 1:
+            # not supported yet
+            continue
+        yield stmt.lvalues[0], stmt.rvalue