GetUser returns the IAM user who owns the access key in the request.

moto/iam/models.py

@@ -1248,5 +1248,13 @@ class IAMBackend(BaseBackend):
                 return saml_provider
         raise IAMNotFoundException("SamlProvider {0} not found".format(saml_provider_arn))
 
+    def get_user_from_access_key_id(self, access_key_id):
+        for user_name, user in self.users.items():
+            access_keys = self.get_all_access_keys(user_name)
+            for access_key in access_keys:
+                if access_key.access_key_id == access_key_id:
+                    return user
+        return None
+
 
 iam_backend = IAMBackend()

moto/iam/responses.py

@@ -1,9 +1,8 @@
 from __future__ import unicode_literals
 
 from moto.core.responses import BaseResponse
-from .models import iam_backend
 
-AVATAO_USER_NAME = "avatao-user"
+from .models import iam_backend, User
 
 
 class IamResponse(BaseResponse):
@@ -428,9 +427,12 @@ class IamResponse(BaseResponse):
     def get_user(self):
         user_name = self._get_param('UserName')
         if not user_name:
-            user_name = AVATAO_USER_NAME
-            # If no user is specified, IAM returns the current user
-        user = iam_backend.get_user(user_name)
+            access_key_id = self.get_current_user()
+            user = iam_backend.get_user_from_access_key_id(access_key_id)
+            if user is None:
+                user = User("default_user")
+        else:
+            user = iam_backend.get_user(user_name)
 
         template = self.response_template(USER_TEMPLATE)
         return template.render(action='Get', user=user)