From ed2b3f2afd7e329455ec0183af8d1ae07baa7714 Mon Sep 17 00:00:00 2001
From: Samuel Colvin <s@muelcolvin.com>
Date: Wed, 8 Nov 2017 15:36:19 +0000
Subject: [PATCH] errors for invalid validator use

---
 HISTORY.rst              |  4 ++++
 pydantic/main.py         |  6 ++++++
 tests/test_validators.py | 20 ++++++++++++++++++++
 3 files changed, 30 insertions(+)

diff --git a/HISTORY.rst b/HISTORY.rst
index e73a58a..99b52aa 100644
--- a/HISTORY.rst
+++ b/HISTORY.rst
@@ -3,6 +3,10 @@
 History
 -------
 
+v0.6.2 (2017-11-XX)
+...................
+* errors for invalid validator use
+
 v0.6.1 (2017-11-08)
 ...................
 * prevent duplicate validators, #101
diff --git a/pydantic/main.py b/pydantic/main.py
index 2144698..f5ea6a0 100644
--- a/pydantic/main.py
+++ b/pydantic/main.py
@@ -318,6 +318,12 @@ def validator(*fields, pre: bool=False, whole: bool=False, always: bool=False):
     :param whole: for complex objects (sets, lists etc.) whether to validate individual elements or the whole object
     :param always: whether this method and other validators should be called even if the value is missing
     """
+    if not fields:
+        raise ConfigError('validator with no fields specified')
+    elif isinstance(fields[0], FunctionType):
+        raise ConfigError("validators should be used with fields and keyword arguments, not bare. "
+                          "E.g. usage should be `@validator('<field_name>', ...)`")
+
     def dec(f):
         ref = f.__module__ + '.' + f.__qualname__
         if ref in _FUNCS:
diff --git a/tests/test_validators.py b/tests/test_validators.py
index fb34820..0520c36 100644
--- a/tests/test_validators.py
+++ b/tests/test_validators.py
@@ -177,6 +177,26 @@ def test_duplicates():
                                    '"tests.test_validators.test_duplicates.<locals>.Model.duplicate_name"')
 
 
+def test_use_bare():
+    with pytest.raises(ConfigError):
+        class Model(BaseModel):
+            a: str
+
+            @validator
+            def checker(cls, v):
+                return v
+
+
+def test_use_no_fields():
+    with pytest.raises(ConfigError):
+        class Model(BaseModel):
+            a: str
+
+            @validator()
+            def checker(cls, v):
+                return v
+
+
 def test_validate_always():
     check_calls = 0