From 881df8bde7bfc90e3a349c420d0b0b7d46237d59 Mon Sep 17 00:00:00 2001
From: Selim Belhaouane <selim.belhaouane@gmail.com>
Date: Thu, 30 Apr 2020 14:05:42 -0400
Subject: [PATCH] move const validator to post validators. fixes #1410 (#1446)

---
 changes/1410-selimb.md | 1 +
 docs/usage/schema.md   | 2 +-
 pydantic/fields.py     | 4 ++--
 tests/test_main.py     | 9 +++++++++
 4 files changed, 13 insertions(+), 3 deletions(-)
 create mode 100644 changes/1410-selimb.md

diff --git a/changes/1410-selimb.md b/changes/1410-selimb.md
new file mode 100644
index 0000000000..3768cf770c
--- /dev/null
+++ b/changes/1410-selimb.md
@@ -0,0 +1 @@
+Move `const` validator to post-validators so it validates the parsed value
diff --git a/docs/usage/schema.md b/docs/usage/schema.md
index 02ba8e0c49..028e7731f2 100644
--- a/docs/usage/schema.md
+++ b/docs/usage/schema.md
@@ -48,7 +48,7 @@ It has the following arguments:
 * `title`: if omitted, `field_name.title()` is used
 * `description`: if omitted and the annotation is a sub-model,
     the docstring of the sub-model will be used
-* `const`: this argument *must* have be the same as the field's default value if present
+* `const`: this argument *must* be the same as the field's default value if present.
 * `gt`: for numeric values (``int``, `float`, `Decimal`), adds a validation of "greater than" and an annotation
   of `exclusiveMinimum` to the JSON Schema
 * `ge`: for numeric values, this adds a validation of "greater than or equal" and an annotation of `minimum` to the
diff --git a/pydantic/fields.py b/pydantic/fields.py
index c444587122..c49ad7f047 100644
--- a/pydantic/fields.py
+++ b/pydantic/fields.py
@@ -510,11 +510,11 @@ def populate_validators(self) -> None:
             )
             self.validators = prep_validators(v_funcs)
 
-        # Add const validator
         self.pre_validators = []
         self.post_validators = []
+
         if self.field_info and self.field_info.const:
-            self.pre_validators = [make_generic_validator(constant_validator)]
+            self.post_validators.append(make_generic_validator(constant_validator))
 
         if class_validators_:
             self.pre_validators += prep_validators(v.func for v in class_validators_ if not v.each_item and v.pre)
diff --git a/tests/test_main.py b/tests/test_main.py
index 71371d3bcc..fdbc9ea3f1 100644
--- a/tests/test_main.py
+++ b/tests/test_main.py
@@ -383,6 +383,15 @@ class Model(BaseModel):
     assert m.a == 3
 
 
+def test_const_validates_after_type_validators():
+    # issue #1410
+    class Model(BaseModel):
+        a: int = Field(3, const=True)
+
+    m = Model(a='3')
+    assert m.a == 3
+
+
 def test_const_with_wrong_value():
     class Model(BaseModel):
         a: int = Field(3, const=True)