Avoid side effect on default factory by calling it only once

fix pydantic#1491

changes/1491-PrettyWood.md

@@ -0,0 +1 @@
+Avoid side effects with `default_factory` by not calling it multiple times

pydantic/fields.py

@@ -271,9 +271,9 @@ def __init__(
         self.model_config.prepare_field(self)
         self.prepare()
 
-    def get_default(self) -> Any:
+    def get_default(self, *, copy_factory: bool = False) -> Any:
         if self.default_factory is not None:
-            value = self.default_factory()
+            value = self.default_factory() if not copy_factory else deepcopy(self.default_factory)()
         elif self.default is None:
             # deepcopy is quite slow on None
             value = None
@@ -296,7 +296,7 @@ def infer(
 
         if isinstance(value, FieldInfo):
             field_info = value
-            value = field_info.default_factory() if field_info.default_factory is not None else field_info.default
+            value = None if field_info.default_factory is not None else field_info.default
         else:
             field_info = FieldInfo(value, **field_info_from_config)
         required: 'BoolUndefined' = Undefined
@@ -341,7 +341,7 @@ def prepare(self) -> None:
         Note: this method is **not** idempotent (because _type_analysis is not idempotent),
         e.g. calling it it multiple times may modify the field and configure it incorrectly.
         """
-        default_value = self.get_default()
+        default_value = self.get_default(copy_factory=True)
         if default_value is not None and self.type_ is None:
             self.type_ = default_value.__class__
             self.outer_type_ = self.type_

tests/test_main.py

@@ -1088,6 +1088,27 @@ class FunctionModel(BaseModel):
     assert m.uid is uuid4
 
 
+def test_default_factory_side_effect():
+    """It should call only once the given factory"""
+
+    class Seq:
+        def __init__(self):
+            self.v = 0
+
+        def __call__(self):
+            self.v += 1
+            return self.v
+
+    class MyModel(BaseModel):
+        id: int = Field(default_factory=Seq())
+
+    m1 = MyModel()
+    assert m1.id == 1
+    m2 = MyModel()
+    assert m2.id == 2
+    assert m1.id == 1
+
+
 @pytest.mark.skipif(sys.version_info < (3, 7), reason='field constraints are set but not enforced with python 3.6')
 def test_none_min_max_items():
     # None default