Repository: marshmallow-code/marshmallow
Issue: Defining fields with dot notations in Meta.exclude only works when initialising first time

Demonstration of the issue
``` python
from marshmallow import Schema, fields


class AuthorSchema(Schema):
    first_name = fields.String(required=True)
    last_name = fields.String(required=True)


class BaseSchema(Schema):
    class Meta:
        exclude = ['author.first_name']

    author = fields.Nested(AuthorSchema, required=True)


data = dict(author=dict(first_name='foo', last_name='bar'))
print(BaseSchema().dump(data))
print(BaseSchema().dump(data))
```
results in
```
{'author': {'last_name': 'bar'}}
{'author': {'first_name': 'foo', 'last_name': 'bar'}}
```

Changed files:
src/marshmallow/schema.py, tests/test_schema.py

diff --git a/src/marshmallow/schema.py b/src/marshmallow/schema.py
index b370035..085c509 100644
--- a/src/marshmallow/schema.py
+++ b/src/marshmallow/schema.py
@@ -346,7 +346,7 @@ class BaseSchema(base.SchemaABC):
         self.declared_fields = copy.deepcopy(self._declared_fields)
         self.many = many
         self.only = only
-        self.exclude = exclude
+        self.exclude = set(self.opts.exclude) | set(exclude)
         if prefix:
             warnings.warn(
                 'The `prefix` argument is deprecated. Use a post_dump '
@@ -719,19 +719,15 @@ class BaseSchema(base.SchemaABC):
             self.__apply_nested_option('only', self.only, 'intersection')
             # Remove the child field names from the only option.
             self.only = self.set_class(
-                [field.split('.', 1)[0] for field in self.only])
-        excludes = set(self.opts.exclude) | set(self.exclude)
-        if excludes:
-            # Apply the exclude option to nested fields.
-            self.__apply_nested_option('exclude', excludes, 'union')
+                [field.split('.', 1)[0] for field in self.only],
+            )
         if self.exclude:
+            # Apply the exclude option to nested fields.
+            self.__apply_nested_option('exclude', self.exclude, 'union')
             # Remove the parent field names from the exclude option.
             self.exclude = self.set_class(
-                [field for field in self.exclude if '.' not in field])
-        if self.opts.exclude:
-            # Remove the parent field names from the meta exclude option.
-            self.opts.exclude = self.set_class(
-                [field for field in self.opts.exclude if '.' not in field])
+                [field for field in self.exclude if '.' not in field],
+            )
 
     def __apply_nested_option(self, option_name, field_names, set_operation):
         """Apply nested options to nested fields"""
@@ -771,9 +767,7 @@ class BaseSchema(base.SchemaABC):
             field_names = self.set_class(self.declared_fields.keys())
 
         # If "exclude" option or param is specified, remove those fields
-        excludes = set(self.opts.exclude) | set(self.exclude)
-        if excludes:
-            field_names = field_names - excludes
+        field_names -= self.exclude
         ret = self.__filter_fields(field_names, obj, many=many)
         # Set parents
         self.__set_field_attrs(ret)
diff --git a/tests/test_schema.py b/tests/test_schema.py
index cccc7c9..b9f7534 100755
--- a/tests/test_schema.py
+++ b/tests/test_schema.py
@@ -938,8 +938,22 @@ def test_meta_nested_exclude():
         blubb = fields.Nested(ChildSchema)
         class Meta:
             exclude = ('blubb.foo',)
-    sch = ParentSchema()
+
     data = dict(bla=1, bli=2, blubb=dict(foo=42, bar=24, baz=242))
+
+    sch = ParentSchema()
+    result = sch.dump(data)
+    assert 'bla' in result.data
+    assert 'blubb' in result.data
+    assert 'bli' in result.data
+    child = result.data['blubb']
+    assert 'foo' not in child
+    assert 'bar' in child
+    assert 'baz' in child
+
+    # Test fields with dot notations in Meta.exclude on multiple instantiations
+    # Regression test for https://github.com/marshmallow-code/marshmallow/issues/1212
+    sch = ParentSchema()
     result = sch.dump(data)
     assert 'bla' in result.data
     assert 'blubb' in result.data