Repository: alteryx/woodwork (python)
Issue: Set time index adds tag to multiple columns after reset_semantic_tags call
If you try to set the time index after a call to reset the semantic tags on a dataframe, the `time_index` tag gets added to multiple columns and can return the wrong value for `df.ww.time_index`.

#### Code Sample

```python
>>> import woodwork as ww
>>> df = ww.demo.load_retail(nrows=100, return_dataframe=True)
df.ww.init(name="retail")
>>> df.ww.init(name="retail")
>>> df.ww.reset_semantic_tags()
>>> df.ww.set_time_index('order_date')
>>> df.ww.time_index
'description'
>>> df.ww
                   Physical Type     Logical Type Semantic Tag(s)
Column                                                           
order_product_id           Int64          Integer     ['numeric']
order_id                   Int64          Integer     ['numeric']
product_id              category      Categorical    ['category']
description               string  NaturalLanguage  ['time_index']
quantity                   Int64          Integer     ['numeric']
order_date        datetime64[ns]         Datetime  ['time_index']
unit_price               float64           Double     ['numeric']
customer_name             string  NaturalLanguage  ['time_index']
country                   string  NaturalLanguage  ['time_index']
total                    float64           Double     ['numeric']
cancelled                boolean          Boolean  ['time_index']
```

Pull request description: Fix bug in _reset_semantic_tags causing columns to share the same semantic tags set
- Fix bug in _reset_semantic_tags causing columns to share the same semantic tags set
- Closes #662 

This PR fixes a bug in `_reset_semantic_tags` that was causing multiple schema columns to share the same semantic tags set after reset. This was being cause by resetting the tags to `LogicalType.standard_tags` without creating a new set, so any columns that shared the same logical type were using the same set after the reset. This fix creates a new set, so that each column now has a unique set, and adds a test to verify this.

Patch:
diff --git a/docs/source/release_notes.rst b/docs/source/release_notes.rst
index d55d956..9ba903b 100644
--- a/docs/source/release_notes.rst
+++ b/docs/source/release_notes.rst
@@ -42,6 +42,7 @@ Release Notes
         * Adds ``__getitem__`` to WoodworkTableAccessor (:pr:`633`)
     * Fixes
         * Create new Schema object when performing pandas operation on Accessors (:pr:`595`)
+        * Fix bug in ``_reset_semantic_tags`` causing columns to share same semantic tags set (:pr:`666`)
     * Changes
         * Move mutual information logic to statistics utils file (:pr:`584`)
         * Bump min Koalas version to 1.4.0 (:pr:`638`)
diff --git a/woodwork/schema_column.py b/woodwork/schema_column.py
index 2cea968..c88dc53 100644
--- a/woodwork/schema_column.py
+++ b/woodwork/schema_column.py
@@ -136,7 +136,7 @@ def _reset_semantic_tags(standard_tags, use_standard_tags):
         use_standard_tags (bool): If True, retain standard tags after reset
     """
     if use_standard_tags:
-        return standard_tags
+        return set(standard_tags)
     return set()
 
 


Test patch:
diff --git a/woodwork/tests/schema/test_schema_column.py b/woodwork/tests/schema/test_schema_column.py
index 2f31a7f..d39661a 100644
--- a/woodwork/tests/schema/test_schema_column.py
+++ b/woodwork/tests/schema/test_schema_column.py
@@ -18,6 +18,7 @@ from woodwork.schema_column import (
     _is_col_categorical,
     _is_col_datetime,
     _is_col_numeric,
+    _reset_semantic_tags,
     _validate_description,
     _validate_logical_type,
     _validate_metadata
@@ -148,3 +149,10 @@ def test_is_col_datetime():
 
     double_column = _get_column_dict('floats', Double)
     assert not _is_col_datetime(double_column)
+
+
+def test_reset_semantic_tags_returns_new_object():
+    standard_tags = {'tag1', 'tag2'}
+    reset_tags = _reset_semantic_tags(standard_tags, use_standard_tags=True)
+    assert reset_tags is not standard_tags
+    assert reset_tags == standard_tags