Added create_index(..., if_not_exists=True), refs #14

docs/python-api.rst

@@ -394,6 +394,8 @@ You can create a unique index by passing ``unique=True``::
 
     dogs.create_index(["name"], unique=True)
 
+Use ``if_not_exists=True`` to do nothing if an index with that name already exists.
+
 Vacuum
 ======
 

sqlite_utils/db.py

@@ -236,19 +236,20 @@ class Table:
         self.exists = True
         return self
 
-    def create_index(self, columns, index_name=None, unique=False):
+    def create_index(self, columns, index_name=None, unique=False, if_not_exists=False):
         if index_name is None:
             index_name = "idx_{}_{}".format(
                 self.name.replace(" ", "_"), "_".join(columns)
             )
         sql = """
-            CREATE {unique}INDEX {index_name}
+            CREATE {unique}INDEX {if_not_exists}{index_name}
                 ON {table_name} ({columns});
         """.format(
             index_name=index_name,
             table_name=self.name,
             columns=", ".join(columns),
             unique="UNIQUE " if unique else "",
+            if_not_exists="IF NOT EXISTS " if if_not_exists else "",
         )
         self.db.conn.execute(sql)
         return self

tests/test_create.py

@@ -1,9 +1,10 @@
 from sqlite_utils.db import Index, Database
 import collections
 import datetime
+import json
 import pathlib
 import pytest
-import json
+import sqlite3
 
 try:
     import pandas as pd
@@ -188,6 +189,17 @@ def test_create_index_unique(fresh_db):
     )
 
 
+def test_create_index_if_not_exists(fresh_db):
+    dogs = fresh_db["dogs"]
+    dogs.insert({"name": "Cleo", "twitter": "cleopaws", "age": 3, "is_good_dog": True})
+    assert [] == dogs.indexes
+    dogs.create_index(["name"])
+    assert 1 == len(dogs.indexes)
+    with pytest.raises(sqlite3.OperationalError):
+        dogs.create_index(["name"])
+    dogs.create_index(["name"], if_not_exists=True)
+
+
 @pytest.mark.parametrize(
     "data_structure",
     (