table.exists() now a documented method, closes #83

docs/python-api.rst

@@ -920,6 +920,13 @@ If you have loaded an existing table or view, you can use introspection to find
     >>> db["PlantType"]
     <Table PlantType (id, value)>
 
+The ``.exists()`` method can be used to find out if a table exists or not::
+
+    >>> db["PlantType"].exists()
+    True
+    >>> db["PlantType2"].exists()
+    False
+
 The ``.count`` property shows the current number of rows (``select count(*) from table``)::
 
     >>> db["PlantType"].count

sqlite_utils/db.py

@@ -226,7 +226,7 @@ class Database:
         extracts = resolve_extracts(extracts)
         for extract_column, extract_table in extracts.items():
             # Ensure other table exists
-            if not self[extract_table].exists:
+            if not self[extract_table].exists():
                 self.create_table(extract_table, {"id": int, "value": str}, pk="id")
             columns[extract_column] = int
             foreign_keys_by_column[extract_column] = ForeignKey(
@@ -348,11 +348,11 @@ class Database:
 
         # Verify that all tables and columns exist
         for table, column, other_table, other_column in foreign_keys:
-            if not self[table].exists:
+            if not self[table].exists():
                 raise AlterError("No such table: {}".format(table))
             if column not in self[table].columns_dict:
                 raise AlterError("No such column: {} in {}".format(column, table))
-            if not self[other_table].exists:
+            if not self[other_table].exists():
                 raise AlterError("No such other_table: {}".format(other_table))
             if (
                 other_column != "rowid"
@@ -416,7 +416,8 @@ class Database:
 
 
 class Queryable:
-    exists = False
+    def exists(self):
+        return False
 
     def __init__(self, db, name):
         self.db = db
@@ -433,7 +434,7 @@ class Queryable:
         return self.rows_where()
 
     def rows_where(self, where=None, where_args=None):
-        if not self.exists:
+        if not self.exists():
             return []
         sql = "select * from [{}]".format(self.name)
         if where is not None:
@@ -445,7 +446,7 @@ class Queryable:
 
     @property
     def columns(self):
-        if not self.exists:
+        if not self.exists():
             return []
         rows = self.db.conn.execute(
             "PRAGMA table_info([{}])".format(self.name)
@@ -486,7 +487,6 @@ class Table(Queryable):
         conversions=None,
     ):
         super().__init__(db, name)
-        self.exists = self.name in self.db.table_names()
         self._defaults = dict(
             pk=pk,
             foreign_keys=foreign_keys,
@@ -506,10 +506,13 @@ class Table(Queryable):
         return "<Table {}{}>".format(
             self.name,
             " (does not exist yet)"
-            if not self.exists
+            if not self.exists()
             else " ({})".format(", ".join(c.name for c in self.columns)),
         )
 
+    def exists(self):
+        return self.name in self.db.table_names()
+
     @property
     def pks(self):
         names = [column.name for column in self.columns if column.is_pk]
@@ -614,7 +617,6 @@ class Table(Queryable):
                 hash_id=hash_id,
                 extracts=extracts,
             )
-        self.exists = True
         return self
 
     def create_index(self, columns, index_name=None, unique=False, if_not_exists=False):
@@ -847,7 +849,7 @@ class Table(Queryable):
             self.db.conn.execute(sql, pk_values)
 
     def delete_where(self, where=None, where_args=None):
-        if not self.exists:
+        if not self.exists():
             return []
         sql = "delete from [{}]".format(self.name)
         if where is not None:
@@ -982,7 +984,7 @@ class Table(Queryable):
         for chunk in chunks(itertools.chain([first_record], records), batch_size):
             chunk = list(chunk)
             if first:
-                if not self.exists:
+                if not self.exists():
                     # Use the first batch to derive the table names
                     self.create(
                         suggest_column_types(chunk),
@@ -1167,7 +1169,7 @@ class Table(Queryable):
     def lookup(self, column_values):
         # lookups is a dictionary - all columns will be used for a unique index
         assert isinstance(column_values, dict)
-        if self.exists:
+        if self.exists():
             self.add_missing_columns([column_values])
             unique_column_sets = [set(i.columns) for i in self.indexes]
             if set(column_values.keys()) not in unique_column_sets:
@@ -1245,7 +1247,8 @@ class Table(Queryable):
 
 
 class View(Queryable):
-    exists = True
+    def exists(self):
+        return True
 
     def __repr__(self):
         return "<View {} ({})>".format(

tests/test_m2m.py

@@ -61,8 +61,8 @@ def test_m2m_lookup(fresh_db):
     people.insert({"name": "Wahyu"}).m2m("tags", lookup={"tag": "Coworker"})
     people_tags = fresh_db["people_tags"]
     tags = fresh_db["tags"]
-    assert people_tags.exists
-    assert tags.exists
+    assert people_tags.exists()
+    assert tags.exists()
     assert [
         ForeignKey(
             table="people_tags",
@@ -94,7 +94,7 @@ def test_m2m_explicit_table_name_argument(fresh_db):
     )
     assert fresh_db["tags"].exists
     assert fresh_db["tagged"].exists
-    assert not fresh_db["people_tags"].exists
+    assert not fresh_db["people_tags"].exists()
 
 
 def test_m2m_table_candidates(fresh_db):
@@ -130,10 +130,10 @@ def test_uses_existing_m2m_table_if_exists(fresh_db):
         foreign_keys=["people_id", "tags_id"],
     )
     people.insert({"name": "Wahyu"}).m2m("tags", lookup={"tag": "Coworker"})
-    assert fresh_db["tags"].exists
-    assert fresh_db["tagged"].exists
-    assert not fresh_db["people_tags"].exists
-    assert not fresh_db["tags_people"].exists
+    assert fresh_db["tags"].exists()
+    assert fresh_db["tagged"].exists()
+    assert not fresh_db["people_tags"].exists()
+    assert not fresh_db["tags_people"].exists()
     assert [{"people_id": 1, "tags_id": 1}] == list(fresh_db["tagged"].rows)