Make merge_tables api more flexible

Allow the target argument to be either a string or wrapped table
and allow the tables sequence to have table names (strings) or
wrapped table instances.

urbansim/sim/simulation.py

@@ -1148,9 +1148,9 @@ def merge_tables(target, tables, columns=None):
 
     Parameters
     ----------
-    target : str
-        Name of the table onto which tables will be merged.
-    tables : list of `DataFrameWrapper` or `TableFuncWrapper`
+    target : str, DataFrameWrapper, or TableFuncWrapper
+        Name of the table (or wrapped table) onto which tables will be merged.
+    tables : list of `DataFrameWrapper`, `TableFuncWrapper`, or str
         All of the tables to merge. Should include the target table.
     columns : list of str, optional
         If given, columns will be mapped to `tables` and only those columns
@@ -1163,6 +1163,15 @@ def merge_tables(target, tables, columns=None):
     merged : pandas.DataFrame
 
     """
+    # allow target to be string or table wrapper
+    if isinstance(target, (DataFrameWrapper, TableFuncWrapper)):
+        target = target.name
+
+    # allow tables to be strings or table wrappers
+    tables = [get_table(t)
+              if not isinstance(t, (DataFrameWrapper, TableFuncWrapper)) else t
+              for t in tables]
+
     merges = {t.name: {} for t in tables}
     tables = {t.name: t for t in tables}
     casts = _get_broadcasts(tables.keys())

urbansim/sim/tests/test_mergetables.py

@@ -123,7 +123,7 @@ def test_merge_tables1(dfa, dfz, dfb):
 def test_merge_tables2(dfa, dfz, dfb, dfc):
     all_broadcasts()
 
-    merged = sim.merge_tables('c', [dfa, dfz, dfb, dfc])
+    merged = sim.merge_tables(dfc, [dfa, dfz, dfb, dfc])
 
     expected = pd.merge(
         dfa.to_frame(), dfb.to_frame(), left_index=True, right_on='a_id')
@@ -177,12 +177,7 @@ def test_merge_tables3():
     sim.broadcast(cast='b', onto='d', cast_index=True, onto_on='b_id')
     sim.broadcast(cast='c', onto='d', cast_index=True, onto_on='c_id')
 
-    a = sim.get_table('a')
-    b = sim.get_table('b')
-    c = sim.get_table('c')
-    d = sim.get_table('d')
-
-    df = sim.merge_tables(target='d', tables=[a, b, c, d])
+    df = sim.merge_tables(target='d', tables=['a', 'b', 'c', 'd'])
 
     expected = pd.merge(df_a, df_b, left_index=True, right_on='a_id')
     expected = pd.merge(expected, df_d, left_index=True, right_on='b_id')