Enable casting to IamDataFrame multiple times

RELEASE_NOTES.md

@@ -1,5 +1,6 @@
 # Next Release
 
+- [#396](https://github.com/IAMconsortium/pyam/pull/396) Enable casting to `IamDataFrame` multiple times
 - [#394](https://github.com/IAMconsortium/pyam/pull/394) Switch CI to Github Actions.
 - [#393](https://github.com/IAMconsortium/pyam/pull/393) Import ABC from collections.abc for Python 3.10 compatibility.
 - [#380](https://github.com/IAMconsortium/pyam/pull/380) Add compatibility with latest matplotlib and py3.8

pyam/core.py

@@ -93,6 +93,14 @@ class IamDataFrame(object):
     """
     def __init__(self, data, **kwargs):
         """Initialize an instance of an IamDataFrame"""
+        if isinstance(data, IamDataFrame) and not kwargs:
+            for attr, value in data.__dict__.items():
+                setattr(self, attr, value)
+        else:
+            self._init(data, **kwargs)
+
+    def _init(self, data, **kwargs):
+        """Process data and set attributes for new instance"""
         # import data from pd.DataFrame or read from source
         if isinstance(data, pd.DataFrame) or isinstance(data, pd.Series):
             _data = format_data(data.copy(), **kwargs)

tests/test_core.py

@@ -46,6 +46,19 @@ def test_init_df_with_index(test_pd_df):
     pd.testing.assert_frame_equal(df.timeseries().reset_index(), test_pd_df)
 
 
+def test_init_from_iamdf(test_df_year):
+    # casting an IamDataFrame instance again works
+    df = IamDataFrame(test_df_year)
+
+    # inplace-operations on the new object have effects on the original object
+    df.rename(scenario={'scen_a': 'scen_foo'}, inplace=True)
+    assert all(test_df_year.scenarios().values == ['scen_b', 'scen_foo'])
+
+    # overwrites on the new object do not have effects on the original object
+    df = df.rename(scenario={'scen_foo': 'scen_bar'})
+    assert all(df.scenarios().values == ['scen_b', 'scen_bar'])
+    assert all(test_df_year.scenarios().values == ['scen_b', 'scen_foo'])
+
 def test_init_df_with_float_cols_raises(test_pd_df):
     _test_df = test_pd_df.rename(columns={2005: 2005.5, 2010: 2010.})
     pytest.raises(ValueError, IamDataFrame, data=_test_df)