Fix parameters to ggplot object

ggplot(data=df) resulted in an exception.

doc/changelog.rst

@@ -14,6 +14,8 @@ v0.2.1
 
 - Fixed bug in :class:`~plotnine.stat_summary` where the you got
   an exception for some types of the `x` aesthetic values.
+
+- Fixed bug where ``ggplot(data=df)`` resulted in an exception.
 
 
 v0.2.0

plotnine/ggplot.py

@@ -52,7 +52,8 @@ class ggplot(object):
 
     def __init__(self, mapping=None, data=None, environment=None):
         # Allow some sloppiness
-        if not isinstance(mapping, aes):
+        if (isinstance(mapping, pd.DataFrame) and
+                (data is None or isinstance(data, aes))):
             mapping, data = data, mapping
         if mapping is None:
             mapping = aes()

plotnine/tests/test_ggplot_internals.py

@@ -52,11 +52,28 @@ def test_labels():
 
 def test_ggplot_parameters():
     p = ggplot(df, aes('x'))
+
     assert p.data is df
     assert p.mapping == aes('x')
     assert p.environment.namespace['np'] is np
     assert p.environment.namespace['pd'] is pd
 
+    p = ggplot(data=df, mapping=aes('x'))
+    assert p.data is df
+    assert p.mapping == aes('x')
+
+    p = ggplot(data=df)
+    assert p.data is df
+    assert p.mapping == aes()
+
+    p = ggplot(mapping=aes('x'))
+    assert p.data is None
+    assert p.mapping == aes('x')
+
+    p = ggplot()
+    assert p.data is None
+    assert p.mapping == aes()
+
     with pytest.raises(PlotnineError):
         ggplot([1, 2, 3], aes('x'))