fix(corpus): register corpus attributes.

When a Corpus would contain a token that is identical to one of the
attribute names (e.g., 'logger'), pandas would store the value assigned to
the attribute in (all rows of) the column of the underlying DataFrame.

delta/corpus.py

@@ -311,7 +311,7 @@ def __init__(self, operation):
 
 
 class Corpus(pd.DataFrame):
-    _metadata = ['metadata']
+    _metadata = ['metadata', 'logger', 'document_describer', 'feature_generator']
 
     def __init__(self, source=None, *, subdir=None, file=None, corpus=None,
                  feature_generator=None,

test/corpus_test.py

@@ -1,3 +1,5 @@
+import numpy as np
+import pandas as pd
 from pytest import approx
 
 import delta as d
@@ -78,4 +80,13 @@ def test_featuredescriber_args(testdir):
 
 def test_parallel_corpus(testdir, corpus):
     parallel_corpus = d.Corpus(testdir, parallel=True)
-    assert (parallel_corpus == corpus).all().all()
+    assert (parallel_corpus == corpus).all().all()
+
+
+@pytest.mark.parametrize("attr_name", ["logger", "metadata", "feature_generator", "document_describer", "save"])
+def test_attribute_names(attr_name):
+    df = pd.DataFrame([[17, 4], [23, 42]],
+                      columns=['foo', attr_name],
+                      index=['doc1', 'doc2'])
+    corpus = d.Corpus(df)
+    assert list(corpus[attr_name]) == [4, 42]