make null lowercase a missing value

doc/source/io.rst

@@ -227,7 +227,7 @@ na_values : scalar, str, list-like, or dict, default ``None``
   Additional strings to recognize as NA/NaN. If dict passed, specific per-column
   NA values. By default the following values are interpreted as NaN:
   ``'-1.#IND', '1.#QNAN', '1.#IND', '-1.#QNAN', '#N/A N/A', '#N/A', 'N/A', 'NA',
-  '#NA', 'NULL', 'NaN', '-NaN', 'nan', '-nan', ''``.
+  '#NA', 'NULL', 'null', 'NaN', '-NaN', 'nan', '-nan', ''``.
 keep_default_na : boolean, default ``True``
   If na_values are specified and keep_default_na is ``False`` the default NaN
   values are overridden, otherwise they're appended to.

doc/source/whatsnew/v0.21.0.txt

@@ -38,7 +38,7 @@ Other Enhancements
 - :func:`read_feather` has gained the ``nthreads`` parameter for multi-threaded operations (:issue:`16359`)
 - :func:`DataFrame.clip()` and :func: `Series.cip()` have gained an inplace argument. (:issue: `15388`)
 - :func:`crosstab` has gained a ``margins_name`` parameter to define the name of the row / column that will contain the totals when margins=True. (:issue:`15972`)
-
+- :func:`read_csv` has gained 'null' as an additional default missing value.(:issue:`16471`)
 .. _whatsnew_0210.api_breaking:
 
 Backwards incompatible API changes

pandas/_libs/parsers.pyx

@@ -277,7 +277,7 @@ DEFAULT_CHUNKSIZE = 256 * 1024
 # no longer excluding inf representations
 # '1.#INF','-1.#INF', '1.#INF000000',
 _NA_VALUES = [b'-1.#IND', b'1.#QNAN', b'1.#IND', b'-1.#QNAN',
-              b'#N/A N/A', b'NA', b'#NA', b'NULL', b'NaN',
+              b'#N/A N/A', b'NA', b'#NA', b'NULL', b'null', b'NaN',
               b'nan', b'']
 
 

pandas/io/common.py

@@ -31,7 +31,7 @@
 # '1.#INF','-1.#INF', '1.#INF000000',
 _NA_VALUES = set([
     '-1.#IND', '1.#QNAN', '1.#IND', '-1.#QNAN', '#N/A N/A', '#N/A',
-    'N/A', 'NA', '#NA', 'NULL', 'NaN', '-NaN', 'nan', '-nan', ''
+    'N/A', 'NA', '#NA', 'NULL', 'null', 'NaN', '-NaN', 'nan', '-nan', ''
 ])
 
 try:

pandas/tests/io/parser/na_values.py

@@ -70,7 +70,7 @@ def test_non_string_na_values(self):
 
     def test_default_na_values(self):
         _NA_VALUES = set(['-1.#IND', '1.#QNAN', '1.#IND', '-1.#QNAN',
-                          '#N/A', 'N/A', 'NA', '#NA', 'NULL', 'NaN',
+                          '#N/A', 'N/A', 'NA', '#NA', 'NULL', 'null', 'NaN',
                           'nan', '-NaN', '-nan', '#N/A N/A', ''])
         assert _NA_VALUES == parsers._NA_VALUES
         nv = len(_NA_VALUES)