ENH: support non default indexes in writing to Parquet (#18629)

fastparquet automatically names an index 'index' if it doesn't
already have a name

doc/source/io.rst

@@ -4504,11 +4504,8 @@ dtypes, including extension dtypes such as datetime with tz.
 
 Several caveats.
 
-- The format will NOT write an ``Index``, or ``MultiIndex`` for the
-  ``DataFrame`` and will raise an error if a non-default one is provided. You
-  can ``.reset_index()`` to store the index or ``.reset_index(drop=True)`` to
-  ignore it.
 - Duplicate column names and non-string columns names are not supported
+- Index level names, if specified, must be strings
 - Categorical dtypes can be serialized to parquet, but will de-serialize as ``object`` dtype.
 - Non supported types include ``Period`` and actual python object types. These will raise a helpful error message
   on an attempt at serialization.

doc/source/whatsnew/v0.21.1.txt

@@ -61,6 +61,9 @@ New features
 Improvements to the Parquet IO functionality
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
+- :func:`DataFrame.to_parquet` will now write non-default indexes when the
+  underlying engine supports it. The indexes will be preserved when reading
+  back in with :func:`read_parquet` (:issue:`18581`).
 - :func:`read_parquet` now allows to specify the columns to read from a parquet file (:issue:`18154`)
 - :func:`read_parquet` now allows to specify kwargs which are passed to the respective engine (:issue:`18216`)
 
@@ -91,8 +94,6 @@ Performance Improvements
 
 Bug Fixes
 ~~~~~~~~~
--
-
 
 Conversion
 ^^^^^^^^^^

pandas/io/parquet.py

@@ -3,7 +3,8 @@
 from warnings import catch_warnings
 from distutils.version import LooseVersion
 from pandas import DataFrame, RangeIndex, Int64Index, get_option
-from pandas.compat import range
+from pandas.compat import string_types
+from pandas.core.common import AbstractMethodError
 from pandas.io.common import get_filepath_or_buffer
 
 
@@ -39,39 +40,75 @@ def get_engine(engine):
         return FastParquetImpl()
 
 
-class PyArrowImpl(object):
+class BaseImpl(object):
+
+    api = None  # module
+
+    @staticmethod
+    def validate_dataframe(df):
+
+        if not isinstance(df, DataFrame):
+            raise ValueError("to_parquet only supports IO with DataFrames")
+
+        # must have value column names (strings only)
+        if df.columns.inferred_type not in {'string', 'unicode'}:
+            raise ValueError("parquet must have string column names")
+
+        # index level names must be strings
+        valid_names = all(
+            isinstance(name, string_types)
+            for name in df.index.names
+            if name is not None
+        )
+        if not valid_names:
+            raise ValueError("Index level names must be strings")
+
+    def write(self, df, path, compression, **kwargs):
+        raise AbstractMethodError(self)
+
+    def read(self, path, columns=None, **kwargs):
+        raise AbstractMethodError(self)
+
+
+class PyArrowImpl(BaseImpl):
 
     def __init__(self):
         # since pandas is a dependency of pyarrow
         # we need to import on first use
-
         try:
             import pyarrow
             import pyarrow.parquet
         except ImportError:
-            raise ImportError("pyarrow is required for parquet support\n\n"
-                              "you can install via conda\n"
-                              "conda install pyarrow -c conda-forge\n"
-                              "\nor via pip\n"
-                              "pip install -U pyarrow\n")
-
-        if LooseVersion(pyarrow.__version__) < LooseVersion('0.4.1'):
-            raise ImportError("pyarrow >= 0.4.1 is required for parquet"
-                              "support\n\n"
-                              "you can install via conda\n"
-                              "conda install pyarrow -c conda-forge\n"
-                              "\nor via pip\n"
-                              "pip install -U pyarrow\n")
-
-        self._pyarrow_lt_050 = (LooseVersion(pyarrow.__version__) <
-                                LooseVersion('0.5.0'))
-        self._pyarrow_lt_060 = (LooseVersion(pyarrow.__version__) <
-                                LooseVersion('0.6.0'))
+            raise ImportError(
+                "pyarrow is required for parquet support\n\n"
+                "you can install via conda\n"
+                "conda install pyarrow -c conda-forge\n"
+                "\nor via pip\n"
+                "pip install -U pyarrow\n"
+            )
+        if LooseVersion(pyarrow.__version__) < '0.4.1':
+            raise ImportError(
+                "pyarrow >= 0.4.1 is required for parquet support\n\n"
+                "you can install via conda\n"
+                "conda install pyarrow -c conda-forge\n"
+                "\nor via pip\n"
+                "pip install -U pyarrow\n"
+            )
+
+        self._pyarrow_lt_060 = (
+            LooseVersion(pyarrow.__version__) < LooseVersion('0.6.0'))
+        self._pyarrow_lt_070 = (
+            LooseVersion(pyarrow.__version__) < LooseVersion('0.7.0'))
+
         self.api = pyarrow
 
     def write(self, df, path, compression='snappy',
               coerce_timestamps='ms', **kwargs):
+        self.validate_dataframe(df)
+        if self._pyarrow_lt_070:
+            self._validate_write_lt_070(df)
         path, _, _ = get_filepath_or_buffer(path)
+
         if self._pyarrow_lt_060:
             table = self.api.Table.from_pandas(df, timestamps_to_ms=True)
             self.api.parquet.write_table(
@@ -85,36 +122,75 @@ def write(self, df, path, compression='snappy',
 
     def read(self, path, columns=None, **kwargs):
         path, _, _ = get_filepath_or_buffer(path)
+        if self._pyarrow_lt_070:
+            return self.api.parquet.read_pandas(path, columns=columns,
+                                                **kwargs).to_pandas()
+        kwargs['use_pandas_metadata'] = True
         return self.api.parquet.read_table(path, columns=columns,
                                            **kwargs).to_pandas()
 
-
-class FastParquetImpl(object):
+    def _validate_write_lt_070(self, df):
+        # Compatibility shim for pyarrow < 0.7.0
+        # TODO: Remove in pandas 0.22.0
+        from pandas.core.indexes.multi import MultiIndex
+        if isinstance(df.index, MultiIndex):
+            msg = (
+                "Multi-index DataFrames are only supported "
+                "with pyarrow >= 0.7.0"
+            )
+            raise ValueError(msg)
+        # Validate index
+        if not isinstance(df.index, Int64Index):
+            msg = (
+                "pyarrow < 0.7.0 does not support serializing {} for the "
+                "index; you can .reset_index() to make the index into "
+                "column(s), or install the latest version of pyarrow or "
+                "fastparquet."
+            )
+            raise ValueError(msg.format(type(df.index)))
+        if not df.index.equals(RangeIndex(len(df))):
+            raise ValueError(
+                "pyarrow < 0.7.0 does not support serializing a non-default "
+                "index; you can .reset_index() to make the index into "
+                "column(s), or install the latest version of pyarrow or "
+                "fastparquet."
+            )
+        if df.index.name is not None:
+            raise ValueError(
+                "pyarrow < 0.7.0 does not serialize indexes with a name; you "
+                "can set the index.name to None or install the latest version "
+                "of pyarrow or fastparquet."
+            )
+
+
+class FastParquetImpl(BaseImpl):
 
     def __init__(self):
         # since pandas is a dependency of fastparquet
         # we need to import on first use
-
         try:
             import fastparquet
         except ImportError:
-            raise ImportError("fastparquet is required for parquet support\n\n"
-                              "you can install via conda\n"
-                              "conda install fastparquet -c conda-forge\n"
-                              "\nor via pip\n"
-                              "pip install -U fastparquet")
-
-        if LooseVersion(fastparquet.__version__) < LooseVersion('0.1.0'):
-            raise ImportError("fastparquet >= 0.1.0 is required for parquet "
-                              "support\n\n"
-                              "you can install via conda\n"
-                              "conda install fastparquet -c conda-forge\n"
-                              "\nor via pip\n"
-                              "pip install -U fastparquet")
-
+            raise ImportError(
+                "fastparquet is required for parquet support\n\n"
+                "you can install via conda\n"
+                "conda install fastparquet -c conda-forge\n"
+                "\nor via pip\n"
+                "pip install -U fastparquet"
+            )
+        if LooseVersion(fastparquet.__version__) < '0.1.0':
+            raise ImportError(
+                "fastparquet >= 0.1.0 is required for parquet "
+                "support\n\n"
+                "you can install via conda\n"
+                "conda install fastparquet -c conda-forge\n"
+                "\nor via pip\n"
+                "pip install -U fastparquet"
+            )
         self.api = fastparquet
 
     def write(self, df, path, compression='snappy', **kwargs):
+        self.validate_dataframe(df)
         # thriftpy/protocol/compact.py:339:
         # DeprecationWarning: tostring() is deprecated.
         # Use tobytes() instead.
@@ -125,7 +201,8 @@ def write(self, df, path, compression='snappy', **kwargs):
 
     def read(self, path, columns=None, **kwargs):
         path, _, _ = get_filepath_or_buffer(path)
-        return self.api.ParquetFile(path).to_pandas(columns=columns, **kwargs)
+        parquet_file = self.api.ParquetFile(path)
+        return parquet_file.to_pandas(columns=columns, **kwargs)
 
 
 def to_parquet(df, path, engine='auto', compression='snappy', **kwargs):
@@ -146,43 +223,7 @@ def to_parquet(df, path, engine='auto', compression='snappy', **kwargs):
     kwargs
         Additional keyword arguments passed to the engine
     """
-
     impl = get_engine(engine)
-
-    if not isinstance(df, DataFrame):
-        raise ValueError("to_parquet only support IO with DataFrames")
-
-    valid_types = {'string', 'unicode'}
-
-    # validate index
-    # --------------
-
-    # validate that we have only a default index
-    # raise on anything else as we don't serialize the index
-
-    if not isinstance(df.index, Int64Index):
-        raise ValueError("parquet does not support serializing {} "
-                         "for the index; you can .reset_index()"
-                         "to make the index into column(s)".format(
-                             type(df.index)))
-
-    if not df.index.equals(RangeIndex.from_range(range(len(df)))):
-        raise ValueError("parquet does not support serializing a "
-                         "non-default index for the index; you "
-                         "can .reset_index() to make the index "
-                         "into column(s)")
-
-    if df.index.name is not None:
-        raise ValueError("parquet does not serialize index meta-data on a "
-                         "default index")
-
-    # validate columns
-    # ----------------
-
-    # must have value column names (strings only)
-    if df.columns.inferred_type not in valid_types:
-        raise ValueError("parquet must have string column names")
-
     return impl.write(df, path, compression=compression, **kwargs)