Have head() traverse all partitions

docs/tutorials/common_data_operations.ipynb

@@ -110,7 +110,7 @@
     "Often, you'll want to peek at your data even though the full-size is too large for memory.\n",
     "\n",
     "> **_Note:_**\n",
-    "By default this only looks at the first partition of data, so any operations that remove all data from the first partition will produce an empty head result. Specify `npartitions=-1` to grab from all partitions.\n"
+    "some partitions may be empty and `head` will have to traverse these empty partitions to find enough rows for your result. An empty table with many partitions (O(100)k) might be costly even for an ultimately empty result. "
    ]
   },
   {
@@ -119,7 +119,7 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "ens.source.head(5, npartitions=-1)  # grabs the first 5 rows\n",
+    "ens.source.head(5)  # grabs the first 5 rows\n",
     "\n",
     "# can also use tail to grab the last 5 rows"
    ]

src/tape/ensemble_frame.py

@@ -845,6 +845,39 @@ def repartition(
         )
         return self._propagate_metadata(result)
 
+    def head(self, n=5, compute=True):
+        """Returns `n` rows of data for previewing purposes.
+
+        Parameters
+        ----------
+        n : int, optional
+            The number of desired rows. Default is 5.
+        compute : bool, optional
+            Whether to compute the result immediately. Default is True.
+
+        Returns:
+            A pandas DataFrame with up to `n` rows of data.
+        """
+        if not compute:
+            # Just use the Dask head method
+            return super().head(n, compute=False)
+
+        if n <= 0:
+            return super().head(0)
+
+        # Iterate over the partitions until we have enough rows
+        dfs = []
+        remaining_rows = n
+        for partition in self.partitions:
+            if remaining_rows == 0:
+                break
+            # Note that partition is itself a _Frame object, so we need to compute to avoid infinite recursion
+            partition_head = partition.compute().head(remaining_rows)
+            dfs.append(partition_head)
+            remaining_rows -= len(partition_head)
+
+        return pd.concat(dfs)
+
 
 class EnsembleSeries(_Frame, dd.Series):
     """A barebones extension of a Dask Series for Ensemble data."""

tests/tape_tests/test_ensemble_frame.py

@@ -1,5 +1,6 @@
 """ Test EnsembleFrame (inherited from Dask.DataFrame) creation and manipulations. """
 
+from math import floor
 import numpy as np
 import pandas as pd
 from tape import (
@@ -470,3 +471,57 @@ def test_partition_slicing(parquet_ensemble_with_divisions):
 
     assert ens.source.npartitions == 2  # should return exactly 2 partitions
     assert len(ens.object) < prior_src_len  # should affect objects
+
+
+@pytest.mark.parametrize(
+    "data_fixture",
+    [
+        "parquet_ensemble",
+        "parquet_ensemble_with_divisions",
+    ],
+)
+def test_head(data_fixture, request):
+    """
+    Tests that head returns the correct number of rows.
+    """
+    ens = request.getfixturevalue(data_fixture)
+
+    # Test witht repartitioning the source frame
+    frame = ens.source
+    frame = frame.repartition(npartitions=10)
+
+    assert frame.npartitions == 10
+
+    # Test inputs that should return an empty frame
+    assert len(frame.head(-100)) == 0
+    assert len(frame.head(0)) == 0
+    assert len(frame.head(-1)) == 0
+
+    assert len(frame.head(100, compute=False).compute()) == 100
+
+    one_res = frame.head(1)
+    assert len(one_res) == 1
+    assert isinstance(one_res, TapeFrame)
+    assert set(one_res.columns) == set(frame.columns)
+
+    def_result = frame.head()
+    assert len(def_result) == 5
+    assert isinstance(one_res, TapeFrame)
+    assert set(one_res.columns) == set(frame.columns)
+
+    def_result = frame.head(24)
+    assert len(def_result) == 24
+    assert isinstance(one_res, TapeFrame)
+    assert set(one_res.columns) == set(frame.columns)
+
+    # Test that we have sane behavior even when the number of rows requested is larger than the number of rows in the frame.
+    assert len(frame.head(2 * len(frame))) == len(frame)
+
+    # Choose a value that will be guaranteed to hit every partition for this data.
+    # Note that with parquet_ensemble_with_divisions some of the partitions are empty
+    # testing that as well.
+    rows = floor(len(frame.compute()) * 0.98)
+    result = frame.head(rows)
+    assert len(result) == rows
+    assert isinstance(result, TapeFrame)
+    assert set(result.columns) == set(frame.columns)