Repository: wireservice/agate

Issue:
agate.First aggregation
I end up doing this all the time:

```
def pick_first(c):
    return c[0]

agate.Summary('Serial_Num', agate.Text(), pick_first)
```

Discussion:

Patch:
diff --git a/agate/aggregations/__init__.py b/agate/aggregations/__init__.py
index e4f40cc..cf82a30 100644
--- a/agate/aggregations/__init__.py
+++ b/agate/aggregations/__init__.py
@@ -21,6 +21,7 @@ from agate.aggregations.all import All  # noqa
 from agate.aggregations.any import Any  # noqa
 from agate.aggregations.count import Count  # noqa
 from agate.aggregations.deciles import Deciles  # noqa
+from agate.aggregations.first import First  # noqa
 from agate.aggregations.has_nulls import HasNulls  # noqa
 from agate.aggregations.iqr import IQR  # noqa
 from agate.aggregations.mad import MAD  # noqa
diff --git a/agate/aggregations/first.py b/agate/aggregations/first.py
new file mode 100644
index 0000000..37e1695
--- /dev/null
+++ b/agate/aggregations/first.py
@@ -0,0 +1,42 @@
+#!/usr/bin/env python
+
+from agate.aggregations.base import Aggregation
+from agate.data_types import Boolean
+
+
+class First(Aggregation):
+    """
+    Returns the first value that passes a test.
+
+    If the test is omitted, the aggregation will return the first value in the column.
+
+    If no values pass the test, the aggregation will raise an exception.
+
+    :param column_name:
+        The name of the column to check.
+    :param test:
+        A function that takes a value and returns `True` or `False`. Test may be
+        omitted when checking :class:`.Boolean` data.
+    """
+    def __init__(self, column_name, test=None):
+        self._column_name = column_name
+        self._test = test
+
+    def get_aggregate_data_type(self, table):
+        return table.columns[self._column_name].data_type
+
+    def validate(self, table):
+        column = table.columns[self._column_name]
+        data = column.values()
+
+        if self._test is not None and len([d for d in data if self._test(d)]) == 0:
+            raise ValueError('No values pass the given test.')
+
+    def run(self, table):
+        column = table.columns[self._column_name]
+        data = column.values()
+
+        if self._test is None:
+            return data[0]
+
+        return next((d for d in data if self._test(d)))


Test patch:
diff --git a/tests/test_aggregations.py b/tests/test_aggregations.py
index 11eefe1..e0dc625 100644
--- a/tests/test_aggregations.py
+++ b/tests/test_aggregations.py
@@ -67,6 +67,17 @@ class TestSimpleAggregation(unittest.TestCase):
         self.assertEqual(All('one', lambda d: d != 5).run(self.table), True)
         self.assertEqual(All('one', lambda d: d == 2).run(self.table), False)
 
+    def test_first(self):
+        with self.assertRaises(ValueError):
+            First('one', lambda d: d == 5).validate(self.table)
+
+        First('one', lambda d: d).validate(self.table)
+
+        self.assertIsInstance(First('one').get_aggregate_data_type(self.table), Number)
+        self.assertEqual(First('one').run(self.table), 1)
+        self.assertEqual(First('one', lambda d: d == 2).run(self.table), 2)
+        self.assertEqual(First('one', lambda d: not d).run(self.table), None)
+
     def test_count(self):
         rows = (
             (1, 2, 'a'),