Add a block() method to dataset objects.

block() return the window and size in bytes of a TIFF block. The
size is undefined for other formats and will be None.

Resolves #1077

CHANGES.txt

@@ -8,10 +8,10 @@ Breaking changes:
 
 - In the GeoJSON output of rio-blocks, the windows are now JSON
   representations of the `Window` class (#1074).
-- The `rasterio.windows.Window` class no longer derives from `tuple`.
-  Comparisons like `Window(0, 0, 1, 1) == ((0, 1), (0, 1))` are no
-  longer possible. Instead, call the `.toranges()` method of the 
-  former or coerce the latter using `Window.from_ranges()` (#1074).
+- The ``rasterio.windows.Window`` class no longer derives from ``tuple``.
+  Comparisons like ``Window(0, 0, 1, 1) == ((0, 1), (0, 1))`` are no
+  longer possible. Instead, call the ``.toranges()`` method of the 
+  former or coerce the latter using ``Window.from_ranges()`` (#1074).
 
 New features:
 
@@ -19,6 +19,7 @@ New features:
   Rasterio (#1074).
 - Use of old style range tuples as windows is deprecated and warned
   against (#1074).
+- Addition of dataset ``block()`` method (#1077).
 
 1.0a9 (2017-06-02)
 ------------------

rasterio/_base.pyx

@@ -449,6 +449,45 @@ cdef class DatasetBase(object):
                     GDALGetRasterUnitType(self.band(j)) for j in self.indexes)
             return self._units
 
+    def block(self, bidx, i, j):
+        """Returns info about a particular block
+
+        Parameters
+        ----------
+        bidx: int
+            Band index, starting with 1.
+        i: int
+            Row index of the block, starting with 0.
+        j: int
+            Column index of the block, starting with 0.
+
+        Returns
+        -------
+        {'window': Window, 'size': int (bytes)}
+        """
+        cdef GDALMajorObjectH obj = NULL
+        cdef char *value = NULL
+        cdef const char *key_c = NULL
+
+        obj = self.band(bidx)
+
+        if self.driver == 'GTiff':
+            key_b = 'BLOCK_SIZE_{0}_{1}'.format(j, i).encode('utf-8')
+            key_c = key_b
+            value = GDALGetMetadataItem(obj, key_c, 'TIFF')
+            size = int(value)
+        else:
+            size = None
+
+        h, w = self.block_shapes[bidx-1]
+        row = i * h
+        height = min(h, self.height - row)
+        col = j * w
+        width = min(w, self.width - col)
+        window = windows.Window(col, row, width, height)
+
+        return {'window': window, 'size': size}
+
     def block_windows(self, bidx=0):
         """Returns an iterator over a band's blocks and their corresponding
         windows.  Produces tuples like ``(block, window)``.  The primary use

tests/test_blocks.py

@@ -162,3 +162,12 @@ def test_block_windows_filtered_none(path_rgb_byte_tif):
         itr = ((ij, win) for ij, win in src.block_windows() if filter_func(win))
         with pytest.raises(StopIteration):
             next(itr)
+
+
+def test_block_tiff(path_rgb_byte_tif):
+    """Without compression a TIFF's blocks are all the same size"""
+    with rasterio.open(path_rgb_byte_tif) as src:
+        block_windows = list(src.block_windows())
+        sizes = [src.block(1, i, j)['size'] for (i, j), w in block_windows]
+        assert sizes.count(2373) == 1
+        assert sizes.count(7119) == len(block_windows) - 1