Ensure our histogram uses a non-zero step

When all allocations are the same size, we were falling back to the size
of the smallest allocation as our step for generating histogram buckets.
When the smallest allocation was zero bytes, that would lead to
a division by zero and an exception.

Fix this by falling back to 1 instead of the size of the smallest
allocation. This will never lead to a zero division error, and still
puts every element in the same bucket because they're all equal
(otherwise we wouldn't have needed the fallback).

Signed-off-by: Matt Wozniski <[email protected]>

docs/news/113.bugfix.rst

@@ -0,0 +1 @@
+Fix the generation of histograms when the tests performed zero-byte allocations.

src/pytest_memray/plugin.py

@@ -78,7 +78,7 @@ def histogram(
     [0, 0, 0, 0, 1, 0, 0, 1, 3, 2]
 
     """
-    step = ((high - low) / bins) or low
+    step = ((high - low) / bins) or low or 1
     dist = collections.Counter((x - low) // step for x in iterable)
     return [dist[b] for b in range(bins)]
 

tests/test_utils.py

@@ -15,6 +15,7 @@
 
 from pytest_memray.utils import WriteEnabledDirectoryAction
 from pytest_memray.utils import parse_memory_string
+from pytest_memray.plugin import cli_hist
 
 
 @pytest.mark.parametrize(
@@ -123,3 +124,25 @@ def test_write_enabled_dir_cannot_create(
             w_dir_check(path)
     finally:
         tmp_path.chmod(tmp_path.stat().st_mode | write)
+
+
+def test_histogram_with_zero_byte_allocations():
+    # GIVEN
+    allocations = [0, 100, 990, 1000, 50000]
+
+    # WHEN
+    histogram = cli_hist(allocations, bins=5)
+
+    # THEN
+    assert histogram == "▄   ▄ █ ▄"
+
+
+def test_histogram_with_only_zero_byte_allocations():
+    # GIVEN
+    allocations = [0, 0, 0, 0]
+
+    # WHEN
+    histogram = cli_hist(allocations, bins=5)
+
+    # THEN
+    assert histogram == "█        "