Descriptive error when subplot spacing impossible

Indicates range of values that are accepted.

Author: nicholas-esterer

Diff for: packages/python/plotly/plotly/subplots.py

@@ -530,19 +530,35 @@ def _checks(item, defaults):
             )
         )
 
+    def _check_hv_spacing(dimsize, spacing, name, dimname):
+        if spacing < 0 or spacing > 1:
+            raise ValueError("%s spacing must be between 0 and 1." % (name,))
+        if dimsize <= 1:
+            return
+        max_spacing = 1 / (dimsize - 1)
+        if spacing > max_spacing:
+            raise ValueError(
+                "%s spacing cannot be greater than (1 / (%s - 1)) = %f."
+                % (name, dimname, max_spacing)
+            )
+
     # ### horizontal_spacing ###
     if horizontal_spacing is None:
         if has_secondary_y:
             horizontal_spacing = 0.4 / cols
         else:
             horizontal_spacing = 0.2 / cols
+    # check horizontal_spacing can be satisfied:
+    _check_hv_spacing(cols, horizontal_spacing, "Horizontal", "cols")
 
     # ### vertical_spacing ###
     if vertical_spacing is None:
         if subplot_titles is not None:
             vertical_spacing = 0.5 / rows
         else:
             vertical_spacing = 0.3 / rows
+    # check vertical_spacing can be satisfied:
+    _check_hv_spacing(rows, vertical_spacing, "Vertical", "rows")
 
     # ### subplot titles ###
     if subplot_titles is None:

Diff for: packages/python/plotly/plotly/tests/test_core/test_subplots/test_make_subplots.py

@@ -1,6 +1,7 @@
 from __future__ import absolute_import
 
 from unittest import TestCase
+import pytest
 from plotly.graph_objs import (
     Annotation,
     Annotations,
@@ -1934,3 +1935,73 @@ def test_if_passed_figure(self):
         fig2sp = subplots.make_subplots(2, 2)
         assert id(fig2sp) != id(figsp)
         assert fig2sp.layout == figsp.layout
+
+
+def test_make_subplots_spacing_error():
+    # check exception describing maximum value for horizontal_spacing or
+    # vertical_spacing is raised when spacing exceeds that value
+    with pytest.raises(
+        ValueError,
+        match=(
+            "^%s spacing cannot be greater than \(1 / \(%s - 1\)\) = %f.$"
+            % ("Vertical", "rows", 0.02)
+        ).replace(".", "\."),
+    ):
+        fig = subplots.make_subplots(51, 1, vertical_spacing=0.0201)
+    with pytest.raises(
+        ValueError,
+        match=(
+            "^%s spacing cannot be greater than \(1 / \(%s - 1\)\) = %f.$"
+            % ("Horizontal", "cols", 0.02)
+        ).replace(".", "\."),
+    ):
+        fig = subplots.make_subplots(1, 51, horizontal_spacing=0.0201)
+    try:
+        fig = subplots.make_subplots(51, 1, vertical_spacing=0.0200)
+    except ValueError:
+        # This shouldn't happen so we assert False to force failure
+        assert False
+    try:
+        fig = subplots.make_subplots(1, 51, horizontal_spacing=0.0200)
+    except ValueError:
+        # This shouldn't happen so we assert False to force failure
+        assert False
+    # make sure any value between 0 and 1 works for horizontal_spacing if cols is 1
+    try:
+        fig = subplots.make_subplots(1, 1, horizontal_spacing=0)
+    except ValueError:
+        # This shouldn't happen so we assert False to force failure
+        assert False
+    try:
+        fig = subplots.make_subplots(1, 1, horizontal_spacing=1)
+    except ValueError:
+        # This shouldn't happen so we assert False to force failure
+        assert False
+    # make sure any value between 0 and 1 works for horizontal_spacing if cols is 1
+    try:
+        fig = subplots.make_subplots(1, 1, horizontal_spacing=0)
+    except ValueError:
+        # This shouldn't happen so we assert False to force failure
+        assert False
+    # make sure any value between 0 and 1 works for horizontal_spacing if cols is 1
+    try:
+        fig = subplots.make_subplots(1, 1, horizontal_spacing=1)
+    except ValueError:
+        # This shouldn't happen so we assert False to force failure
+        assert False
+    with pytest.raises(
+        ValueError, match="^Horizontal spacing must be between 0 and 1\.$"
+    ):
+        fig = subplots.make_subplots(1, 1, horizontal_spacing=-0.01)
+    with pytest.raises(
+        ValueError, match="^Horizontal spacing must be between 0 and 1\.$"
+    ):
+        fig = subplots.make_subplots(1, 1, horizontal_spacing=1.01)
+    with pytest.raises(
+        ValueError, match="^Vertical spacing must be between 0 and 1\.$"
+    ):
+        fig = subplots.make_subplots(1, 1, vertical_spacing=-0.01)
+    with pytest.raises(
+        ValueError, match="^Vertical spacing must be between 0 and 1\.$"
+    ):
+        fig = subplots.make_subplots(1, 1, vertical_spacing=1.01)