Trying to set the labels without setting ticks through pyplot now raises TypeError with a proper message

rscmendes · rscmendes · commit 86dd139af4f1 · 2019-12-16T00:23:11.000Z
diff --git a/doc/api/next_api_changes/behaviour.rst b/doc/api/next_api_changes/behaviour.rst
@@ -41,3 +41,12 @@ did nothing, when passed an unsupported value. It now raises a ``ValueError``.
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 ``backend_pgf.LatexManager.latex`` is now created with ``encoding="utf-8"``, so
 its ``stdin``, ``stdout``, and ``stderr`` attributes are utf8-encoded.
+
+``pyplot.xticks()`` and ``pyplot.yticks()``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Previously, passing labels without passing the ticks to either `.pyplot.xticks`
+and `.pyplot.yticks` would result in
+
+    TypeError: object of type 'NoneType' has no len()
+
+It now raises a ``TypeError`` with a proper description of the error.
diff --git a/lib/matplotlib/pyplot.py b/lib/matplotlib/pyplot.py
@@ -1394,14 +1394,17 @@ def xticks(ticks=None, labels=None, **kwargs):
     """
     ax = gca()
 
-    if ticks is None and labels is None:
+    if ticks is None:
         locs = ax.get_xticks()
-        labels = ax.get_xticklabels()
-    elif labels is None:
+        if labels is not None:
+            raise TypeError("xticks(): Parameter 'labels' can't be set "
+                            "without setting 'ticks'")
+    else:
         locs = ax.set_xticks(ticks)
+
+    if labels is None:
         labels = ax.get_xticklabels()
     else:
-        locs = ax.set_xticks(ticks)
         labels = ax.set_xticklabels(labels, **kwargs)
     for l in labels:
         l.update(kwargs)
@@ -1451,14 +1454,17 @@ def yticks(ticks=None, labels=None, **kwargs):
     """
     ax = gca()
 
-    if ticks is None and labels is None:
+    if ticks is None:
         locs = ax.get_yticks()
-        labels = ax.get_yticklabels()
-    elif labels is None:
+        if labels is not None:
+            raise TypeError("yticks(): Parameter 'labels' can't be set "
+                            "without setting 'ticks'")
+    else:
         locs = ax.set_yticks(ticks)
+
+    if labels is None:
         labels = ax.get_yticklabels()
     else:
-        locs = ax.set_yticks(ticks)
         labels = ax.set_yticklabels(labels, **kwargs)
     for l in labels:
         l.update(kwargs)
